Class 7 Acids, Bases and Salts

📝 Class 7 Acids, Bases, and Salts

Some materials like lemon, vinegar, and tamarind have a sour taste. These materials taste sour due to the presence of chemicals called acids.  Other materials like baking soda and soap, have a bitter taste. It is due to the presence of chemicals called bases. 

➤The term acid has been derived from the Latin word 'acidus' which means sour. All sour-tasting materials contain acids. 

Acids are classified into two categories:

(i) Mineral Acids

(ii) Organic Acids

(i) Mineral Acids

Acids, which are prepared from the minerals present in the earth, are called mineral acids. Hydrochloric acid (`HCl`), sulphuric acid (`H_2SO_4`), nitric acid (`HNO_3`), sulphurous acid (`H_2SO_3`) and phosphoric acid (`H_3PO_4`) are some of the well-known mineral acids.

(ii) Organic Acids

Acids, which are naturally occurring and are found in plants and animals, are called organic acids. For example, guava contains oxalic acid. Orange, lemon, and amla (Indian Gooseberry) contain citric acid. Grapes, tamarind contain tartaric acid. Sour milk contains lactic acids. Vinegar contains acetic acid (`CH_3COOH`).

Bases and Alkalies

Bases are substances that react with acids to form water and salt. Bases are bitter in taste and have a soapy feel.

Some bases are soluble in water. Such bases are called alkalies. For example, sodium hydroxide `NaOH` and aluminium hydroxide `Al(OH)_3` are both bases. However, sodium hydroxide is also termed as an alkali because it is soluble in water. 

Strong bases ➤ Sodium hydroxide `NaOH`, potassium hydroxide `KOH` 

Weak bases ➤ Ammonium hydroxide `NH_4OH`, aluminium hydroxide `Al(OH)_3` and copper hydroxide `Cu(OH)_2`

Indicators

Acids and bases can be identified not only through their taste but also with the help of substances called indicators. An indicator is a substance that shows different colours in an acidic and a basic medium. The colour of the indicator can thus, help in identify acids and bases.  

Kinds of Indicators 

1. Litmus solution
2. Litmus paper
3. Turmeric
4. Juice of China Rose
5. Phenolphthalein

1. Litmus Solution 

The most commonly used natural indicator is litmus. It is extracted from lichens (Fig. a). It has a purple colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution.

Lichen 

2. Litmus Paper

Litmus is also available in the form of strips of paper, known as litmus paper. Generally, it is available in red and blue litmus paper. (Fig. b). 

● Blue litmus paper ➨ turns red in an acidic medium

● Red litmus paper ➨ turns blue in basic medium

Litmus paper 

3. Turmeric 

● It turns red  ➨ in a basic medium

● Remains yellow ➨ in an acidic medium 

4. Juice of China Rose (Hibiscus) 

Collect some China rose (Gudhal) petals and place them in a beaker. Add some warm water. Keep the mixture for some time till the water becomes coloured. Use the coloured water as an indicator.

● It turns dark pink (magenta)  ➨ in an acidic medium

● Turns green ➨ in a basic medium 

China rose flower and indicator prepared from it

5. Phenolphthalein

It is colourless in an acidic medium and turns pink in a basic medium. 

Neutralisation and Foramtion of Salts

Acid + Base `rightarrow` Salt + Water 

A chemical reaction between an acid and a base is known as neutralisation reaction. When such a reaction takes place, water and salts are formed. During neutralisation, the acid and base cancel the effect of each other. 

Examples:

`NaOH + HCl  rightarrow NaCl + H_2O`

(Sodium hydroxide + Hydrochloric acid `rightarrow` Sodium Chloride + Water) 

`2KOH + H_2SO_4 rightarrow K_2SO_4 + 2H_2O`

(Potassium hydroxide + Sulphuric acid `rightarrow` Potassium sulphate + Water)

`Mg(OH)_2 + 2HNO_3 rightarrow Mg(NO_3)_2 + 2H_2O`

(Magnesium hydroxide + Nitric acid `rightarrow` Magnesium nitrate + Water) 

Naming of Salts

● Salts of sulphuric acids `(H_2SO_4)` are named as sulphates.

● Salts of hydrochloric acid `(HCl)` are named as chloride. 

● Salts of nitric acid `(HNO_3)` are named as nitrates.

● Salts of sulphurous acid `(H_2SO_3)` are named as sulphites.

● Salts of carbonic acid `(H_2CO_3)` are named as carbonates.

● Salts of acetic acid `(CH_3COOH)` are named as acetates. 

For example, `CaSO_4` is a salt formed from calcium hydroxide and sulphuric acid. It is named as calcium sulphate. Similarly `NaNO_3` is named as sodium nitrate; `NaCl` is named as sodium chloride; `Na_2SO_3` is named as sodium sulphite; `Na_2CO_3` is named as sodium carbonate and `CH_3COONa` is named as sodium acetate. 

Properties of Salts

1. Salts are formed through reactions between acids and bases.

2. Most of the salts are soluble in water.

3. Salts do not conduct electricity in their solid-state. However, molten salts and solution of salts in water conduct electricity. 

Solution of salts conduct electricity 

Classification of Salts

1. Neutral salts

2. Acidic salts

3. Basic salts

1. Neutral salts

Salts formed by the reaction of a strong acid with a strong base, are known as neutral salts. The solution of neutral salt in water is neutral, that is neither acidic nor basic. Hence, such a salt solution does not change the colour of the litmus paper.  

Sodium chloride `(NaCl)` is a neutral salt as it is formed by the reaction of strong acid `(HCl)` and a strong base `(NaOH)`. `KCl, KNO_3, Na_2SO_4` are some of the other samples of neutral salts. 

2. Acidic Salts

Salts, formed by the reaction of a strong acid and a weak base, are acidic salts. The solution of an acidic salt in water is acidic, that is, it would change the colour of blue litmus paper to red. 

Aluminium chloride (`AlCl_3`) is an acidic salt as it is formed by the reaction of a strong acid (`HCl`) and a weak base (`Al(OH)_3`). `NH_4NO_3, ZnSO_4, CuCl_2` are some other examples of acidic salts. 

3. Basic Salts

Salts, formed by the reaction of a weak acid and a strong base, are basic salts. Solution of a basic salt in water is basic, that is it change the colour of red litmus paper to blue. 

Sodium acetate (`CH_3COONa`) is a basic salt as it formed by the reaction of a weak acid (`CH_3COOH`) and strong base (`NaOH`). `CH_3COOK, HCOONa, Na_2CO_3` are some other examples of basic salts.