It is true that when a candle burns, both physical and chemical changes occur. initially, candle burns to produce heat and light. It is a chemical change because candle burns with the formation of gases. Due to the heat produced, some candles simultaneously melts. This is a physical change as the molten candle wax behaves like original candle.

These are chemical changes :
Magnesium + Oxygen⟶ Magnesium oxide
Magnesium oxide + Water ⟶ Magnesium hydroxide

Differences between ice, water and steam:

Difference between physical and chemical changes.

When an iron blade of a knife is dipped in a copper sulphate solution, then iron blade is coated with reddish brown deposits of copper.
And the blue colour of copper sulphate solution changes to light green due to the formation of iron sulphate. So, it is a chemical change.

In a chemical change, the products are quite different from the reactants. Therefore, a chemical change cannot be normally reversed.

(i) Burning of a candle Chemical change as well as physical change and energy evolved.
(ii) Lightning of a bulb Physical change, energy evolved.
(iii) Preparation of food by green plants Chemical change, energy absorbed.
(iv) Volcanic eruption Chemical change, energy evolved.
(v) Evaporation of petrol Physical change as no new chemical substance is formed, energy absorbed.
(vi) Burning of LPG Chemical change because LPG on burning form CO₂ and H₂O, energy absorbed.

The ozone layer protects us from the harmful ultraviolet radiations which come from the sun. Ozone absorbs ultraviolet radiations coming from the sun and breaks down to form oxygen.
In this way, ozone layer absorbs harmful ultraviolet radiations.

In the process of digestion, the various food materials break down to form new substances which can be absorbed by the body. So, the process of digestion is a chemical change.

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.

Differences between physical and chemical changes are

Formation of clouds is a physical change. Clouds are formed by the condensation of water vapours present in the atmosphere. When rainwater goes back on the earth, no new product is formed. Therefore, it is a physical change.

(c) Both processes A and B are chemical changes. Bacteria act on waste and converts it to biogas (change A). Hence, it is a chemical change. During biogas production, it works as fuel(change Bl, and produces CO₂ and heat. Hence, A and B both are chemical changes.

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.