Acids, bases and salts are among the most important and commonly encountered classes of chemical compounds. In our daily lives we come across substances such as lemon juice, curd, vinegar, soap and baking soda, which behave either as acids or as bases. The sour taste of citrus fruits comes from citric acid, while the bitter taste of soap arises from its basic nature. Studying acids, bases and salts helps us understand why certain substances taste the way they do, why antacids relieve acidity, and how important industrial chemicals such as sodium hydroxide and common salt are manufactured.
An acid is a substance that releases hydrogen ions (H+) when dissolved in water, while a base is a substance that releases hydroxide ions (OH-) in water. Bases that are soluble in water are called alkalis. Acids are sour in taste and turn blue litmus red, whereas bases are bitter in taste, feel soapy to touch, and turn red litmus blue. These properties are tested with indicators, which are substances that change colour in acidic and basic media.
In this chapter, we will study the properties of acids and bases, understand their chemical reactions including neutralisation, learn the pH scale and its importance, and explore the preparation, uses and importance of various salts such as sodium chloride, washing soda, baking soda, bleaching powder and plaster of Paris.
When an acid reacts with an active metal, hydrogen gas is produced. A metal reacts with the base to produce hydrogen gas as well, but this is observed only with metals like aluminium and zinc because of the amphoteric nature of these metals:
$$2\text{Al} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow 2\text{NaAlO}_2 + 3\text{H}_2$$
The reaction between an acid and a base is called neutralisation. The acidic and basic properties are destroyed, and a salt and water are formed:
$$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{Heat}$$
If the acid is strong and the base is strong, the solution formed is neutral (pH 7).
Acids react with metal carbonates and metal hydrogencarbonates to produce salt, carbon dioxide gas and water:
$$\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \uparrow$$
$$\text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 \uparrow$$
Carbon dioxide turns lime water milky and extinguishes a burning splinter, which serves as its test.
The strength of an acid or a base in solution is expressed in terms of pH. The pH scale ranges from 0 to 14.
$$\text{pH} = -\log[\text{H}^+]$$
A universal indicator gives different colours at different pH values, and a pH paper can be used to measure the approximate pH of a solution.
Our stomach produces hydrochloric acid to digest food. When too much acid is produced, we suffer from acidity. Antacids, which are mild bases such as magnesium hydroxide, neutralise the excess acid and provide relief. Since the cells lining our stomach also produce an enzyme that works best in an acidic medium, we should not take a strong alkali to neutralise the acid.
A salt is formed when an acid reacts with a base. Sodium chloride, NaCl, is the common salt used in cooking. Common salt is obtained from seawater and is the starting material for the manufacture of many other chemicals.
Sodium hydroxide is manufactured by the electrolysis of a concentrated solution of sodium chloride (brine). This process is called the chlor-alkali process because chlorine and sodium hydroxide (an alkali) are produced:
$$2\text{NaCl} + 2\text{H}_2\text{O} \xrightarrow{\text{Electric current}} 2\text{NaOH} + \text{Cl}_2 + \text{H}_2$$
The products are hydrogen gas at the cathode and chlorine gas at the anode, with sodium hydroxide remaining in solution. Sodium hydroxide is a strong alkali used in the manufacture of soap, paper and textiles.
Bleaching powder, CaOCl2, is made by passing chlorine gas over dry slaked lime:
$$\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$$
It is used for bleaching cotton and linen in the textile industry, as an oxidising agent, and as a disinfectant for water and in the bleaching of wood pulp.
Sodium hydrogencarbonate, NaHCO3, is commonly called baking soda. It is prepared by passing carbon dioxide through a concentrated solution of sodium chloride (saturated brine) in the presence of ammonia (Solvay process). It is used in baking to make cakes fluffy, in the manufacture of soft drinks, and as an ingredient in antacids and fire extinguishers.
$$\text{NaCl} + \text{NH}_3 + \text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{NaHCO}_3 + \text{NH}_4\text{Cl}$$
Sodium carbonate decahydrate, Na2CO3.10H2O, is called washing soda. Recrystallisation of sodium carbonate gives washing soda crystals. It is used in the manufacture of glass, soap and paper, and as a cleaning agent for domestic purposes. Washing soda and baking soda are chemically related; on heating, baking soda decomposes to form sodium carbonate, water and carbon dioxide:
$$2\text{NaHCO}_3 \xrightarrow{\text{Heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$
Plaster of Paris is calcium sulphate hemihydrate, CaSO4.1/2H2O. It is prepared by heating gypsum (calcium sulphate dihydrate) to about 373 K:
$$\text{CaSO}_4.2\text{H}_2\text{O} \xrightarrow{\text{373 K}} \text{CaSO}_4.\tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\text{H}_2\text{O}$$
Plaster of Paris is a white powder which, on mixing with water, sets into a hard mass as it absorbs water to become gypsum again. It is used for making toys, plaster casts for fractured bones, and for false ceilings and decorative designs.
The fixed number of water molecules chemically combined with a salt in its crystalline structure is called water of crystallisation. For example, CuSO4.5H2O (blue vitriol) contains five molecules of water, and Na2CO3.10H2O contains ten. On heating, these water molecules are lost and the salt becomes anhydrous. Hydrated copper sulphate is blue, while anhydrous copper sulphate is white.
| Property | Acid | Base |
|---|---|---|
| Taste | Sour | Bitter |
| Touch | Corrosive | Soapy |
| Litmus test | Blue litmus turns red | Red litmus turns blue |
| pH value | Less than 7 | More than 7 |
| Ions in water | H+ | OH- |
| Reaction with metals | Liberates H2 gas | Liberates H2 with amphoteric metals |
| Examples | HCl, H2SO4, CH3COOH | NaOH, KOH, Ca(OH)2 |
| Salt | Chemical Name | Common Name | Use |
|---|---|---|---|
| NaCl | Sodium chloride | Common salt | Cooking, preserving food |
| NaOH | Sodium hydroxide | Caustic soda | Soap, paper and textile industry |
| CaOCl2 | Calcium oxychloride | Bleaching powder | Bleaching and disinfection |
| NaHCO3 | Sodium hydrogencarbonate | Baking soda | Baking, antacids, fire extinguishers |
| Na2CO3.10H2O | Sodium carbonate decahydrate | Washing soda | Cleaning agent, glass industry |
| CaSO4.1/2H2O | Calcium sulphate hemihydrate | Plaster of Paris | Plaster casts, decoration |
Acids, bases and salts are not just chapters in a textbook; they are substances we encounter daily. Understanding their properties, the concept of pH, and the neutralisation reaction helps explain everything from the sourness of lemon to the relief given by an antacid. The important salts, including common salt, caustic soda, baking soda, washing soda, bleaching powder and plaster of Paris, connect classroom chemistry to major industries. The pH of a solution governs many biological and environmental processes, from soil fertility to tooth health. A strong command over indicators, pH, neutralisation and the chlor-alkali process will not only secure good marks in the examination but also build the foundation for understanding more advanced chemistry in higher classes.