Level: Chapter

Chemical Changes are changes in which the composition and chemical properties of the substance get changed.

In chemical changes new substances are produced. The most of the chemical changes are irreversible and permanent.

Note: Some chemical changes are reversible, known as reversible chemical changes.

Example: burning of a candle, formation of curd from milk, ripening of fruits.

Some Chemical Reactions in daily life:

Rusting of Iron: Rusting is the process in which iron turns into iron oxide.

It happens when iron comes into contact with water and oxygen. The process is a type of corrosion that occurs easily under natural conditions.

Prevention of Rusting:

1. By Painting
2. By Oiling and greasing
3. By Chromium plating
4. By Galvanizing
5. By Alloying

Cooking of food: Cooking causes breakdown of complex molecules of carbohydrates, fats and proteins into smaller molecules.

It is regarded as a decomposition reaction.

Cooked food is easier to digest than uncooked food.

3. Decay of Organic Substances: Microorganisms like fungi and bacteria produce enzymes which break down complex organic compounds into smaller substances.

It is also regarded as a decomposition reaction.

Some substances can be obtained in pure state from their solutions by crystallization.

Types of Acids:

(i) Mineral Acids: These are acids prepared from minerals present in the earth’s crust.

(ii) Organic Acids: These are acids produced by plants and animals (except hydrochloric acid).

3. Weak Acids: These do not dissociate completely in solution.
4. Examples: tartaric acid, lactic acid.
5. Strong Acids: These dissociate completely in solution. Example: nitric acid, sulphuric acid.

Neutralization: It is the reaction between an acid and a base which results in formation of salt and water.

Acid + Base ———-> Salt + Water

Example: HCl + NaOH ———-> NaCl + H₂O

Neutralisation in Everyday Life:

Indigestion: Too much acid in stomach causes indigestion. It is neutralized by taking an antacid like milk of magnesia.

Ant sting: When an ant bites, it injects formic acid into the skin. The effect is neutralized by rubbing moist baking soda (sodium hydrogen carbonate) or calamine (containing zinc carbonate).

(iii) Soil treatment: When the soil is too acidic, it is neutralized by treating with

quicklime (calcium oxide) or slaked lime (calcium hydroxide).

Bases: Bases are bitter in taste and soapy to touch.

Types of Bases:

1. Weak Bases: These naturally produce less hydroxide ions in solution. Example: magnesium hydroxide, ammonium hydroxide.
2. Strong Bases: These produce more number of hydroxide ions on dissolving in water. Example: Sodium hydroxide(NaOH), Potassium hydroxide (KOH)

Substances which are neither acidic nor basic are called neutral.

An acid and a base neutralise each other and form a salt. A salt may be acidic, basic or neutral in nature.

Solutions of substances that show different colour in acidic, basic and neutral solutions are called indicators.

Indicators: It is special chemical that changes its colour to indicate the presence of a chemical substance.

It is used to confirm the presence of an acid, a base or a neutral solution.

Classification of Indicators:

Natural Indicators:

1. Litmus: It is extracted from lichens. It is available in the form of strips of paper or in the form of a solution.· Acid turns blue litmus red. Bases turn red litmus blue.
2. Turmeric: It remains yellow in neutral and acidic solutions but turns red in alkaline solutions.
3. China rose: It turns acidic solutions to dark pink (magenta) and basic solution to green.
4. Red cabbage: It turns acidic solutions to red and basic solutions to blue.

Other Indicators:

1. Methyl Orange: It gives pinkish red colour with acidic solutions and yellow colour with bases.
2. Phenolphthalein: It is an acid-base indicator. It is colourless in acidic solutions but turns pink in alkali solutions.