Acids

Acids

The term acid is derived from the Latin word ‘acidus’ which means sour. Thus, the chemical compounds which have sour taste are generally called as acids. All acids contain one or more replaceable hydrogen atoms in their molecules and when dissolved in water they release H+ ions. For example, Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄) and Nitric acid (HNO₃) release hydrogen ions (H⁺) when dissolved in water.

Hydrochloric acid --^Water→ Hydrogen ion + Chloride ion

HCl ---^H2O→ H⁺ + Cl^–

 Sulphuric acid --^Water→ Hydrogen ion + Sulphate ion

 H₂SO₄ ---^H2O→ 2H⁺ + SO₄^2–

Swedish chemist Svante Arrheniu proposed a theory on acids. According to him,an acid is a substance which furnishes H⁺ ions or H₃O⁺ ions in aqueous solution.

Thus, acids are defined as the chemical substances which release hydrogen ions when dissolved in water.

Acids can be classified into organic acids and inorganic acids depending on the sources. Some acids occur naturally in fruits and vegetables. These are called organic acids. Examples: Citric acid, tartaric acid etc. ,

On the other hand, acids are produced artificially in industries. These acids are called mineral acids or inorganic acids. Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃) etc. , There are many more classifications of acids. You will study about them in your higher classes.

Properties of Acids

a. Physical properties

Acids are sour in taste.

* They are corrosive in nature. Strong acids can spoil substances like human skin, clothes and paper.

* Generally acids exist in liquid state but few acids exist in solid state as well. E.g. Benzoic acid

* Acids are colourless.

* Acids change the colour of the indicators. Blue litmus paper turns red and methyl orange turns pink when treated with acids.

* They are soluble in water.

* Solutions of acids conduct electricity due to ionisation in water.

We feel hungry due to thecorrosive action of hydrochloric acid on the inner lining of the stomach. When the level of hydrochloric acidgoes higher, it causes ulcer.

b. Chemical properties

i. Reaction with metals

Metals like zinc, magnesium, aluminum, iron etc. , react with acids like hydrochloric acid, sulphuric acid to form metal salts and release hydrogen gas.

Metal + Dilute acids → Metal salt + Hydrogen

Examples

Zinc + Hydrochloric acid → Zinc chloride + Hydrogen

 Zn + 2HCl → ZnCl₂ + H₂↑

 Iron + Sulphuric acid → Ferrous sulphate + Hydrogen

Fe + H₂SO₄ → FeSO₄ + H₂↑

Activity 1

Take a clean test tube with holder and pour some dilute hydrochloric acid. Add few pieces of magnesium ribbon slowly. What do you observe? Now show a burning match stick near the mouth of the test tube. Do you hear any sound? The gas burns with a pop sound. From this it is observed that hydrogen gas is formed due to the reaction between acid and metal (Do it under the supervision of the teacher).

Copper or brass cooking vessels are coated with tin metal (eyam). If it is not coated the organic acids present in the food materialswill react with copper and make the food poisonous. The tin isolates the vessel from the action of acids and prevents food poisoning.

Answer: The gas burns with a pop sound. From this it is observed that hydrogen gas has been formed due to the reaction between acid and metal.

ii. Reaction with metal carbonates and bicarbonates

When carbonates and bicarbonates come into contact with dilute acids carbon dioxide is given out along with water. For example, limestone (calcium carbonate) reacts with dilute sulphuric acid to form calcium sulphate, carbon dioxide and water.

Calcium carbonate + dil Sulphuric acid → Calcium sulphate + Carbon dioxide + Water

CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O

Activity 2

Take some lemon juice in a tumbler and add baking soda slowly. What do you see? What do you infer from this?

Answer:

Inference : When lemon juice is mixed with baking soda, the new product CO₂ is formed with water and salt.

iii. Reaction with metal oxide

Oxides of various metals react with dilute acids to form their metallic salts and water.

Metal oxides + dilute Acid → Metal salts + Water

Example:

Calcium oxide + Hydrochloric acid → Calcium chloride + Water

CaO + 2HCl → CaCl₂ + H₂O

Uses of Acids

* Hydrochloric acid present in our stomach helps in the digestion of food materials.

* Vinegar (acetic acid) is used to preserve food materials.

* Benzoic acid is also used to preserve food materials like pickles.

* Sodium or potassium salts of higher fatty acids are used to make washing and bathing soaps.

* Sulphuric acid is called the king of chemicals. It is an effective dehydrating agent. It is used in various industries to make detergents, paints, fertilizers and many more chemicals.

* Hydrochloric acid, nitric acid and sulphuric acid are important laboratory reagents.

* Cells of all living organisms contain the fundamental nuclear material called nucleic acids. Animals have deoxy ribo nucleic acid (DNA) whereas plants contain ribo nucleic acid (RNA) .

Pickles remain in good condition for long time because they contain vinegar (acetic acid) or benzoic acid.