During photosynthesis, the plants intake carbon dioxide and water in the presence of chlorophyll and sunlight to form two new substances, i.e. glucose (food) and oxygen gas. So, photosynthesis 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.

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.

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

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.

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

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.

Carbon dioxide gas (CO₂) turns lime water milky.

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

(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.