Welcome dear students! Today we are going to learn about Acids, Bases and Salts from Class 7 Science. We use in our daily life a large number of substances such as lemon, tamarind, common salt, sugar and vinegar. Do they have the same taste? Let us recall the tastes of some edible substances. You will find that some taste sour, some taste bitter, some taste sweet, and some taste salty. If you have not tasted these, please do so safely and record your results. Lemon juice, orange juice, vinegar, curd, tamarind, amla, grapes, and unripe mango taste sour. Sugar and grapes taste sweet. Common salt tastes salty. Baking soda tastes bitter. Cucumber has a mild taste. [CHECKPOINT]
Section four point one is Acids and Bases. Curd, lemon juice, orange juice and vinegar taste sour because they contain acids. The chemical nature of such substances is acidic. The word acid comes from the Latin word acere which means sour. The acids in these substances are natural acids. What about baking soda? It does not taste sour, which means it has no acids in it. It is bitter in taste. If you rub its solution between your fingers, it feels soapy. Generally, substances which are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be basic. If we cannot taste every substance, how do we find its nature? Special substances are used to test whether a substance is acidic or basic. These are known as indicators. Indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, and China rose petals, also known as Gudhal, are naturally occurring indicators. [CHECKPOINT]
Please remember the caution: Do not taste anything unless asked to do so. Do not touch anything unless asked to do so. Let us look at the Do you know box. Acids found in nature include: Acetic acid in vinegar, Formic acid in ant sting, Citric acid in citrus fruits, Lactic acid in curd, Oxalic acid in spinach, Ascorbic acid in amla and citrus fruits, and Tartaric acid in tamarind, grapes, and unripe mangoes. Bases found in nature include: Calcium hydroxide in lime water, Ammonium hydroxide in window cleaner, Sodium hydroxide and Potassium hydroxide in soap, and Magnesium hydroxide in milk of magnesia. [CHECKPOINT]
Section four point two covers Natural Indicators Around Us. The most commonly used natural indicator is litmus. It is extracted from lichens. In distilled water, it has a mauve or purple colour. When added to an acidic solution, it turns red. When added to a basic solution, it turns blue. It is available as a solution or as strips of paper known as litmus paper, usually red and blue. Now, let us perform Activity four point one step by step. Mix some water with lemon juice in a plastic cup or test tube. Put a drop of this solution on a strip of red litmus paper using a dropper. Observe any colour change. Repeat the same exercise with a strip of blue litmus paper. Note down the changes. Perform this exact same activity with tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, and lime water. Record your observations in Table four point two. [CHECKPOINT]
In Table four point two, you will note the effect on red and blue litmus paper for each test solution. Acidic solutions like lemon juice, vinegar, and aerated drinks will turn blue litmus red, while red litmus remains unchanged. Basic solutions like soap solution, detergent, baking soda, milk of magnesia, washing soda, and lime water will turn red litmus blue, while blue litmus remains unchanged. In your table, you will notice substances like tap water, sugar solution, and common salt solution do not change the colour of either red or blue litmus paper. The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic. [CHECKPOINT]
Turmeric is another natural indicator. Let us do Activity four point two. Take a tablespoonful of turmeric powder. Add a little water and make a paste. Make turmeric paper by depositing the paste on blotting paper or filter paper and drying it. Cut thin strips of the yellow paper obtained. Put a drop of soap solution on a strip. You will observe the yellow paper turns red. You can prepare a greeting card for your mother by applying turmeric paste on plain white paper, drying it, and drawing a flower with soap solution using a cotton bud. The soap will turn the yellow turmeric red. Now, test the solutions in Table four point three. Lemon juice, orange juice, vinegar, sugar, and common salt have no effect on turmeric. Milk of magnesia, baking soda, and lime water turn turmeric red. This explains why a turmeric stain on a white shirt turns red when washed with soap, because soap solution is basic. [CHECKPOINT]
Now, let us use China Rose as an indicator in Activity four point three. Collect some China rose or Gudhal petals and place them in a beaker. Add some warm water. Keep the mixture for some time till the water becomes coloured. Use this coloured water as your indicator. Add five drops of this indicator to each solution in Table four point four. China rose indicator turns acidic solutions to dark pink or magenta, and basic solutions to green. For example, shampoo and sodium hydrogencarbonate solution turn green. Lemon juice, soda water, and vinegar turn dark pink. Sugar and common salt solutions show no colour change. [CHECKPOINT]
Paheli brought a riddle: Coffee is brown and bitter in taste. Is it an acid or a base? Do not give the answer without any test, you are in the dark with its taste. Also, a student noted that solid baking soda on dry litmus paper gives no result. This is because indicators require a solution to work. Make a solution of baking soda and then try. Activity four point four is a teacher demonstration. The teacher uses dilute solutions of hydrochloric acid, sulphuric acid, nitric acid, acetic acid, sodium hydroxide, ammonium hydroxide, and calcium hydroxide. Record observations in Table four point five. Dilute acids turn blue litmus red, have no effect on turmeric, and turn China rose solution dark pink. Bases turn red litmus blue, turn turmeric red, and turn China rose green. [CHECKPOINT]
Section four point three is Neutralisation. We know acids turn blue litmus red and bases turn red litmus blue. Let us see what happens when an acid is mixed with a base. We will use phenolphthalein. Activity four point five is demonstrated by the teacher. Fill one fourth of a test tube with dilute hydrochloric acid. Note its colour. Note the colour of phenolphthalein solution. Add two to three drops of the indicator to the acid and shake gently. No colour change occurs. Add a drop of sodium hydroxide solution by a dropper and stir gently. Continue adding sodium hydroxide drop by drop while stirring till the pink colour just appears. Now add one more drop of dilute hydrochloric acid. The solution becomes colourless. Add one drop of sodium hydroxide again. It becomes pink. When the solution is basic, phenolphthalein gives a pink colour. When acidic, it remains colourless. [CHECKPOINT]
When an acidic solution is mixed with a basic solution, both neutralise the effect of each other. When mixed in suitable amounts, both the acidic nature of the acid and the basic nature of the base are destroyed. The resulting solution is neither acidic nor basic. Touch the test tube immediately after neutralisation. You will feel it is warm. In a neutralisation reaction, heat is always produced, or evolved. The evolved heat raises the temperature. A new substance is also formed, called salt. Salt may be acidic, basic, or neutral. Thus, neutralisation can be defined as follows: The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat. Acid plus Base gives Salt plus Water. Heat is evolved. For example: Hydrochloric acid, HCl, plus Sodium hydroxide, NaOH, gives Sodium chloride, NaCl, plus Water, H₂O. Boojho asks: If dilute sulphuric acid is added to lime water, will it become hot or cool? It becomes hot, as neutralisation always evolves heat. [CHECKPOINT]
Section four point four covers Neutralisation in Everyday Life. First, Indigestion. Our stomach contains hydrochloric acid to digest food. Too much acid causes painful indigestion. We take an antacid like milk of magnesia, which contains magnesium hydroxide, to neutralise the excess acid. Second, Ant bite. An ant injects formic acid into the skin. The effect is neutralised by rubbing moist baking soda, sodium hydrogencarbonate, or calamine solution containing zinc carbonate. Third, Soil treatment. Excessive chemical fertilisers make soil acidic. Plants do not grow well in highly acidic or basic soil. Acidic soil is treated with quick lime, calcium oxide, or slaked lime, calcium hydroxide. Basic soil is treated with organic matter or compost, which releases acids to neutralise it. [CHECKPOINT]
Fourth, Factory wastes. Factory wastes contain acids. If released into water bodies, they kill fish and organisms. Therefore, factory wastes are neutralised by adding basic substances. Are you familiar with acid rain? Rain containing excess acids is called acid rain. It forms when carbon dioxide, sulphur dioxide, and nitrogen dioxide pollutants dissolve in rain drops to form carbonic acid, sulphuric acid, and nitric acid. Acid rain damages buildings, monuments, plants, and animals. Great care must be taken while handling laboratory acids and bases because they are corrosive, irritating, and harmful to skin. [CHECKPOINT]
Let us review what we have learnt. Acids are sour in taste. Generally, bases are bitter in taste and soapy to touch. Acid turns blue litmus red. Bases turn red litmus blue. Substances which are neither acidic nor basic are called neutral. Solutions that show different colours in acidic, basic, and neutral solutions are called indicators. An acid and a base neutralise each other and form a salt. A salt may be acidic, basic, or neutral. [CHECKPOINT]
Now, let us solve the exercises completely. Exercise one: State differences between acids and bases. Acids taste sour, turn blue litmus red, have no effect on turmeric, and turn China rose indicator dark pink. Bases taste bitter, feel soapy, turn red litmus blue, turn turmeric red, and turn China rose indicator green. Exercise two: Ammonia turns red litmus blue. What is its nature? It is basic. Exercise three: Source of litmus and its use? Litmus is obtained from lichens. It is used to test if a solution is acidic or basic. Exercise four: Is distilled water acidic, basic, or neutral? How to verify? It is neutral. Verify by dipping both red and blue litmus paper; neither changes colour. Exercise five: Describe neutralisation with an example. It is the reaction between an acid and a base to form salt and water with heat evolution. Example: HCl plus NaOH gives NaCl plus H₂O. [CHECKPOINT]
Exercise six: True or False. One: Nitric acid turns red litmus blue. False. Two: Sodium hydroxide turns blue litmus red. False. Three: Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. True. Four: Indicator shows different colours in acidic and basic solutions. True. Five: Tooth decay is caused by a base. False. Exercise seven: Dorji has unlabelled drinks. He tests each with litmus paper. The drink turning blue litmus red is acidic. The one turning red litmus blue is basic. The one causing no change in either is neutral. Exercise eight: Explain why. A: Antacid contains a base to neutralise excess stomach acid. B: Calamine contains zinc carbonate, a base that neutralises ant bite formic acid. C: Factory waste is neutralised to prevent harming aquatic life. [CHECKPOINT]
Exercise nine: Identify hydrochloric acid, sodium hydroxide, and sugar solution using only turmeric. Dip turmeric paper in each. The one turning it red is sodium hydroxide. The other two show no change. Take the red turmeric paper and dip it in the remaining two. The one turning it back to yellow is hydrochloric acid. The one keeping it red is sugar solution. Exercise ten: Blue litmus remains blue. The solution is either basic or neutral. Exercise eleven: Which statements are correct? Only statement d is correct: Change of colour depends on the type of indicator. So option four, only d, is correct. [CHECKPOINT]
Extended learning activities. One: Write a secret message with baking soda and beet root. Baking soda solution is basic. Write with it on white paper. Rub fresh beet root over it. Beet root acts as an indicator and changes colour on the basic message, revealing it. Two: Prepare red cabbage juice by boiling cabbage in water. Use it as an indicator to test acidic and basic solutions. Three: Test local soil. If acidic, farmers add lime. If basic, they add compost. Four: Visit a doctor. Antacids treat acidity. Prevention includes avoiding spicy food and eating on time. Finally, a Did you know fact. Each cell contains deoxyribonucleic acid or DNA, controlling body features. Proteins are made of amino acids. Fats contain fatty acids. Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]