Acids: Types and Chemical Properties

Acids: Acid has sour taste and it turns blue litmus paper to red.

If a substance tastes sour then it must contain acid. Example: curd, tamarind, lemon, etc. tase sour because they contain acid.

Types of Acids

Acids are divided into two types on the basis of their occurrence, viz. Natural acids and Mineral acids.

1. Natural Acid: Acids which are obtained from natural sources are called natural acid or organic acid. Examples: vinegar, citric acid, tartaric acid, etc.
2. Mineral Acids: Acids which are prepared from mineral are known as mineral acids. They are also called inorganic acids, man-made acids or synthetic acid. Examples: hydrochloric acid, sulphuric acid, nitric acid, etc.

Some organic acids and their sources are given in the following table.

Chemical Properties of Acid

Reaction of acids with metal:

What happens when an acid reacts with metals? Acids give hydrogen gas along with respective salt when they react with a metal.

Metal + Acid ⇨ Salt + Hydrogen

Example 1

Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal.

Zn + 2HCl ⇨ ZnCl₂ + H₂

Example 2

Hydrogen gas and sodium chloride are formed when hydrochloric acid reacts with sodium metal.

2Na + 2HCl ⇨ 2NaCl + H₂

Example 3

Hydrogen gas and iron chloride are formed when hydrochloric acid reacts with iron.

Fe + 2HCl ⇨ FeCl₂ + H₂

Example 4

Hydrogen gas and zinc sulphate are formed when zinc metal reacts with sulphuric acid

Zn + H₂SO₄ ⇨ ZnSO₄ + H₂

Test for hydrogen gas: The gas evolved after reaction of acid with metal can be tested by bringing a burning candle near it. If the candle burns with pop sound, then it confirms the evolution of hydrogen gas. This is the characteristic test for hydrogen gas.

Reaction of acids with metal carbonate:

What happens when an acid reacts with metal carbonate? Acids give carbon dioxide gas and respective salts along with water when they react with metal carbonates.

Metal carbonate + Acid ⇨ Salt + Carbon dioxide + Water

Example 1

Hydrochloric acid gives carbon dioxide gas, sodium chloride along with water when it reacts with sodium carbonate.

Na₂CO₃ + 2HCl ⇨ 2NaCl + CO₂ + H₂O

Example 2

Hydrochloric acid gives carbon dioxide, magnesium chloride and water when it reacts with magnesium carbonate.

MgCO₃ + 2HCl ⇨ MgCl₂ + CO₂ + H₂O

Example 3

Hydrochloric acid gives carbon dioxide gas, calcium chloride and water when it reacts with calcium carbonate.

CaCO₃ + 2HCl ⇨ CaCl₂ + CO₂ + H₂O

Example 4

Sulphuric acid gives calcium sulphate, carbon dioxide gas, calcium sulphate and water when it reacts with calcium carbonate.

CaCO₃ + H₂SO₄ ⇨ CaSO₄ + CO₂ + H₂O

Example 5

Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium carbonate.

Na₂CO₃ + H₂SO₄ ⇨ Na₂SO₄ + CO₂ + H₂O

Example 6

Nitric acid gives sodium nitrate, water and carbon dioxide gas when it reacts with sodium carbonate.

2HNO₃ + Na₂CO₃ ⇨ NaNO₃ + 2H₂O + CO₂

Reaction of acid with hydrogen carbonates (bicarbonates):

What happens when an acid reacts with metal bicarbonate? Acids give carbon dioxide gas, respective salt and water when they react with metal hydrogen carbonate.

Acid + Metal hydrogen carbonate ⇨ Salt + Carbon dioxide + Water

Example 1

Hydrochloric acid gives carbon dioxide, sodium chloride and water when it reacts with sodium bicarbonate.

NaHCO₃ + HCl ⇨ NaCl + CO₂ + H₂O

Example 2

Sulphuric acid gives sodium sulphate, carbon dioxide gas and water when it reacts with sodium bicarbonate.

2NaHCO₃ + H₂SO₄ ⇨ Na₂SO₄ + 2CO₂ + 2H₂O

Sodium bicarbonate is also known as sodium hydrogen carbonate, baking soda, baking powder, bread soda and bicarbonate of soda.

Test for evolution of carbon dioxide gas: Carbon dioxide turns lime water milky when passed through it. This is the characteristic test for carbon dioxide gas.

The gas evolved because of reaction of acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of formation of white precipitate of calcium carbonate.

Ca(OH)₂ + CO₂ ⇨ CaCO₃ + H₂O

CaCO₃ + CO₂ + H₂O ⇨ Ca(HCO₃ )₂

But when excess of carbon dioxide is passed through lime water, it makes milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus the milky colour of solution mixture disappears.

Reaction of acid with marble and egg shell: Since, marble and egg shell are made of calcium carbonate, hence when acid is poured over marble or egg shell, bubbles of carbon dioxide are formed.

Historical Monuments and Acid Rain:

Burning of fossil fuels releases oxides of sulphur and nitrogen. Nitrogen oxide and sulphur dioxide form nitric acid and sulphuric acid on reaction with water. When rain droplets mix with these gases (present in atmosphere because of pollution) they form acid rain.

Acid rain causes damage to historical monuments and other buildings. For example, the Taj Mahal, which is made of marble, is getting damaged because of reaction with acid rain. Marble is calcium carbonate which reacts with the acid and thus gets corroded.

Acid & Base and Fire Extinguisher

Metal carbonate or metal hydrogen carbonate and acid are used in fire extinguisher to produce carbon dioxide gas. Acid and metal carbonate or bicarbonate are kept in separate chambers in a fire extinguisher. On emergency they are allowed to react with one another. The carbon dioxide gas so produced is poured over fire. As carbon dioxide does not support burning, it puts off the fire.