Physical And Chemical Changes

Rusting of iron, combustion of magnesium ribbon and burning of candle are chemical changes while melting of wax is physical change because only its physical state is changing from solid to liquid.

Cooking of vegetables consist of many chemical changes.

Change in colour or temperature or evolution of gas may accompany a chemical change. Combination of any of these may occur during a chemical change.

When milk is converted into curd, it doesn’t changes its colour. It remains white but it changes its state and taste and this change can’t be reversed.

Sunlight doesn’t harm iron. So there is no need to protect it from sun. Paint can’t avoid dust. So main reason behind painting the gate is to prevent it from rusting and to make it look beautiful.

Mixing other metals in iron resulting in its composition make it resistant of rusting. Therefore, Iron pillars of Qutub Minar has not rusted from such a long period.

To prevent rusting of iron, we apply layer of zinc on its surface. This process is called galvanization.

Forming crystals of a substance from its solution is known as crystallisation. It is a physical process and can be reversed.

Heat can be absorbed as well as evolved during a chemical reaction.

Crystallisation is a physical change and also can be reversed. Therefore, both the steps are physical change.

(a) True

Melting of wax is a physical change while burning of wax is a chemical change.

(b) True

Anaerobic bacteria decompose the animal waste and it produces biogas.

(c) True

The water of the sea contains many salts. The salt water makes the process of rust formation faster. Therefore, ships suffer a lot of damage from rusting though they are painted.

(d) False

Stretching of rubber is a physical change. It just changes its size when we stretch it and comes to actual size when we leave it.

Melting of water is also a change where water changes its state from solid to liquid.

Tearing of paper is a physical change. It changes the size of paper but it can’t be reversed.

a) Sugarcane’s juice is crystallised to form large crystals of sugar candy.

b) Process of depositing a layer of zinc on iron is called Galvanisation.

c) Souring of milk is a chemical change. It changes its odour and taste.

d) Carbon dioxide form calcium carbonate [CaCO₃] with lime water [Ca(OH)₂] and turns it milky.

e) During rusting, iron reacts with moist oxygen to form iron oxide, which is called rust.

f) Dissolving common salt in water is a physical change.

(a) Making sugar solution is a physical change.

Here sugar crystals are changing their state from solid to liquid. Hence it is a physical change.

(b) A physical change is generally reversible.

Most of the physical changes are reversible.

(c) Grinding of wheat grain changes its size. It is a physical change.

Size is a physical property. Change in size is a physical change.

(d) Iron benches kept in lawns and gardens get rusted. It is a chemical change because a new substance is formed.

Iron benches kept in lawns and gardens get rusted due to the presence of air and water. It is a chemical change in which iron oxide is formed as a new substance.

(i) Physical change

Beating of aluminium metal to make aluminium foil consist of a change in only shape and size.

(ii) Chemical change

Digestion of food involves various chemical reaction inside our stomach.

(iii) Physical change

Cutting off a log of wood into pieces just changes its physical properties like shape and size. Therefore, it is a physical change.

(iv) Chemical change

Crackers contains substances which are explosive and during burning explosive reaction took place. Hence, it is a chemical change.

(i) Copper sulphate + iron —> iron sulphate + copper

(ii) Iron + air + water —> iron oxide

(a) When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes lime water milky. The turning of lime water into milky is a standard test of carbon dioxide.

Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] —> Calcium Carbonate (CaCO₃) + Water (H₂O)

(b) When baking soda is added to the acetic acid then carbon dioxide gas is formed which results in formation of bubbles.

Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) —> Carbon dioxide + other substances

(a) Examples of physical changes which are reversible:

i) Melting of wax.

ii) Formation of sugar crystals from its solution.

(b) Examples of physical changes which are not reversible:

i) Tearing of paper

ii) Cutting log of wood

(c) Examples of chemical changes:

i) Reaction between lime water and carbon dioxide

ii) Burning of magnesium ribbon

(a) Chemical reaction between iron and copper sulphate. When iron is added to blue colour solution of copper sulphate, it changes its colour to green due to formation of iron sulphate.

Copper Sulphate solution (blue) + Iron —> Iron Sulphate solution (green) + Copper (brown deposit)

(b) Reaction between acetic acid and baking soda. When vinegar (acetic acid) is mixed with baking soda, chemical reaction takes place and bubbles of carbon dioxide gas are formed along with some other substances.

Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) —> Carbon dioxide + other substances

(c) Explosion of fireworks produces heat, light, sound. Explosion of firework is a chemical change.

(a) Yes. Rust is iron oxide (Fe₂O₃) which is not same as Iron (Fe). Both are different substance.

(b) No. Rusting of iron is a chemical change resulting in the formation of new substance Iron oxide as product. It can’t be reversed by any simple method.

(c) Yes, rusting of iron is a chemical change. Iron combines with oxygen in presence of water to form a new substance ‘iron oxide’. The iron oxide is rust.

Iron (Fe) + Oxygen (O2, from the air) + water (H2O) —> rust (iron oxide Fe2O3)

It can’t be reversed. Hence, Rusting is a chemical change.

(d) Examples of similar type of chemical change are:

i) Burning of magnesium ribbon to form magnesium oxide.

ii) Souring of milk.

(a) We expect following changes:

i) Blue colour of the solution changes to green due to formation of new substance ‘iron sulphate’.

ii) There will be brown deposit of copper on iron.

(b) Yes, these changes are chemical in nature as new substances, iron sulphate and copper are formed during the reaction.

(c) The reaction is as follows:

Copper Sulphate solution (blue) + Iron —> Iron Sulphate solution (green) + Copper (brown deposit)