Hello, and welcome to today's chemistry lesson. In this session, we will explore one of the most fundamental areas of chemistry — acids, bases, and salts. By the end of this lesson, you will understand what makes a substance acidic or basic, how we measure acidity using the pH scale, and how salts are formed through various chemical reactions. We will also learn about the different types of salts and the methods used to prepare them in the laboratory.
Let us begin with acids and bases. Acids are compounds that contain hydrogen and, when dissolved in water, produce hydronium ions H₃O⁺ as the only positively charged ions. The hydronium ion is represented as H₃O⁺, which forms when a hydrogen ion H⁺ combines with a water molecule H₂O. For simplicity, we often write H⁺ in equations, though the actual ion present in solution is always the hydronium ion.
Acids can be classified in several ways. Based on source, we have organic acids, which are obtained from plants and contain carbon along with hydrogen, such as acetic acid CH₃COOH found in vinegar, and citric acid found in citrus fruits. These are generally weak acids that do not ionise completely. Inorganic or mineral acids, such as hydrochloric acid HCl, sulphuric acid H₂SO₄, and nitric acid HNO₃, are strong acids that ionise completely in solution, producing a high concentration of hydronium ions. These acids contain only ions in solution.
Acids can also be classified by their basicity. The basicity of an acid is defined as the number of hydronium ions H₃O⁺ that can be produced by the ionisation of one molecule of that acid in aqueous solution. Monobasic acids like hydrochloric acid produce one hydronium ion H₃O⁺ per molecule. Dibasic acids like sulphuric acid produce two hydronium ions H₃O⁺, and tribasic acids like phosphoric acid H₃PO₄ produce three. It is important to note that basicity depends on the number of ionisable hydrogen atoms, not the total number of hydrogen atoms in the molecule. For example, acetic acid contains four hydrogen atoms but is monobasic because only one hydrogen is ionisable.
Now let us turn to bases. A base is defined as a metallic oxide, a metallic hydroxide, or ammonium hydroxide which reacts with the hydronium ions of an acid to form salt and water only. The word "only" is crucial here — if other products like chlorine gas are formed, the substance is not classified as a base.
An alkali is a basic hydroxide which, when dissolved in water, produces hydroxyl ions OH⁻ as the only negatively charged ions. Common alkalis include sodium hydroxide NaOH, potassium hydroxide KOH, calcium hydroxide Ca(OH)₂, and ammonium hydroxide NH₄OH. All alkalis are bases, but not all bases are alkalis — insoluble metallic hydroxides like ferric hydroxide Fe(OH)₃ and cupric hydroxide Cu(OH)₂ are bases but not alkalis because they are insoluble in water.
Bases can be strong or weak. Strong alkalis like sodium hydroxide and potassium hydroxide undergo almost complete dissociation in water, producing a high concentration of hydroxyl ions. Weak alkalis like ammonium hydroxide and calcium hydroxide undergo only partial dissociation.
The acidity or basicity of a solution can be measured using the pH scale. The pH of a solution is defined as the negative logarithm to the base 10 of the hydrogen ion concentration expressed in moles per litre. Mathematically, pH equals minus log to the base 10 of the hydrogen ion concentration.
The pH scale normally ranges from 0 to 14. A pH of 7 represents a neutral solution. Values below 7 indicate increasing acidity, with lower numbers meaning stronger acids. Values above 7 indicate increasing alkalinity, with higher numbers meaning stronger bases. We can test pH using universal indicator, which gives a spectrum of colours — from red and pink for acids, through green for neutral, to blue and violet for bases.
The pH scale has enormous practical importance. Our body functions within a narrow pH range of 7.0 to 7.8. Crops grow best in specific pH ranges — rice prefers slightly acidic soil, while citrus fruits thrive in alkaline soil. Acid rain, with pH below 5.6, damages soil fertility and marine life. In medicine, pH values of blood and urine help diagnose diseases. Even tooth decay is pH-related — when bacteria reduce mouth pH below 5.5, tooth enamel begins to corrode.
Now we come to salts. A salt is defined as a compound formed by the partial or total replacement of the ionisable hydrogen atoms of an acid by a metallic ion or an ammonium ion. Alternatively, a salt is an ionic compound that dissociates in water to yield a positive ion other than hydrogen and a negative ion other than hydroxyl.
There are several types of salts. Normal salts are formed by complete replacement of ionisable hydrogen atoms by a metallic or ammonium ion — examples include sodium chloride NaCl and sodium sulphate Na₂SO₄. Acid salts are formed by partial replacement of the ionisable hydrogen atoms of a polybasic acid by a metal or ammonium ion — examples include sodium hydrogen sulphate NaHSO₄ and sodium dihydrogen phosphate NaH₂PO₄. Acid salts ionise in water to give hydronium ions and therefore show all the properties of an acid. Basic salts are formed by partial replacement of the hydroxyl group of a diacidic or triacidic base by an acid radical — examples include basic lead chloride Pb(OH)Cl and basic copper nitrate Cu(OH)NO₃.
Let us examine the methods of preparing normal salts. First, direct combination or synthesis, where a metal reacts directly with a non-metal. For example, molten sodium reacts with chlorine gas to form sodium chloride: two Na plus Cl₂ gives two NaCl.
Iron and aluminium also combine directly with chlorine to form their chlorides.
Second, simple displacement, where an active metal reacts with a dilute acid to produce a salt and hydrogen gas. Zinc with dilute sulphuric acid gives zinc sulphate and hydrogen: Zn plus H₂SO₄ gives ZnSO₄ plus H₂.
This method is used for preparing soluble salts of active metals.
Third, decomposition, where dilute acids react with carbonates, bicarbonates, sulphites, or sulphides. Calcium carbonate with hydrochloric acid produces calcium chloride, water, and carbon dioxide: CaCO₃ plus two HCl gives CaCl₂ plus H₂O plus CO₂.
Note that if the salt produced is insoluble, the reaction does not proceed.
Fourth, neutralisation of an insoluble base, where an acid reacts with an insoluble metallic oxide or hydroxide. Copper oxide with sulphuric acid gives copper sulphate and water: CuO plus H₂SO₄ gives CuSO₄ plus H₂O.
Fifth, neutralisation of an alkali by titration, where a soluble acid and soluble base react. This requires careful measurement using an indicator to determine the endpoint — phenolphthalein turns from pink to colourless, while methyl orange changes from orange to pink. Sodium hydroxide with sulphuric acid produces sodium sulphate and water: two NaOH plus H₂SO₄ gives Na₂SO₄ plus two H₂O.
Insoluble salts are prepared differently, primarily through precipitation or double decomposition. In this method, two soluble salts in solution exchange ions to form an insoluble salt that precipitates out. For example, barium chloride solution with sulphuric acid produces a white precipitate of barium sulphate: BaCl₂ plus H₂SO₄ gives BaSO₄ precipitate plus two HCl.
Some salts require special preparation techniques. Lead chloride, for instance, is insoluble in cold water but dissolves in hot water. Lead chloride requires special preparation due to its unique solubility properties.
Salts exhibit several important properties. They are electrovalent compounds that conduct electricity in molten state and in aqueous solution due to dissociation into ions. Most salts are soluble in water, though solubility varies with temperature.
The nature of a salt solution depends on its parent acid and base. Salts from strong bases and weak acids give alkaline solutions with pH above 7, such as sodium carbonate. Salts from strong acids and weak bases give acidic solutions with pH below 7, such as FeCl₃ and CuSO₄.
This phenomenon, where salts of weak acids or weak bases produce non-neutral solutions, is called hydrolysis. Salts from strong acids and strong bases give neutral solutions with pH 7, such as sodium chloride and potassium nitrate.
Many salts contain water of crystallisation — water molecules loosely combined in definite proportions. Examples include blue vitriol, CuSO₄·5H₂O, which contains five water molecules per formula unit, green vitriol FeSO₄·7H₂O with seven water molecules, and washing soda, Na₂CO₃·10H₂O, with ten water molecules. When heated, these salts lose their water of crystallisation and may change colour — blue copper sulphate crystals become white anhydrous powder.
Some salts exhibit efflorescence, losing water of crystallisation when exposed to dry air and becoming powdery. Others are hygroscopic, absorbing moisture from air without dissolving. Deliquescent substances go further — they absorb so much moisture that they ultimately dissolve in the absorbed water, forming a saturated solution. Examples of deliquescent substances include caustic soda NaOH, caustic potash KOH, and iron three chloride FeCl₃.
Because iron three chloride is highly deliquescent, it is kept dry with fused calcium chloride.
Finally, we distinguish between drying agents and dehydrating agents. Drying agents like anhydrous calcium chloride CaCl₂ and concentrated sulphuric acid remove moisture from substances — this is a physical change. Dehydrating agents like concentrated sulphuric acid remove the elements of water from compounds — this is a chemical change. Concentrated sulphuric acid can thus act as both a drying agent and a dehydrating agent.
Let us recap the key takeaways from today's lesson. First, acids produce hydronium ions H₃O⁺ in water, while alkalis produce hydroxyl ions OH⁻; the pH scale from 0 to 14 measures acidity and alkalinity, with 7 being neutral. Second, salts are formed by replacement of hydrogen in acids by metal or ammonium ions, and they can be normal, acid, or basic depending on the extent of replacement. Third, salts are prepared by various methods including direct combination, displacement, decomposition, neutralisation, and double decomposition or precipitation. Fourth, the nature of a salt solution — acidic, basic, or neutral — depends on the strength of its parent acid and base. Fifth, salts may contain water of crystallisation and can exhibit efflorescence, hygroscopy, or deliquescence.
That concludes our lesson on acids, bases, and salts. I hope this has given you a clear understanding of these fundamental chemical concepts and their practical applications in everyday life. Keep exploring, keep questioning, and remember — chemistry is all around you, from the food you eat to the medicines you take. Until next time, stay curious and keep learning.