Acids Bases and Salts:

In this article, the notes of acids bases and salts for class 10 is provided. It will make you easy to revise throughout quickly for the purpose of your exams.

Types of Indicator:

Indicators	Category
Litmus	Natural Indicator
Turmeric	Natural Indicator
Methyl Orange	Synthetic Indicator
Phenolphthalein	Synthetic Indicator

The following types of indicator are used to test whether one is acid or base. Each has its own action on acid and base. As blue litmus is turned to red when subjected to acids and on the other hand when red litmus is subjected to base it turns to blue. Similarly, all the indicators have the same role.

Acid:

A compound, which on dissolving in water gives hydronium ions (H₃O⁺) as the only positive ions are acid.

Chemical properties of acids:

• The word ‘acid’ is derived from a Latin word ‘acidus’, which means “sour”.
• Acidic nature of a substance is due to the formation of H+(aq) ions in solution.

1. Reaction of acid with metal:

Acid + Metal → Salt + Hydrogen gas

When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.
Example:
2NaOH + Zn → K₂ZnO₃ + H₂

2. Reaction of acid with metal carbonate and metal hydrogen carbonates:

Acid + Metal carbonate / Metal hydrogen Carbonate → Salt + CO₂ + H₂O

When an acid reacts with a metal carbonate (MCO₃) or metal hydrogen carbonate (MHCO₃), it gives the corresponding salt, carbon dioxide(CO₂) gas and water.
Example:
Na₂CO₃(S) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
NaHCO₃ (s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂ (g)

3. Reaction of acid with base:

Acid + Base→ Salt + Water

Acid and base together react to produce salt and water. This reaction is called neutralization reaction.
Example: KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)

4. Reaction of Metallic Oxides with Acids:

Metal oxide + Acid → Salt + Water

Acids react with metal oxides to produce salt and water.
Example: CuO + HCl → CuCl₂ + H₂O

5. Conduction of electricity:

Acidic solutions in water conduct electricity because they produce hydronium ions. Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H⁺(aq) or hydronium ion (H₃O⁺). i.e. H⁺ + H₂O → H₃O⁺.

6.Strength of acid:

We can test the strength of an acid by using a scale called the pH scale which gives the measure of hydrogen ion concentration in a solution.
pH value of acids is always less than 7.

Base:

A compound, which on dissolving in water gives hydroxide ions (OH^–) as the only negative ions is base or simply we can say that a base is ‘a compound which reacts with an acid to give salt and water only.

• Bases are bitter to taste. They are soapy and slippery to touch.
• The presence of basic property in bases is determined by the concentration of OH^– ion in water.

1. Base in water:

Bases generate hydroxide (OH^–) ions in water. Water soluble bases are called alkalis.
Example: Mg(OH)₂(s)  + H₂O → Mg⁺²(aq)+ OH (aq) + 2OH (aq)

2. Reaction of base with metal:

When base is reacted with some metals it forms hydrogen gas. However, these reactions are not possible with all the metals.
Example: 2NaOH + Zn → Na₂ZnO₂ + H₂

3. Reaction of a Non-metallic Oxide with Base:

Non-metallic oxides react with bases to form salt and water.
Example: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(S) +  H₂O(l)
From this reaction, it is clear that Non-metallic oxides are acidic in nature.

4. Conduction of electricity:

Bases in water also conduct electricity as the acids do. As bases also dissociate into hydroxide ions in water thus conducts electricity. The process of dissolving a base in water is a highly exothermic one.

pH Scale:

pH Scale is a scale for measuring hydrogen ion concentration in a solution.

• The p in pH stands for ‘potenz’ in German, meaning ‘power’.
• The pH scale is marked with numbers 0 to 14, we should think pH simply as a number which indicates the acidic or basic nature of a solution.
• The pH varies from 0 (very acidic) to 7(neutral) and 14 (very alkaline).
• Higher will be the hydronium ion concentration, lower will be its pH value.

Importance of pH in everyday life:

1. Plants and animals are pH sensitive. If we talk of human body it works within the pH range of 7.0 to 7.8. Acid rain occurs when pH is less than 5.6 which affects the aquatic life.
2. For better growth of plants, the soil should have a specific pH range.
3. pH has a great role in our digestive system. Our stomach produces HCl acid which helps in the digestion of food without any harm.
4. When our mouth has pH less than 5.5 then our tooth decay starts. After eating, some food particles and sugar left in the mouth and with the help of it bacteria present in our mouth releases acid. Thus cause tooth decay.
5. Honeybee, ants and many other when bites us, causes pain and irritation. This is because they leave acid in our body. Some plants also cause the same. They use it for their self-defence.

Salts:

When an acid reacts with a base then it neutralizes to form salt and water. This reaction is called neutralization reaction.

• The H⁺ ion of acid and OH^– ions of base combine to form water(H₂O). The non-metallic ions of the acid and the metal ions of the base form the salt.
• Salts always stays in solution. Salts become solid when water is removed from it. Some salts are insoluble. They precipitate out when the acid and base are added together.
• The salt we eat is sodium chloride. It is formed by the combination of hydrochloric acid and sodium hydroxide solution.

Some more salts:

• Rock salt:
  Salt when present in its natural form as a crystalline mineral is rock salt or halite.
  When seas of bygone ages dried up, beds of rock salt were formed in the form of large brown crystals (brown due to impurities present in it).
• Common salt:
  Salt whose chemical composition is mainly NaCl is common salt.

Chlor-alkali process:

When we pass electricity through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide.
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

• Bleaching powder:
  Bleaching powder (CaOCl₂) is produced by the action of chlorine (formed in chlor-alkali process) on dry slaked lime [Ca(OH)2].
  Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses of bleaching powder:

1. To bleach cotton and linen in the textile industry.
2. Many chemical industries use it as an oxidising agent.
3. To make drinking water free of germs by disinfecting it.

• Baking soda:
  When common salt is treated with Ammonia in the presence of water and carbon dioxide then baking soda (NaHCO₃) is formed.
  NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

Uses of baking soda:

1. It is used for making baking powder (baking soda + mild edible acid).
2. Baking soda is also used in making antacids.
3. soda-acid fire extinguishers also use it as the main ingredient. 

• Washing soda:
  When we heat baking soda (NaHCO₃) it forms sodium carbonate (Na₂CO₃). When it combines with the excess of water than washing soda is obtained.
  2NaHCO₃ + Heat → Na₂CO₃ + H₂O + CO₂
  Na₂CO₃ + 10H₂O → Na₂CO₃ ∙ 10H₂O

Uses of washing soda:

1. Washing soda has main use in glass, paper and soap industries.
2. It is used in the manufacture of borax and many other sodium compounds.
3. We use it as a cleaning agent for domestic purposes.
4. Washing soda is used for removing permanent hardness of water.

• Plaster of Paris:
  When we heat gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO₄ . ½ H₂O). This is Plaster paris.

Uses and properties of plaster of paris:

1. Doctors mainly use it for supporting fractured bones in the right position.
2. It is a white powder but on mixing with water, it changes to hard solid mass i.e gypsum again.

Practice Questions on Acids, Bases and Salts  –   Click Here