Get a small trip on ribbon of magnesium. Clean its tip with sand paper. Bring the tip near a candle flame. It burns with a brilliant white light. After combustion, white powdery ash is left, it is called magnesium oxide.
Equation: 2Mg         +  O₂         \(\rightarrow\)          2MgO
          Magnesium                   Magnesium oxide  

Ozone layer is called natural shield against radiations. It protects us from the harmful ultraviolet radiations which come from the sun. Ozone absorbs these radiations and breaks down to oxygen. If ultraviolet radiations come on earth then they can cause harm to us and other life forms.

Characteristics of a chemical change:
(i) Mostly chemical reactions are irreversible.
(ii) They cause release or absorption of energy.
(iii) New substances with new properties are formed.
(iv) The properties of reactants are different from the products.

Ships are made of wood and iron and a part of them remains in water. On the part above water, water keeps clinging. This part is exposed to air. Moreover, the water of sea contains many salts which makes the process of rust formation faster. Therefore, ships suffer a lot of damage from rusting inspite being painted regularly. So, a fraction of ship's iron has to be replaced every year.

When a new substance is formed in a chemical reaction, heat, light is produced or used up. The chemical properties of the reaction mixture becomes different from the reactions.

When a cracker explodes, heat, light and smoke comes out due to the formation of new substances. Thus, it is a chemical change.

(i) lighting of bulb.
(ii) motion of fan.
(iii) sound produced by radio.
(iv changing of day and night.
(v) changes in the shape of moon.
(vi) changing in size of the baby dog.
(vii) blooming of flower.
(viii) melting of ice.
(ix)vaporisation of water.
(x) lightning.

The two methods to prevent rusting are
(i) Painting the iron articles.
(ii) Greasing or oiling the iron articles.

Metals like chromium and nickel are mixed (alloyed) with iron to make stainless steel.

Magnesium oxide is basic in nature because it turns red litmus solution to blue.

When magnesium is burnt in the air then a brilliant white flame is obtained.

When iron nails are dipped in copper sulphate solution, then the colour of the solution changes to green.

Carbon dioxide gas (CO₂) turns lime water milky.

Melting of ice is also a change where solid changes into liquid state.

Tearing of paper is a physical change although, it cannot be reversed.