(b) Process B is a chemical change. Process A is a physical change. The LPG in the cylinder is in liquid form because of high pressure. When it comes from the cylinder, it turns into gas. It is a physical change. Process B is a chemical change because the burning of gas is a chemical change.

We know that for rusting, the presence of both oxygen and water (water vapours) is essential.
Thus, in coastal areas, the air contains high moisture which means more humid environment and rusting becomes faster.
Whereas in deserts, moisture in air is less, hence rusting of iron is very slow there.

Rusting of iron is due to the exposure of iron to the atmosphere for some time. To prevent rusting of iron gate, we paint it. Painting prevents the iron gate to come in direct contact of atmosphere, water or both. Thus, a coat of paint cuts the contact between iron and atmosphere and it prevents rusting of iron.
Also we know that
Iron (Fe)+ Oxygen (O₂)+ Water (H₂O) \(\longrightarrow\) Rust (iron oxide, Fe₂O₃)
Thus, it is necessary to cut the oxygen contact with iron to prevent rusting.

Physical Changes in burning candle On heating, candle's wax melts, it is a physical change. Since it again turns into solid wax on cooling hence, the change is reversible.
Chemical changes in burning candle The wax near to flame burns and gives new substances like carbon dioxide, carbon soot. water vapours, heat and light.
Cooking of food, boiling of eggs are examples of both physical and chemical changes. In both cases, the physical appearance of the substances change and new substances are formed.

When baking soda (sodium bicarbonate) is mixed with lemon juice (citric acid), a chemical change occurs. In this reaction, new substances like carbon dioxide is formed and heat is evolved. This change is irreversible. It is a chemical change.
Sodium bicarbonate (NaHCO₃)+ Citric acid \(\longrightarrow\) Sodium citrate + Carbon dioxide\(\uparrow\) + Water + Heat

(a) Photosynthesis - Chemical change
(b) Dissolving sugar in water - Physical change
(c) Burning of coal - Chemical change
(d) Melting of wax - Physical change
(e) Beating aluminium to make aluminium foil - Physical change
(f) Digestion of food - 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.

Ripening of a fruit is a chemical change because after ripening, a new product with different properties is formed.

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

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

Carbon dioxide gas (CO₂) turns lime water milky.

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

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.

(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.