Welcome dear students! Today we are going to learn about Acids, Bases and Salts from Class 10 Science. You have learnt in your previous classes that the sour and bitter tastes of food are due to acids and bases, respectively, present in them. If someone in the family is suffering from a problem of acidity after overeating, which of the following would you suggest as a remedy: lemon juice, vinegar or baking soda solution? Which property did you think of while choosing the remedy? Surely you must have used your knowledge about the ability of acids and bases to nullify each other’s effect. Recall how we tested sour and bitter substances without tasting them. You already know that acids are sour in taste and change the colour of blue litmus to red, whereas bases are bitter and change the colour of red litmus to blue. Litmus is a natural indicator, and turmeric is another such indicator. Have you noticed that a stain of curry on a white cloth becomes reddish-brown when soap, which is basic in nature, is scrubbed on it? It turns yellow again when the cloth is washed with plenty of water. You can also use synthetic indicators such as methyl orange and phenolphthalein to test for acids and bases. In this Chapter, we will study the reactions of acids and bases, how acids and bases cancel out each other’s effects and many more interesting things that we use and see in our day-to-day life.
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Do You Know? Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta, and is commonly used as an indicator. When the litmus solution is neither acidic nor basic, its colour is purple. There are many other natural materials like red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a solution. These are called acid-base indicators or sometimes simply indicators. Now let us address the first question provided in the text. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? First, dip the red litmus paper into each test tube one by one. The test tube in which the red litmus paper turns blue contains the basic solution. Take the now blue litmus paper and dip it into the remaining two test tubes. The test tube in which the blue litmus turns red contains the acidic solution. The test tube that causes no colour change in either red or blue litmus contains distilled water.
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Let us move to section two point one, Understanding the Chemical Properties of Acids and Bases. We begin with two point one point one, Acids and Bases in the Laboratory. Activity two point one requires you to collect the following solutions from the science laboratory: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃), acetic acid (CH₃COOH), sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)₂], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)₂], and ammonium hydroxide (NH₄OH). Put a drop of each of the above solutions on a watch-glass one by one and test with a drop of the indicators shown in Table two point one. You must observe the colour change with red litmus, blue litmus, phenolphthalein and methyl orange solutions for each solution. Tabulate your observations. These indicators tell us whether a substance is acidic or basic by change in colour. There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators. Let us try out some of these indicators in Activity two point two. Take some finely chopped onions in a plastic bag along with some strips of clean cloth. Tie up the bag tightly and leave overnight in the fridge. The cloth strips can now be used to test for acids and bases. Take two of these cloth strips and check their odour. Keep them on a clean surface and put a few drops of dilute HCl solution on one strip and a few drops of dilute NaOH solution on the other. Rinse both cloth strips with water and again check their odour. Note your observations. Now take some dilute vanilla essence and clove oil and check their odour. Take some dilute HCl solution in one test tube and dilute NaOH solution in another. Add a few drops of dilute vanilla essence to both test tubes and shake well. Check the odour once again and record changes in odour, if any. Similarly, test the change in the odour of clove oil with dilute HCl and dilute NaOH solutions and record your observations. Based on your observations, vanilla, onion and clove can be used as olfactory indicators.
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Let us do some more activities to understand the chemical properties of acids and bases in section two point one point two, How do Acids and Bases React with Metals? Activity two point three requires caution and the teacher’s assistance. Set the apparatus as shown in Figure two point one. In this diagram, we see a test tube held in a stand. Inside the test tube is dilute sulphuric acid and zinc granules. A delivery tube connects the test tube to a container of soap solution. Bubbles of hydrogen gas form in the soap solution. A burning candle is brought near a gas filled bubble to test the gas. Take about five millilitres of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it. Observe the surface of zinc granules. Pass the gas being evolved through the soap solution. Bubbles are formed in the soap solution because hydrogen gas is produced. Take a burning candle near a gas filled bubble. You will observe the hydrogen gas burning with a pop sound. Repeat this activity with some more acids like HCl, HNO₃ and CH₃COOH. The observations in all cases are the same. Note that the metal in the above reactions displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt. Thus, the reaction of a metal with an acid can be summarised as: Acid + Metal → Salt + Hydrogen gas. You can now write the equations for the reactions you have observed.
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Next is Activity two point four. Place a few pieces of granulated zinc metal in a test tube. Add two millilitres of sodium hydroxide solution and warm the contents of the test tube. Repeat the rest of the steps as in Activity two point three and record your observations. The reaction that takes place can be written as follows: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g). The product Na₂ZnO₂ is sodium zincate. You find again that hydrogen is formed in the reaction. However, such reactions are not possible with all metals. Let us proceed to section two point one point three, How do Metal Carbonates and Metal Hydrogencarbonates React with Acids? Activity two point five instructs you to take two test tubes, label them as A and B. Take about zero point five grams of sodium carbonate (Na₂CO₃) in test tube A and about zero point five grams of sodium hydrogencarbonate (NaHCO₃) in test tube B. Add about two millilitres of dilute HCl to both the test tubes. Observe the reaction. Pass the gas produced in each case through lime water, which is calcium hydroxide solution, as shown in Figure two point two. In this figure, a test tube containing the reacting mixture is connected via a delivery tube to another test tube containing clear lime water. The gas bubbles through the lime water. Record your observations. The reactions occurring are: Test tube A: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g). Test tube B: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g). On passing the carbon dioxide gas evolved through lime water, the reaction is: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). Lime water turns milky due to the white precipitate of calcium carbonate.
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On passing excess carbon dioxide, the following reaction takes place: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq), which is soluble in water. Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water. Thus, the reaction can be summarised as: Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water. Now we move to section two point one point four, How do Acids and Bases React with each other? Activity two point six: Take about two millilitres of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution. The solution turns pink. Add dilute HCl solution to the above solution drop by drop. The pink colour disappears. Why did the colour of phenolphthalein change after the addition of an acid? Because the base is neutralised by the acid. Now add a few drops of NaOH to the above mixture. The pink colour of phenolphthalein reappears because the base is in excess again. In this activity, we have observed that the effect of a base is nullified by an acid and vice-versa. The reaction taking place is: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as: Base + Acid → Salt + Water.
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Next is section two point one point five, Reaction of Metallic Oxides with Acids. Activity two point seven: Take a small amount of copper oxide in a beaker and add dilute hydrochloric acid slowly while stirring. Note the colour of the solution. The copper oxide dissolves and the solution becomes blue-green. The blue-green colour is due to the formation of copper(II) chloride. The general reaction between a metal oxide and an acid can be written as: Metal oxide + Acid → Salt + Water. Now write and balance the equation for the above reaction: CuO + 2HCl → CuCl₂ + H₂O. Since metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides. Section two point one point six covers Reaction of a Non-metallic Oxide with Base. You saw the reaction between carbon dioxide and calcium hydroxide, lime water, in Activity two point five. Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. Since this is similar to the reaction between a base and an acid, we can conclude that non-metallic oxides are acidic in nature.
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Let us answer the questions from this section. Question one: Why should curd and sour substances not be kept in brass and copper vessels? Answer: Curd and sour substances contain acids. These acids react with brass and copper to form toxic salts, which can contaminate the food and cause poisoning. Question two: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas? Answer: Hydrogen gas is usually liberated. For example, zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas: Zn + H₂SO₄ → ZnSO₄ + H₂. To test for hydrogen gas, pass it through soap solution to form bubbles. Bring a burning candle near the bubble. The gas burns with a pop sound, confirming hydrogen. Question three is a worked example. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. Answer: The gas that extinguishes a burning candle and causes effervescence with acid is carbon dioxide. This means compound A is a carbonate. Since calcium chloride is formed, A must be calcium carbonate, CaCO₃. The balanced equation is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
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Now we proceed to section two point two, What do all Acids and all Bases have in Common? In section two point one we saw that all acids have similar chemical properties. What leads to this similarity? We saw in Activity two point three that all acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids. Let us perform Activity two point eight to investigate whether all compounds containing hydrogen are acidic. Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, and so on. Fix two nails on a cork, and place the cork in a one hundred millilitre beaker. Connect the nails to the two terminals of a six volt battery through a bulb and a switch, as shown in Figure two point three. In this diagram, a beaker contains two nails connected to a battery and a bulb. Pour some dilute HCl in the beaker and switch on the current. The bulb glows. Repeat with dilute sulphuric acid. The bulb glows again. Repeat the experiment separately with glucose and alcohol solutions. The bulb does not glow. Glowing of the bulb indicates that there is a flow of electric current through the solution. The electric current is carried through the acidic solution by ions. Acids contain H⁺ ion as cation and anion such as Cl⁻ in HCl, NO₃⁻ in HNO₃, SO₄²⁻ in H₂SO₄, CH₃COO⁻ in CH₃COOH. Since the cation present in acids is H⁺, this suggests that acids produce hydrogen ions, H⁺(aq), in solution, which are responsible for their acidic properties. Repeat the same activity using alkalis such as sodium hydroxide, calcium hydroxide, and so on. The bulb glows, showing bases also produce ions.
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Section two point two point one asks, What Happens to an Acid or a Base in a Water Solution? Do acids produce ions only in aqueous solution? Let us test this in Activity two point nine. Take about one gram solid NaCl in a clean and dry test tube and set up the apparatus as shown in Figure two point four. This figure shows a test tube with solid NaCl and concentrated sulphuric acid, connected to a delivery tube for gas collection. Add some concentrated sulphuric acid to the test tube. A gas comes out of the delivery tube. Test the gas evolved successively with dry and wet blue litmus paper. The dry blue litmus paper does not change colour. The wet blue litmus paper turns red. On the basis of this activity, we infer that dry HCl gas is not acidic, but HCl solution is acidic. This experiment suggests that hydrogen ions in HCl are produced in the presence of water. The separation of H⁺ ion from HCl molecules cannot occur in the absence of water. The equation is: HCl + H₂O → H₃O⁺ + Cl⁻. Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H⁺(aq) or hydronium ion, H₃O⁺. The equation is: H⁺ + H₂O → H₃O⁺. We have seen that acids give H₃O⁺ or H⁺(aq) ion in water. Let us see what happens when a base is dissolved in water. NaOH(s) → H₂O → Na⁺(aq) + OH⁻(aq) in water. KOH(s) → H₂O → K⁺(aq) + OH⁻(aq) in water. Mg(OH)₂(s) → H₂O → Mg²⁺(aq) + 2OH⁻(aq) in water. Bases generate hydroxide, OH⁻, ions in water. Bases which are soluble in water are called alkalis.
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Do You Know? All bases do not dissolve in water. An alkali is a base that dissolves in water. They are soapy to touch, bitter and corrosive. Never taste or touch them as they may cause harm. Which of the bases in Table two point one are alkalis? Sodium hydroxide, calcium hydroxide, potassium hydroxide, magnesium hydroxide, and ammonium hydroxide are all alkalis. Now as we have identified that all acids generate H⁺(aq) and all bases generate OH⁻(aq), we can view the neutralisation reaction as follows: Acid + Base → Salt + Water. In ionic form: H⁺(aq) + OH⁻(aq) → H₂O(l). Let us see what is involved when water is mixed with an acid or a base in Activity two point ten. Take ten millilitres water in a beaker. Add a few drops of concentrated H₂SO₄ to it and swirl the beaker slowly. Touch the base of the beaker. The beaker becomes hot. This is an exothermic process. Repeat with sodium hydroxide pellets. The beaker becomes hot again. The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Look out for the warning sign shown in Figure two point five on the can of concentrated sulphuric acid and on the bottle of sodium hydroxide pellets. Mixing an acid or base with water results in decrease in the concentration of ions, H₃O⁺ or OH⁻, per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.
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Let us answer the questions from section two point two. Question one: Why do HCl, HNO₃, and so on, show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character? Answer: HCl and HNO₃ dissociate in water to produce H⁺ ions, which give acidic character. Alcohol and glucose do not dissociate to release H⁺ ions in water, so they do not show acidic character. Question two: Why does an aqueous solution of an acid conduct electricity? Answer: Because it contains freely moving ions, specifically H⁺ and anions, which carry electric current. Question three: Why does dry HCl gas not change the colour of the dry litmus paper? Answer: Dry HCl gas does not produce H⁺ ions. Ions are only formed in the presence of water. Question four: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? Answer: Adding water to concentrated acid releases a large amount of heat suddenly, which can cause splashing and burns. Adding acid to water slowly dissipates the heat safely. Question five: How is the concentration of hydronium ions, H₃O⁺, affected when a solution of an acid is diluted? Answer: The concentration of hydronium ions decreases per unit volume. Question six: How is the concentration of hydroxide ions, OH⁻, affected when excess base is dissolved in a solution of sodium hydroxide? Answer: The concentration of hydroxide ions increases.
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We now move to section two point three, How Strong are Acid or Base Solutions? We know how acid-base indicators can be used to distinguish between an acid and a base. We have also learnt about dilution and decrease in concentration of H⁺ or OH⁻ ions in solutions. Can we quantitatively find the amount of these ions present in a solution? Can we judge how strong a given acid or base is? We can do this by making use of a universal indicator, which is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution. A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for potenz in German, meaning power. On the pH scale we can measure pH generally from zero, very acidic, to fourteen, very alkaline. pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is seven. Values less than seven on the pH scale represent an acidic solution. As the pH value increases from seven to fourteen, it represents an increase in OH⁻ ion concentration in the solution, that is, increase in the strength of alkali. This is shown in Figure two point six, which illustrates a scale from zero to fourteen with H⁺ concentration decreasing and OH⁻ concentration increasing. Generally paper impregnated with the universal indicator is used for measuring pH.
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Activity two point eleven instructs you to test the pH values of solutions given in Table two point two. Record your observations and determine the nature of each substance. Table two point two lists: Saliva before meal, Saliva after meal, Lemon juice, Colourless aerated drink, Carrot juice, Coffee, Tomato juice, Tap water, one M NaOH, and one M HCl. You will test these with pH paper and note the colour and approximate pH. Figure two point seven shows a pH paper scale with common substances: Gastric juice about one point two, Lemon juice about two point two, Pure water seven, Blood seven point four, Milk of magnesia ten, and Sodium hydroxide solution about fourteen. The strength of acids and bases depends on the number of H⁺ ions and OH⁻ ions produced, respectively. If we take hydrochloric acid and acetic acid of the same concentration, say one molar, then these produce different amounts of hydrogen ions. Acids that give rise to more H⁺ ions are said to be strong acids, and acids that give less H⁺ ions are said to be weak acids. Weak and strong bases are defined similarly based on OH⁻ ion production.
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Section two point three point one discusses the Importance of pH in Everyday Life. Are plants and animals pH sensitive? Our body works within the pH range of seven point zero to seven point eight. Living organisms can survive only in a narrow range of pH change. When pH of rain water is less than five point six, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers becomes difficult. Do You Know? The atmosphere of Venus is made up of thick white and yellowish clouds of sulphuric acid. Do you think life can exist on this planet? What is the pH of the soil in your backyard? Plants require a specific pH range for their healthy growth. To find out the pH required for the healthy growth of a plant, you can collect the soil from various places and check the pH in the manner described in Activity two point twelve. Put about two grams soil in a test tube and add five millilitres water to it. Shake the contents of the test tube. Filter the contents and collect the filtrate in a test tube. Check the pH of this filtrate with the help of universal indicator paper. You can conclude about the ideal soil pH for the growth of plants in your region. Also, note down which plants are growing in the region from which you have collected the soil.
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pH in our digestive system: It is very interesting to note that our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach. During indigestion the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, people use bases called antacids. One such remedy must have been suggested by you at the beginning of this Chapter. These antacids neutralise the excess acid. Magnesium hydroxide, Milk of magnesia, a mild base, is often used for this purpose. pH change as the cause of tooth decay: Tooth decay starts when the pH of the mouth is lower than five point five. Tooth enamel, made up of calcium hydroxyapatite, a crystalline form of calcium phosphate, is the hardest substance in the body. It does not dissolve in water, but is corroded when the pH in the mouth is below five point five. Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay. Self defence by animals and plants through chemical warfare: Have you ever been stung by a honey-bee? Bee-sting leaves an acid which causes pain and irritation. Use of a mild base like baking soda on the stung area gives relief. Stinging hair of nettle leaves inject methanoic acid causing burning pain.
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Do You Know? Nature provides neutralisation options. Nettle is a herbaceous plant which grows in the wild. Its leaves have stinging hair, which cause painful stings when touched accidentally. This is due to the methanoic acid secreted by them. A traditional remedy is rubbing the area with the leaf of the dock plant, which often grows beside the nettle in the wild. Can you guess the nature of the dock plant? It is basic in nature. So next time you know what to look out for if you accidentally touch a nettle plant while trekking. Table two point three lists some naturally occurring acids. Vinegar contains acetic acid. Sour milk, curd, contains lactic acid. Orange contains citric acid. Lemon contains citric acid. Tamarind contains tartaric acid. Ant sting contains methanoic acid. Tomato contains oxalic acid. Nettle sting contains methanoic acid. Let us answer the questions from section two point three. Question one: You have two solutions, A and B. The pH of solution A is six and pH of solution B is eight. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic? Answer: Solution A has more hydrogen ion concentration. Solution A is acidic and solution B is basic. Question two: What effect does the concentration of H⁺(aq) ions have on the nature of the solution? Answer: Higher H⁺ concentration makes the solution more acidic, while lower concentration makes it less acidic or basic. Question three: Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic? Answer: Yes, basic solutions have H⁺ ions, but they have a higher concentration of OH⁻ ions than H⁺ ions, making them basic. Question four: Under what soil condition do you think a farmer would treat the soil of his fields with quick lime, calcium oxide, or slaked lime, calcium hydroxide, or chalk, calcium carbonate? Answer: When the soil is too acidic, the farmer treats it with these basic substances to neutralise the acidity.
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We now enter section two point four, More About Salts. In the previous sections we have seen the formation of salts during various reactions. Let us understand more about their preparation, properties and uses. Section two point four point one covers Family of Salts. Activity two point thirteen: Write the chemical formulae of the salts given below. Potassium sulphate is K₂SO₄. Sodium sulphate is Na₂SO₄. Calcium sulphate is CaSO₄. Magnesium sulphate is MgSO₄. Copper sulphate is CuSO₄. Sodium chloride is NaCl. Sodium nitrate is NaNO₃. Sodium carbonate is Na₂CO₃. Ammonium chloride is NH₄Cl. Identify the acids and bases from which the above salts may be obtained. For example, NaCl comes from HCl and NaOH. Salts having the same positive or negative radicals are said to belong to a family. For example, NaCl and Na₂SO₄ belong to the family of sodium salts. Similarly, NaCl and KCl belong to the family of chloride salts. You can identify multiple families among these salts.
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Section two point four point two discusses pH of Salts. Activity two point fourteen: Collect the following salt samples: sodium chloride, potassium nitrate, aluminium chloride, zinc sulphate, copper sulphate, sodium acetate, sodium carbonate and sodium hydrogencarbonate. Check their solubility in water using distilled water only. Check the action of these solutions on litmus and find the pH using a pH paper. Determine which salts are acidic, basic or neutral. Identify the acid or base used to form the salt. Salts of a strong acid and a strong base are neutral with pH value of seven. On the other hand, salts of a strong acid and weak base are acidic with pH value less than seven and those of a strong base and weak acid are basic in nature, with pH value more than seven. You will record these in Table two point four. Section two point four point three covers Chemicals from Common Salt. The salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that you use in food. It is a neutral salt. Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal. You must have heard about Mahatma Gandhi’s Dandi March. Sodium chloride was such an important symbol in our struggle for freedom.
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Common salt is a raw material for chemicals. It is used to make sodium hydroxide, baking soda, washing soda, bleaching powder and many more. Sodium hydroxide: When electricity is passed through an aqueous solution of sodium chloride, called brine, it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed: chlor for chlorine and alkali for sodium hydroxide. The equation is: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode. Figure two point eight shows the chlor-alkali process setup. Chlorine is used for water treatment, swimming pools, PVC, disinfectants, CFCs, and pesticides. Hydrogen is used for fuels, margarine, and ammonia for fertilisers. Sodium hydroxide is used for de-greasing metals, soaps and detergents, paper making, and artificial fibres. Bleaching powder: Chlorine produced during electrolysis is used for bleaching powder. It is produced by the action of chlorine on dry slaked lime, Ca(OH)₂. Bleaching powder is represented as CaOCl₂, though the actual composition is quite complex. The equation is: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Bleaching powder is used for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry. It is used as an oxidising agent in many chemical industries. It is used to make drinking water free from germs.
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Baking soda: Commonly used in the kitchen for making tasty crispy pakoras. The chemical name is sodium hydrogencarbonate, NaHCO₃. It is produced using sodium chloride as one of the raw materials. The equation is: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. Ammonium chloride and sodium hydrogencarbonate are formed. It is a mild non-corrosive basic salt. When heated during cooking: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Uses of baking soda: For making baking powder, which is a mixture of baking soda and a mild edible acid such as tartaric acid. When heated or mixed in water: NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt of acid. Carbon dioxide causes bread or cake to rise. It is an ingredient in antacids to neutralise excess acid. It is used in soda-acid fire extinguishers. Washing soda: Another chemical from sodium chloride is Na₂CO₃·10H₂O. Sodium carbonate is obtained by heating baking soda. Recrystallisation gives washing soda. It is a basic salt. The equation is: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. What does 10H₂O signify? Does it make Na₂CO₃ wet? We will address this next. Uses of washing soda: Used in glass, soap and paper industries. Used in manufacture of sodium compounds such as borax. Used as a cleaning agent for domestic purposes. Used for removing permanent hardness of water.
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Section two point four point four asks, Are the Crystals of Salts really Dry? Activity two point fifteen: Heat a few crystals of copper sulphate in a dry boiling tube. Figure two point nine shows a boiling tube held by tongs over a burner, with water droplets forming on the cooler upper part. What is the colour of the copper sulphate after heating? It turns white. Do you notice water droplets in the boiling tube? Where have these come from? They come from the water of crystallisation in the salt. Add two to three drops of water on the heated sample. The blue colour is restored. Copper sulphate crystals which seem to be dry contain water of crystallisation. When heated, this water is removed and the salt turns white. Moistening restores the blue colour. Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is CuSO₄·5H₂O. Now you would be able to answer the question whether the molecule of Na₂CO₃·10H₂O is wet. It is not wet; the water is chemically bound in the crystal structure. One other salt possessing water of crystallisation is gypsum. It has two water molecules. Chemical formula is CaSO₄·2H₂O.
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Plaster of Paris: On heating gypsum at three hundred seventy three K, it loses water molecules and becomes calcium sulphate hemihydrate, CaSO₄·½H₂O. This is called Plaster of Paris, used as plaster for supporting fractured bones. It is a white powder. On mixing with water, it changes to gypsum again giving a hard solid mass. Equation: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Note that only half a water molecule is shown. It is written this way because two formula units of CaSO₄ share one molecule of water. Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth. Let us answer the questions from section two point four. Question one: What is the common name of the compound CaOCl₂? Answer: Bleaching powder. Question two: Name the substance which on treatment with chlorine yields bleaching powder. Answer: Slaked lime, calcium hydroxide. Question three: Name the sodium compound which is used for softening hard water. Answer: Sodium carbonate, washing soda. Question four: What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation. Answer: It decomposes to sodium carbonate, water and carbon dioxide. 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Question five: Write an equation to show the reaction between Plaster of Paris and water. Answer: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
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Let us review what you have learnt. Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases. Acidic nature of a substance is due to the formation of H⁺(aq) ions in solution. Formation of OH⁻(aq) ions in solution is responsible for the basic nature of a substance. When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed. When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen. When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives the corresponding salt, carbon dioxide gas and water. Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively. The strength of an acid or an alkali can be tested by using a scale called the pH scale, zero to fourteen, which gives the measure of hydrogen ion concentration in a solution. A neutral solution has a pH of exactly seven, while an acidic solution has a pH less than seven and a basic solution a pH more than seven. Living beings carry out their metabolic activities within an optimal pH range. Mixing concentrated acids or bases with water is a highly exothermic process. Acids and bases neutralise each other to form corresponding salts and water. Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Salts have various uses in everyday life and in industries.
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Now let us solve the exercises completely. Exercise one: A solution turns red litmus blue, its pH is likely to be. Answer: d, ten. Exercise two: A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains. Answer: b, HCl. Exercise three: Ten millilitres of a solution of NaOH is found to be completely neutralised by eight millilitres of a given solution of HCl. If we take twenty millilitres of the same solution of NaOH, the amount HCl solution required to neutralise it will be. Answer: d, sixteen millilitres. Exercise four: Which one of the following types of medicines is used for treating indigestion? Answer: c, Antacid. Exercise five: Write word equations and then balanced equations for the reaction taking place when dilute sulphuric acid reacts with zinc granules. Answer: Zinc plus sulphuric acid yields zinc sulphate plus hydrogen. Zn + H₂SO₄ → ZnSO₄ + H₂. Dilute hydrochloric acid reacts with magnesium ribbon. Answer: Magnesium plus hydrochloric acid yields magnesium chloride plus hydrogen. Mg + 2HCl → MgCl₂ + H₂. Dilute sulphuric acid reacts with aluminium powder. Answer: Aluminium plus sulphuric acid yields aluminium sulphate plus hydrogen. 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂. Dilute hydrochloric acid reacts with iron filings. Answer: Iron plus hydrochloric acid yields iron two chloride plus hydrogen. Fe + 2HCl → FeCl₂ + H₂.
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Exercise six: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it. Answer: Set up the circuit with two nails in a beaker connected to a battery and bulb. Pour glucose solution. Switch on. Bulb does not glow. Repeat with alcohol. Bulb does not glow. Pour dilute HCl. Bulb glows. This proves acids produce ions in water, while glucose and alcohol do not dissociate into ions. Exercise seven: Why does distilled water not conduct electricity, whereas rain water does? Answer: Distilled water is pure and lacks ions. Rain water contains dissolved salts and atmospheric gases that form ions, allowing conduction. Exercise eight: Why do acids not show acidic behaviour in the absence of water? Answer: Acids require water to dissociate and produce H⁺ or H₃O⁺ ions. Without water, no ions are formed, so no acidic behaviour is shown. Exercise nine: Five solutions A, B, C, D and E when tested with universal indicator showed pH as four, one, eleven, seven and nine, respectively. Which solution is neutral? Answer: D, pH seven. Strongly alkaline? Answer: C, pH eleven. Strongly acidic? Answer: B, pH one. Weakly acidic? Answer: A, pH four. Weakly alkaline? Answer: E, pH nine. Arrange the pH in increasing order of hydrogen-ion concentration. Answer: One, four, seven, nine, eleven. Exercise ten: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid is added to A, acetic acid to B. Amount and concentration same. In which test tube will the fizzing occur more vigorously and why? Answer: Test tube A. HCl is a strong acid and produces more H⁺ ions than weak acetic acid, leading to faster hydrogen gas evolution.
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Exercise eleven: Fresh milk has a pH of six. How do you think the pH will change as it turns into curd? Explain your answer. Answer: pH will decrease below six. Bacteria produce lactic acid during fermentation, increasing acidity. Exercise twelve: A milkman adds a very small amount of baking soda to fresh milk. Why does he shift the pH of the fresh milk from six to slightly alkaline? Answer: Baking soda is basic. It neutralises some acid, raising pH. Why does this milk take a long time to set as curd? Answer: The added base neutralises the lactic acid produced by bacteria, delaying the pH drop needed for curdling. Exercise thirteen: Plaster of Paris should be stored in a moisture-proof container. Explain why? Answer: It reacts with moisture in air to form gypsum and hardens prematurely, losing its usefulness. Exercise fourteen: What is a neutralisation reaction? Give two examples. Answer: Reaction between an acid and a base to form salt and water. Examples: NaOH + HCl → NaCl + H₂O. Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. Exercise fifteen: Give two important uses of washing soda and baking soda. Answer: Washing soda: glass and soap manufacture, removing permanent hardness. Baking soda: antacid, baking powder ingredient.
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Now let us cover the Group Activities. Group Activity one: Prepare your own indicator. Crush beetroot in a mortar. Add sufficient water to obtain the extract. Filter the extract. Collect the filtrate to test substances. Arrange four test tubes labelled A, B, C and D. Pour two millilitres each of lemon juice solution, soda-water, vinegar and baking soda solution in them respectively. Put two to three drops of the beetroot extract in each test tube and note the colour change if any. Write your observation in a Table. You can prepare indicators by using other natural materials like extracts of red cabbage leaves, coloured petals of some flowers such as Petunia, Hydrangea and Geranium. Group Activity two: Preparing a soda-acid fire extinguisher. The reaction of acids with metal hydrogencarbonates is used in fire extinguishers which produce carbon dioxide. Take twenty millilitres of sodium hydrogencarbonate solution in a wash-bottle. Suspend an ignition tube containing dilute sulphuric acid in the wash-bottle, as shown in Figure two point ten. In this figure, a wash-bottle contains the hydrogencarbonate solution, with a small ignition tube holding acid suspended inside. Close the mouth of the wash-bottle. Tilt the wash-bottle so that the acid mixes with the sodium hydrogencarbonate solution below. You will notice discharge coming out of the nozzle. Direct this discharge on a burning candle. The flame is extinguished due to carbon dioxide gas.
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Finally, let us work through the two worked examples mentioned in the chapter. Worked example one: How will you test for the presence of this gas? Referring to the gas evolved when an acid reacts with a metal. Solution: Pass the gas through soap solution to form bubbles. Bring a burning candle near the bubble. If the gas burns with a characteristic pop sound, it confirms the presence of hydrogen gas. Worked example two: NaCl and Na₂SO₄ belong to the family of sodium salts. Similarly, NaCl and KCl belong to the family of chloride salts. How many families can you identify among the salts given in Activity two point thirteen? Solution: The salts are K₂SO₄, Na₂SO₄, CaSO₄, MgSO₄, CuSO₄, NaCl, NaNO₃, Na₂CO₃, NH₄Cl. Families identified: Sodium salts family includes Na₂SO₄, NaCl, NaNO₃, Na₂CO₃. Sulphate salts family includes K₂SO₄, Na₂SO₄, CaSO₄, MgSO₄, CuSO₄. Chloride salts family includes NaCl, NH₄Cl. Thus, we can identify at least three distinct families based on common cations or anions.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]