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.

When sugar is heated strongly, it becomes foggy due to water vapour and a black powder is left behind. The black substance is charcoal. This is called charring of sugar and it cannot be reversed. Thus, this is a chemical change.

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.

Clouds are formed by the condensation of water vapours present in the atmosphere. It comes back to the atmosphere/earth as rain water. No new product is formed. So it is a physical change.

We should eat freshly cut apple because if we leave the apple after cutting, it starts turn to brownish due to the oxidation of the essential nutrients present in it and its food value decreases.

Ca (OH₂)           +          CO₂              \(\longrightarrow\)        CaCO₃         +       H₂O
Calcium hydroxide   Carbon dioxide             Calcium carbonate    Water

Melting of ice to form water is a physical change which occurs by the action of heat.

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

(a) Solid calcium carbonate on heating gives carbon dioxide gas. If this gas is passed through lime water, it turns milky. 
CaCO₃                      \(\overset{Heat}{\xrightarrow{}}\)        CaO       +            CO₂
Calcium Carbonate             Calcium oxide       Carbon dioxide
(b) When white lead nitrate Pb(N03h is heated, reddish coloured nitrogen dioxide (N02) is evolved and yellow coloured lead monoxide  (PbO) is left behind.
2Pb(NO₃)₂  \(\rightarrow\)  2PbO + 4NO₂ + O₂
(c) When colourless solutions of silver nitrate (AgNO₃) and sodium bromide (NaBr) are mixed, yellow coloured silver bromide (AgBr) gets precipitated
AgNO₃ (aq) + NaBr (aq) \(\rightarrow\) AgBr + NaNO₃ (aq)
(d) When water is added to quick lime (CaO), heat energy is evolved with the formation of calcium hydroxide, Ca(OH)₂.
CaO + H₂O ⟶ Ca (OH)₂ + Heat
(e) When cooked food containing oils and fats is kept in the open for long, pleasant smell changes into foul smell and the food gets spoiled.

If we leave a piece of iron in the open for some time, it acquires a coating of brownish substances. This substance is called rust and the process of its formation is called rusting. This is the change that effects iron articles and slowly destroys them. Since iron is used in making bridges, ships, cars, truck bodies and many other articles. The monetary loss due to rusting is huge. For rusting the presence of both oxygen and water (or water vapour) is essential. Infact, if the moisture content in air is high which means if it is more humid, rusting becomes faster.
Methods of preventing rust formation: Rusting of an iron object can be prevented or reduced by not allowing air and moisture to come in contact with the surface of iron objects. Following are the methods of prevention form rusting.
(i) Putting oil or paint over the surface of iron implements.
(ii) Galvanization: In this method, surface of iron is covered with a layer of more active metal like zinc. Zinc metal looses electrons in preference to iron and hence, prevents the rusting of iron.
(iii) By tin-plating 
(iv) Use of anti-rust solutions: Alkaline chromate and alkaline phosphate solution act as anti-rust solutions. When iron objects are dipped into a boiling and strongly alkaline solution of sodium phosphate, a protective insoluble film of iron phosphate is formed on them. This film protects the article form rusting.

Bum a piece of soft coke in hard glass test tube and pass the gas produced in water.

Then add blue litmus solution (to test the formation of acid) to the test-tube in which carbon dioxide gas is dissolved. The colour of the solution turns red. The following chemical reactions are going on during this experiment.
(i) C + O₂ ⟶CO₂ (Carbon dioxide)
(ii) CO₂ + H₂O ➝H₂CO₃ (Carbonic acid)

When baking soda and vinegar are mixed with each other then carbon dioxide gas is released. When carbon dioxide is passed through lime water, then calcium carbonate is formed. Calcium carbonate makes lime water appear milky. The turning of lime water into milky solution is a standard test of carbon dioxide. The following reactions take place
(i) Vinegar + Baking soda ⟶Carbon dioxide + other substances 
(ii) Carbon dioxide (CO₂) + lime water Ca(OH)_2⟶Calcium carbonate (CaCO₃) + water (H₂O)

(a) Yes, rust is iron oxide (Fe₂O₃). Thus, rust and iron are not the same substance.
(b) No, rusting of iron is a chemical change because in this reaction, a new substance, rust (iron oxide) is formed. It cannot be reversed by any method.
(c) Yes, rusting of iron is a chemical change. During the rusting of iron, it combines with the oxygen in the presence of water (moisture) to form a new compound 'iron oxide'. This iron oxide is a rust.
Iron + Oxygen + Water \(\longrightarrow\) Iron oxide
(From damp air)                    (Rust)
It is a permanent change which cannot be reversed back. So, rusting of iron is a chemical change.
(d) Two other examples are
(i) Setting of curd from milk.
(ii) Burning of magnesium ribbon to form magnesium oxide.

(a) Seawater is collected in shallow pits. It is then allowed to evaporate in the sun. As the water evaporates, the salt solution becomes supersaturated (concentrated). This supersaturated solution cannot hold the excess salt. Thus, it separates out in the form of salt crystals. These salt crystals are collected and are redissolved in water and filtered to remove insoluble impurities. The clear solution is again evaporated to obtain the crystals of pure salt.
(b) Salt is obtained from sea water by the process of evaporation.
(c) The values shown by Karan are curious, aware and intelligence

(a) Rahul's cycle was kept in the open for a longer time. As air contains both oxygen and moisture. Thus, in the presence of oxygen and water, his cycle slowly gets rusted. Iron(Fe)+ Oxygen + Water \(\longrightarrow\) Rust (Iron oxide)
(b) If iron objects are left in humid conditions for a longer time, they get covered with reddish brown ferric oxide (Fe₂O₃) layer. This is called rusting of iron.
(c) Rahul's father is caring, aware and intelligent.

(a) Chemical reaction between copper sulphate solution and iron metal. In this reaction, blue colour of copper sulphate solution changes to light green colour due to the formation of iron sulphate.
Copper sulphate + Iron \(\longrightarrow\) Iron sulphate + Copper
(Blue)                    (Grey)       (Light green)     (Brown)
(b) When baking soda and vinegar are mixed together then a chemical change takes place and bubbles of carbon dioxide gas are formed along with some other substances.
Baking soda              +                Vinegar          \(\longrightarrow\)     Sodium acetate + Carbon dioxide + Water
(Sodium hydrogen carbonate)   (Acetic acid)
(c) Explosion of a firework produces heat, light, sound and unpleasant gases. Explosion of a firework is a chemical change.