Harmful Changes are
(i) Rusting of iron.
(ii) Decaying of fruits.
Rusting of iron is harmful because it slowly destroys iron articles and makes them useless. Since, iron is used in making large number of objects or articles such as bridges, grills, railings, gates and bodies of cars, buses, trucks and ships, etc. Rusting of iron causes a great loss over a period of time.
Prevention Rusting can be prevented by oiling, greasing or painting. It can also be prevented by galvanisation.
Decaying of fruits causes health hazards. Due to decaying of fruits, there is a lot of monetary loss in food industry.
Prevention Fruits can be preserved by keeping them at low temperature and by using some specific preservatives.

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.

Crystals of copper sulphate are prepared in the following manner:
(i) Take a cup full of water in a beaker.
(ii) Add a few drops of dilute sulphuric acid.
[Caution Never add water to the acid as the acid may splash on you].
(iii) Heat the water.
(iv) When it starts boiling, add copper sulphate powder slowly with constant stirring.
(v) Continue adding copper sulphate powder till no more powder can be dissolved.
(vi) Filter the solution.
(vii) Allow it to cool.
(viii) Do not disturb the solution, when it is cooling.
(ix) Look at the solution after some time and wait till it changes into crystals.

Burning of wood is a chemical change while cutting of wood is a physical change because, during burning, new substances are formed. After burning, we cannot get original substance (i.e. wood) back. Cutting of wood into small pieces is a physical change because no new substance is formed.

(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

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

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

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

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.

(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.