Physical vs Chemical Changes class 7 notes

Class 7 Science Notes

1. Physical vs Chemical Changes
Introduction
There are several changes we come across daily, for example, dissolving sugar in water or flattening
a metal rod by beating it. These involve changes in the form of the substance. Changes can be
classified as follows:
(i) Physical
(ii) Chemical
Physical properties
Physical properties include the size, shape, colour and state (solid/liquid/gas) of a substance.
Physical change
Any change to the physical properties of a substance is called a physical change.
Physical changes are usually reversible as no new substance is formed. It is the same substance but with
changed physical properties.
Chemical change
A change in which one or more new substances are formed is called a chemical change.
Usually, a chemical change involves a chemical reaction, which forms new products.
Example: Rusting of Iron or burning wood.
Differences Between Physical and Chemical Change
Physical Change Chemical Change
Physical change is a temporary change. A chemical change is a permanent change.
A Physical change affects only physical
properties i.e. shape, size, etc.
Chemical change both physical and chemical properties
of the substance including its composition
A physical change involves very little to no
absorption of energy.
During a chemical reaction, absorption and evolution of
energy take place.
Some examples of physical change are freezing
of water, melting of wax, boiling of water, etc.
A few examples of chemical change are digestion of food,
burning of coal, rusting, etc.
Generally, physical changes do not involve the
production of energy.
Chemical changes usually involve the production of
energy (which can be in the form of heat, light, sound,
etc.)
In a physical change, no new substance is
formed.
A chemical change is always accompanied by one or
more new substance(s).
Physical change is easily reversible i.e original
substance can be recovered.
Chemical changes are irreversible i.e. original substance
cannot be recovered.
What are the examples of physical and chemical changes?

2. Examples of chemical changes would be burning, cooking, rusting, and rotting.
Examples of physical changes could be boiling, melting, freezing, and shredding.
Most physical changes can be reversed if sufficient energy is provided.
Metallic Oxides
The formation of metal oxides is an example of chemical changes. They are formed by the reaction of
oxygen in the air.
– Burning of Magnesium ribbon:
– 2Mg + O2 → 2MgO
– The product formed is the oxide of magnesium, which is in the form of ash. It does not look anything
like the magnesium ribbon used for burning.
Reaction of metallic oxides with water
The reaction of metal oxides with water forms metal hydroxides.
For example, dissolving Magnesium oxide in water by stirring the ash very well with water.
MgO + H2O → Mg(OH) 2
The product formed is basic in nature and turns red litmus paper → blue
Reaction between baking soda and vinegar
When a pinch of baking soda is added to vinegar, we hear a hissing sound and observe the formation
of bubbles.
– Vinegar (Acetic Acid) + Baking Soda (Sodium bicarbonate)  → CO2 (Carbon dioxide) + Other
products
The carbon dioxide produced during the reaction of Vinegar and baking soda, when passed through
lime water, gives calcium carbonate as follows:
– CO2 + Ca(OH)2 (lime water) → Calcium Carbonate (CaCO3) + H2O
– The calcium carbonate turns lime water milky.
Observations that indicate a chemical change
Heat or light is absorbed or given out during a chemical reaction.
Production of sound
Production of gases or precipitates
Production of smell
A colour change may occur
Rusting
When substances made of Iron are exposed to oxygen and moisture in the atmosphere, it forms a red layer,
which is called rust.
The formation of rust can be represented by the following reaction:
4Fe + 3O2 → 2Fe2O3. The chemical formula for rust is Fe2O3.nH2O. More the moisture in the air, the quicker
the formation of rust.

3. Galvanization
– The process of depositing zinc on the surface of Iron to prevent rusting is
called galvanisation.
– Example: Iron water pipes are galvanised. Ships are made out of iron which is galvanised.
Due to the presence of salts in seawater, the process of rusting is hastened. Hence ships need
to replace their iron body every year.
Crystallisation
– The process of separation of salts from their solution is called crystallisation. It is a
purification technique that purifies seawater or separates crystals from impure samples. It is a
physical change.
Q - What is meant by the corrosion of metals?
Corrosion is the deterioration of a material as a result of its interaction with its surroundings and can occur at
any point or at any time during petroleum and natural gas processing.
3. Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of
_________.
(b) The chemical name of baking soda is _________.
(c) Two methods by which rusting of iron can be prevented are _________ and _________.
(d) Changes in which only _________ properties of a substance change are called physical
changes.
(e) Changes in which new substances are formed are called _________ changes.
Solution:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium
carbonate.
(b) The chemical name of baking soda is Sodium hydrogen carbonate.
(c) Two methods by which rusting of iron can be prevented are painting and galvanisation.
(d) Changes in which only physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called chemical changes.

4. 4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a
gas. What type of change is it? Explain.
Solution:
When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a carbon
dioxide gas. This is a chemical change.
Lemon juice + Baking soda → Carbon dioxide + other substances
5. When a candle burns, both physical and chemical changes take place. Identify these
changes. Give another example of a familiar process in which both chemical and physical
changes take place.
Solution:
Candle Burning
Physical change is the melting of the wax.
The chemical change is the burning of gas with the evolution of CO2.
Digestion of Food
Physical change is the breakdown of larger food molecules into simpler ones.
The chemical change is the digestion of food by the action of HCl and enzymes.
6. How would you show that the setting of curd is a chemical change?
Solution:
The setting of curd is a chemical change because curd cannot be turned back to milk. The properties
of milk and curd are different.
7. Explain why burning wood and cutting it into small pieces are considered two different
types of changes.
Solution:
Cutting wood is a physical change as it does not change the nature of the wood. On the other hand,
the burning of wood is a chemical change as wood is converted to charcoal with the liberation of CO2.

5. 8. Describe how crystals of copper sulphate are prepared.
Solution:
Crystals of copper sulphate are prepared using the crystallisation method, which is described as
follows:
Take a cupful of water in a beaker.
Add a few drops of dilute sulphuric acid to this.
Heat the water, and when it starts boiling, add copper sulphate powder while still stirring.
Add the copper sulphate powder till the solution becomes saturated. Filter into a china dish and allow it to
cool.
The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.
9. Explain how painting an iron gate prevents it from rusting.
Solution:
Rusting of iron requires contact with water (moisture) and oxygen. By painting an iron gate, we
prevent contact between iron, Oxygen and water (moisture); this helps in preventing rusting of iron.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Solution:
Rusting of iron requires contact with water (moisture) and oxygen. In coastal areas, humidity is more
in comparison to that in deserts; hence, rusting of iron objects is faster in coastal areas than in
deserts.
11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it
exists as a liquid. When it comes out from the cylinder, it becomes a gas (Change – A) then it
burns (Change – B). The following statements pertain to these changes. Choose the correct
one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Solution:
The answer is (ii) Process – B is a chemical change.

6. 12. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is
then burnt as fuel (Change – B). The following statements pertain to these changes. Choose
the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Solution:
The answer is (iii) Both processes A and B are chemical changes.