CBSE Class 10 Chemistry Acids Bases And Salts Notes Set 07

These CBSE Class 10 Science notes for Chapter 2, Acids, Bases and Salts, cover acids, bases and indicators, the reactions of acids and bases with metals, carbonates, oxides and each other, ions in solution, the pH scale and its role in everyday life, dilution and neutralisation, the pH of salts and water of crystallisation. They also give the preparation and uses of common salt, sodium hydroxide, washing soda, baking soda, bleaching powder and Plaster of Paris.

Important Notes and Formulas

Acids, Bases and Indicators

1. Acids taste sour, bases taste bitter, and the compounds (salts) formed when acids react with bases taste salty. Some salts have a very unpleasant taste and may be poisonous also. Never taste an unknown chemical.
2. The acids produced by plants and animals are known as organic acids, for example acetic acid, citric acid, lactic acid and formic acid. The acids prepared from minerals present in the earth's crust are known as mineral acids, for example HCl, H₂SO₄ and HNO₃.
3. Indicator: An indicator is a special chemical that changes its colour to indicate the presence of an acid or a base. Indicators are dyes or mixtures of dyes, and they show different colours in acidic and basic media. Examples are litmus solution, methyl orange, phenolphthalein and universal indicator.
4. Olfactory indicators: There are some substances whose odour changes in acidic or basic medium, such as onion, vanilla and clove oil. They are called olfactory indicators.

Chemical Properties of Acids and Bases

1. Reaction with metals: Metals react with acids to give a salt and hydrogen gas, although there are exceptions to this. Hydrogen burns with a pop sound.
   Metal + Acid → Salt + Hydrogen
   Zn + H₂SO₄(dil) → ZnSO₄ + H₂↑
2. When a base reacts with a metal such as zinc, hydrogen gas is evolved along with a salt whose negative ion contains the metal and oxygen.
   Zn + 2NaOH → Na₂ZnO₂ + H₂↑ (sodium zincate)
3. Reaction with metal carbonates and hydrogencarbonates: A metal carbonate or hydrogencarbonate reacts with an acid to give the corresponding salt, carbon dioxide and water.
   Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
   NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
4. Test for carbon dioxide: When the gas is passed through lime water, it turns milky because of the formation of calcium carbonate: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). On passing excess carbon dioxide, the milkiness disappears and the solution becomes colourless due to the formation of soluble calcium hydrogencarbonate: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq).
5. Metal oxides and non-metal oxides: Metal oxides are basic and react with acids to form salt and water: CuO + 2HCl → CuCl₂ + H₂O. Non-metal oxides are acidic and react with bases to form salt and water: 2NaOH + CO₂ → Na₂CO₃ + H₂O.
6. Neutralisation: All bases react with acids to form salts and water. During neutralisation, the H⁺(aq) ions of the acid combine with the OH⁻ ions of the base to form water, while the negative ion of the acid and the positive ion of the base form the salt.
   Acid + Base → Salt + Water: NaOH + HCl → NaCl + H₂O

Acids and Bases in Water

1. When acids dissolve in water, the hydrogen is released into the solution as H⁺(aq) or H₃O⁺ ions. It is the H⁺(aq) or H₃O⁺ ions that make a solution acidic. The formation of OH⁻(aq) ions in solution is responsible for the basic nature of a substance. Acids show acidic behaviour only in the presence of water; dry HCl gas does not change the colour of dry blue litmus.
2. Pure water is a neutral liquid because it contains exactly the same numbers of H⁺ and OH⁻ ions. Acid solutions contain more hydrogen ions than water, and alkaline solutions have a greater concentration of hydroxide ions than water.
3. Bases that dissolve in water are called alkalis, for example NaOH and KOH. All alkalis are bases but all bases are not alkalis.

Dilution

1. The process of dissolving an acid or a base in water is highly exothermic. Mixing an acid or base with water decreases the concentration of ions (H₃O⁺ or OH⁻) per unit volume, and the process is called dilution.
2. 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.

The pH Scale

1. The pH scale is a scale for measuring the hydrogen ion concentration in a solution. The pH of a solution is defined as the negative logarithm (to the base 10) of its hydrogen ion concentration: pH = −log[H⁺]. The higher the hydrogen ion concentration, the lower is the pH value.
2. The pH scale runs from 0 (very acidic) to 14 (very alkaline). A neutral solution has pH = 7, an acidic solution has pH less than 7, and a basic solution has pH more than 7. Each unit change in pH means a tenfold change in H⁺ concentration.
3. A pH paper or universal indicator is used to find the approximate pH: the colour produced is matched with the colour chart.

Importance of pH in Everyday Life

1. In the human body: Human blood, tears and saliva have a pH of about 7.0 to 7.8, and the survival of living organisms is possible only within this narrow range of pH.
2. In gardening and farming: The best crops are usually obtained with neutral or slightly acidic soil (pH 6.5 to 7.0). Below pH 6.5 the soil is not suitable for normal growth of plants, particularly vegetables. Acidic soil is treated with quick lime, slaked lime or chalk.
3. In the stomach: HCl produced in the stomach is important for digestion. Hyperacidity (an increase in the amount of acid during indigestion) causes pain and irritation. The excess acid is neutralised by antacids such as magnesium hydroxide (milk of magnesia).
4. In the mouth: When the pH inside the mouth becomes lower than 5.5, it causes tooth decay. Toothpastes are generally basic, so they neutralise the excess acid and prevent tooth decay.
5. Stings: Stings of insects such as bees and ants and the hairs of the nettle plant inject methanoic (formic) acid, which causes pain and irritation. A mild base such as baking soda gives relief.
6. Acid rain: When the pH of rain water is less than 5.6, it is called acid rain. When it flows into rivers, it lowers the pH of the river water and makes the survival of aquatic life difficult.

pH of Salts

Salts having the same positive or negative ion are said to belong to the same family. For example, NaCl and Na₂SO₄ belong to the sodium family.

Water of Crystallisation

1. The fixed number of water molecules chemically attached to each formula unit of a salt in its crystalline form is called water of crystallisation. Salts that contain water of crystallisation are called hydrated salts.
2. Blue crystals of copper sulphate, CuSO₄·5H₂O, lose their water of crystallisation on heating and turn white (CuSO₄). The blue colour returns when water is added. Other examples: washing soda Na₂CO₃·10H₂O, gypsum CaSO₄·2H₂O, green vitriol FeSO₄·7H₂O.

Common Salt (Sodium Chloride)

1. Chemical name: sodium chloride. Chemical formula: NaCl. It is the salt we use in food. Large crystals of common salt found as underground deposits are called rock salt; they are often brown because of impurities.
2. Preparation: Sodium chloride can be made by the combination of sodium hydroxide and hydrochloric acid: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). It is also obtained from sea water.
3. Uses: it is used in cooking food and as a raw material for making a large number of useful chemicals in industry, such as sodium hydroxide, baking soda, washing soda and bleaching powder.

Caustic Soda (Sodium Hydroxide) and the Chlor-Alkali Process

1. Chemical name: sodium hydroxide. Chemical formula: NaOH.
2. Preparation: When electricity is passed through a concentrated solution of sodium chloride (brine), it decomposes to form sodium hydroxide, chlorine and hydrogen. The process is called the chlor-alkali process because of the products formed: chlor for Cl₂ and alkali for NaOH.
   2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
3. Chlorine gas is produced at the anode and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.
4. Uses of sodium hydroxide: in making soaps and detergents, paper, and rayon and other artificial fibres, for degreasing metals and in the manufacture of bleach.

1. Uses of hydrogen: making margarine from vegetable oils, as a fuel (rocket fuel), in the manufacture of ammonia for fertilisers, and in the manufacture of hydrochloric acid (H₂ + Cl₂ → 2HCl).
2. Uses of chlorine: killing bacteria in drinking water and swimming pools, making bleach and bleaching powder, making hydrochloric acid, and in making PVC and other chlorine compounds.

Washing Soda

1. Chemical name: sodium carbonate decahydrate. Chemical formula: Na₂CO₃·10H₂O.
2. Preparation: Sodium carbonate, obtained by heating baking soda, is recrystallised by dissolving it in water: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂; Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
3. Uses: softening hard water (removing permanent hardness), laundry for washing clothes, paper, paint and textile industries, and manufacture of glass, borax and caustic soda.

Baking Soda

1. Chemical name: sodium hydrogencarbonate. Chemical formula: NaHCO₃.
2. Preparation: It is produced on a large scale by reacting a cold and concentrated solution of sodium chloride (brine) with ammonia and carbon dioxide: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl.
3. On heating, it decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Baking powder is a mixture of baking soda and a mild edible acid such as tartaric acid. The carbon dioxide released makes cakes and bread soft and spongy.
4. Uses: preparation of baking powder, soda water and other effervescent drinks, as an antacid to remove acidity of the stomach, and in soda-acid fire extinguishers.

Bleaching Powder

1. Chemical name: calcium oxychloride. Chemical formula: CaOCl₂.
2. Preparation: It is made by passing chlorine gas over dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
3. Uses: bleaching cotton and linen in the textile industry, bleaching wood pulp in the paper industry, bleaching washed clothes in laundry, as an oxidising agent in chemical industries and for disinfecting drinking water to make it germ-free.

Plaster of Paris

1. Chemical name: calcium sulphate hemihydrate. Chemical formula: CaSO₄·½H₂O. Two formula units of CaSO₄ share one molecule of water.
2. Preparation: It is made by heating gypsum at 373 K in a kiln. The temperature must not be raised above 373 K, otherwise all the water is lost: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.
3. On mixing with water it sets into a hard solid mass of gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. So it must be stored in a moisture-proof container.
4. Uses: making statues, toys, models and other decorative materials, making fireproof materials, and setting fractured bones in the right position.

Summary of Important Compounds