Take a piece of sodium and drop it in water. You will see that sodium melts to form a silvery globule and darts on the surface of the water. Part of sodium vapourises and burns along with hydrogen gas giving a golden yellow flame.

$2\mathrm{Na}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH}+{\mathrm{H}}_2\uparrow$

Now answer the following question:

The change is _____ because the products formed can't be reversed.

Take luke warm milk in a beaker. To this milk add a spoon of curd and keep it overnight. Check it in the morning. The milk will change to _____. The chemical reaction taking placs is:

$\mathrm{Lactose} \mathrm{sugar}\underset{\mathrm{Bacteria} \mathrm{present} \mathrm{in} \mathrm{curd}}{\overset{\mathrm{Lactobacillus}}{\to }}\mathrm{Lactic} \mathrm{acid}$

Classify the following as physical or chemical change. Think of a reason in your mind.

Heating wax

Classify the following as physical or chemical change. Think of a reason in your mind.

Switching an electric bulb

Classify the following as physical or chemical change. Think of a reason in your mind.

Baking of potatoes

Classify the following as physical or chemical change. Think of a reason in your mind.

Ripening of a fruit

Classify the following as physical or chemical change. Think of a reason in your mind.

Making butter from cream

Classify the following as physical or chemical change. Think of a reason in your mind.

Burning of coal