- Books Name
- CBSE Class 7 Science Book
- Param Publication
- CBSE Class 7
The properties such as size, shape colour and state of a substance are called its physical properties.
When a substance undergoes a change in its physical properties, that change is said to be physical change. During a physical change, no new substance is formed. Physical changes are generally reversible changes.
• Reversible change:
A change in which we can get back the initial substance by reversing the action is an reversible change.
To observe a physical change with the help the of ice cubes.
• Take a few ice cubes in a beaker.
• Keep them in the open for 4–5 minutes.
Now ,we will observe that ice (solid) changes into water (liquid). There is a change in the state of ice from soild to liquid.
• Now pour this water back into the ice tray and keep it in the freezer for 30 minutes.
Now, we will get back the ice. Therefore, it is a physical change.
To observe a physical change with the help of a sugar solution.
• Take 100 mL of water in a beaker.
• Dissolve a spoonful of sugar into it.
• Now, we will get a solution of sugar and water.
• Now, heat this solution over a burner for sometime.
• Now, we will observe that slowly the water evaporates and sugar is left at the solution.
• We will be able to get back sugar. Therefore, it is a physical change.
Characteristics of physical change
(i) No new or different product is formed : The composition of molecules of the substance remains unaltered.
Example : Ice melts to form water. In this example only the appearance (state) of matter has changed from solid to liquid. However, the composition of the molecules of ice or water remains same, i.e., for every 1 g of hydrogen, 8 g of oxygen is required . Thus, only a physical change has occurred.
(ii) The change is temporary and is usually reversible : It means the change can be reversed by altering the causes which produce the change.
Example : The water formed from ice can be changed back to ice by placing it in a freezing mixture (a mixture of ice and common salt).
(iii) There is no change in the weight of substance : During a physical change it is only the energy which is added or removed. No matter is added during a physical change. Similarly, no matter is removed during a physical change. Therefore, mass of the substance remains the same.
Some Examples Involving Physical Changes :
Some Common Examples of Physical Changes
• Formation of dew.
• Evaporation of water.
• Crystallisation of sugar from its solution.
• Ringing of an electric bell.
• Breaking of a glass pane.
• Freezing of ice cream.
• A rock rolling down a hill.
• Bending of a glass tube by heating.
• Melting of wax.
• Sublimation of camphor.