Class 7 Science
Physical and Chemical Changes
We see different types of changes; every day in our surrounding. Growth of tree, rising of sun, setting of sun, different shape and size of moon, burning of coal, paper, wood, etc. are the examples of changes around us.
Before knowing the scientific meaning of changes; it is necessary to understand some terms, i.e. physical properties, chemical properties, reversible and irreversible.
Physical Properties: Shape, Size and State of substance are known as physical properties. For example; when a sheet of paper is folded, its shape changes and this is an example of change in physical property.
Chemical Properties: The internal properties of a substance are known as chemical properties. For example curd is the product of milk but the internal properties of milk and curd are completely different.
Reversible: Things or processes which can be reversed are called reversible. For example, a folded sheet of paper can be unfolded and hence folding a sheet of paper is reversible.
Irreversible: Things or processes which cannot be reversed are called irreversible. For example, when milk turns into curd; it cannot be changed back to milk and hence is an irreversible change.
Change in which only physical properties of a substance change and no new substance is formed is called Physical Change.
Most of the physical changes are reversible, this means we can get the substance back even after the change.
Folding of a paper sheet: A paper can be folded or unfolded, and hence it is an example of physical change. Moreover, no new substance is formed in this process, so it is a physical change.
Tearing of paper sheet: Even after being torn in very small pieces; each piece is a piece of paper. Since no new substance is formed in this process, so it is a physical change.
Melting of wax: In melting, only the state of the wax changes; from solid to liquid. Solid wax can be obtained from molten wax. On melting, only the state of the wax changes from solid to liquid. Solid wax can be obtained from molten wax. This is an example of physical and reversible change.
Melting of ice into water: In melting, only the state of water changes. Water can be changed to ice and vice-versa. This is an example of physical and reversible change.
Freezing of water: After freezing, water changes into ice. In this change; there is only the change in the state of water. Water can be obtained back from ice; by melting.
Change of water into vapour (Vaporisation): Vapour is another state of water. Water can be obtained after condensation of vapour. This is a physical and reversible change.
Change of vapour into water (Condensation): This is also a physical and reversible change as water can be changed into vapour again by the process of vaporization.
Stretching of a rubber band: In this change, only the size of the rubber band changes. The rubber band comes back in its original shape and size, once it is released. This is a physical and reversible change.