Class 7 Science Chapter 6: Physical and Chemical Changes - NCERT Solutions

(a) Photosynthesis - Chemical change
Those changes in which new substances are formed, are called chemical changes. Photosynthesis is a chemical change because chemical reaction occur in it which is completed in the presence of sunlight.

(b) Dissolving sugar in water - Physical change
The physical changes are temporary changes which can be easily reversed to form the original substances. Physical changes are those changes which can be easily reversed by physical methods: evaporization and condensation.

(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

(a) False
This staement is false because cutting the log of wood into pieces is a physical change. Here, no new substances are formed.

(b) False
This staement is false because the formation from manure from leaves is not a physical change. It is the chemical change which involves decomposers and some microorganisms. 

(c) True
This staement is true because the coating of zinc protect iron pipe from rusting. It protects the iron pipes to come in contact with air and moisture. 

(d) False
This staement is false because iron and rust are the different substances, because rusting occurs when iron is oxidized. Iron is (Fe) and rust is (Fe₂O₃).

(e) True
This staement is true because condensation of steam is a physical change. Here, no new substances are formed.

(a) Calcium carbonate (CaCO₃)
CO₂ that is Carbon dioxide reacts with Ca(OH)₂ (Calcium hydroxide) that is lime water, to form a white precipitate of CaCO₃ (Calcium carbonate).

(b) Sodium hydrogen carbonate
The chemical name of baking soda is (NaHCO₃) Sodium hydrogen carbonate (Sodium bicarbonate) .Sodium bicarbonate comes with sodium, acid and hydrogen. Sodium carbonate is diprotic and a strong base compound.

(c) Painting and galvanization                                                                                                                                                                                                      Metals which cannot wear easily are melted into their molten form and are used to cover metals which rust easily. The surface of iron or metal are coated with paint or grease. Galvanization is the process of applying a protective layer of zinc on a metal. It is a very common method of preventing the rusting of iron.

(d) Physical
The change in which physical properties of the substance or a material change, this is called physical change. In physical changes, no new substances are formed.

(e) Chemical
The changes in which cannot be seen by naked eyes are called chemical changes. The chemical changes cannot be reversed.

When baking soda (Sodium bicarbonate) is mixed with lemon juice (citric acid), bubbles formed because of evolution of carbon dioxide gas. When baking soda is mixed with citric acid (lemon juice), formation of bubble is occur. These bubbles are formed due to the presence of carbon dioxide presence and this shows the chemical change.

Lemon Juice + Baking soda (Sodium bicarbonate) → Carbon dioxide + other substances

When a candle burns both physical and chemical changes takes place. When a candle is burnt, melting of wax occurs; this is a physical change. And, the burning of wax shows the chemical change. 

Digestion of food is the familiar process which shows both chemical and physical changes. In starting, breaking down of large food particles into the smaller ones shows the physical change. The process of digestion of food is a chemical change in which many new substances are formed.

Setting of curd is a chemical change because this process cannot be reversed, we cannot obtain milk from this again. Both have different nature. Setting of curd is done by the Lacto Bacillus bacteria, this shows setting of the curd is a chemical change. 

Burning of wood and cutting it into small pieces are two different type of changes. When we burn wood, the new substance 'coal' is formed; it is a chemical change and this is not reversible. Whereas, cutting of wood into small pieces is considered as physical change because there is no formation of new substance occurs. Physical change means change in the physical properties of the substance. Here, cutting of wood into small pieces changes the size and the shape of the wood.

Crystals of copper sulphate can be obtained by crystallization method. The formation of crystals are as follows:

1. Take some water in beaker.

2. Now add few drops of sulphuric acid to it.

3. Heat the water until its boiling point.

4. Now add copper sulphate powder to it with constant stirring.

5. Add the copper sulphate powder till the saturation point is obtained.

6. Now, filter the solution into a china dish and allowed it to cool.

7. The solution should be left undisturbed for 24 hours.

8. After 24 hours, separate the crystals carefully. 

Painting prevents rusting of iron. The process of depositing a layer of zinc on iron called galvanization. Painting prevents iron from the moisture and oxygen present in the air. When iron comes in contact with the oxygen and the moisture from the environment. it gets oxidised and makes ferric oxide (Fe₂O₃). 

Rusting of iron occurs faster in coastal area as compared to the deserts because in deserts less moisture is present.

(ii) Process - B is a chemical change

LPG is present in liquid form in the cylinder and when it comes in the contact of environment it converts into gaseous form which is a physical change. But When it burns, it converts in the from of heat and light, then this is a chemical change; this cannot be reversed. 

(iii) Both process A and B are chemical change.
Anaerobic bacteria are those which works in the absence of oxygen. Burning the biogas as fuel is a chemical change because it is an irreversible process. Anaerobic bacteria digest animal waste and produce biogas as the final product in the absence of oxygen. Biogas is a natural gas, which is produced by bacteria.