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Here is a quick visual recap of the chapter, followed by short definitions of every key term.
Mind Map
The whole chapter at a glance — tap any branch to jump to that concept.
CHANGES
AROUND US
AROUND US
Physical & Chemical
5.1
Physical Change
Only the appearance changes — no new substance.
5.2
Chemical Change
A new substance is formed.
5.3.1
Rusting
Iron + moist air → brown iron oxide.
5.3.2
Combustion
Fuel + oxygen + heat → new substance + heat & light.
5.4
Both Together
A burning candle: melting (physical) + burning of vapour (chemical).
5.5
Reversibility
Some changes can be undone; others cannot.
5.6
Desirability
Some changes are useful, some are harmful.
5.7.1
Weathering
Rocks slowly break and chemically change → soil.
5.7.2
Erosion
Wind and flowing water move broken material to new places.
Key Terms and Definitions
All the important words from this chapter, with simple definitions.
- Physical Change
- It is a change in which a substance changes only its physical properties (shape, size, or state). No new substance is formed.
- Chemical Change
- It is a change in which one or more new substances are formed. It involves a chemical reaction and can be written as a chemical equation.
- Chemical Reaction
- It is the process by which a new substance is formed during a chemical change.
- Chemical Equation
- It is the short way of writing a chemical reaction, with reactants on the left and products on the right, joined by an arrow (→).
- Combustion
- It is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light.
- Combustible Substance
- It is a substance that can undergo combustion (that is, can burn). Examples are wood, paper, cotton, kerosene, LPG, and coal.
- Ignition Temperature
- It is the lowest temperature at which a substance catches fire. Below this temperature, the substance will not burn on its own.
- Fire Triangle
- It is the set of three things needed for combustion to happen: a combustible substance (fuel), oxygen, and heat. If any one is removed, combustion stops.
- Rusting
- It is the chemical change in which iron, in the presence of moisture and air, slowly forms a brown layer of iron oxide (rust) on its surface.
- Reversible Change
- It is a change in which the original substance can be brought back. For example, melting ice can be refrozen into ice.
- Irreversible Change
- It is a change in which the original substance cannot be brought back. For example, once paper is burned, the paper cannot be recovered.
- Desirable Change
- It is a change that is useful for us. Examples are milk turning into curd, ripening of fruits, and cooking of food.
- Undesirable Change
- It is a change that is harmful or wasteful for us. Examples are rusting of iron tools and decay of food.
- Weathering
- It is the collective name for the physical and chemical changes that slowly break rocks into smaller pieces. Over thousands of years, weathering forms soil.
- Erosion
- It is the process by which rock pebbles, soil, and sediments are broken down and moved from one place to another by natural forces like wind and flowing water.
