NCERT Gist: Physical and Chemical Changes | Additional Study Material for UPSC PDF Download

- Properties such as shape, size, colour and state of a substance are called its physical properties.
- A change in which a substance undergoes a change in its physical properties is called a physical change.
- A physical change is generally reversible. In such a change no new substance is fonned.
- A change with which you are quite familiar is the rusting of iron.
- When we leave a piece of iron in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting where magnesium hydroxide is a base.
- Magnesium oxide is a new substance formed on burning of magnesium.
- Magnesium hydroxide is another new substance formed by mixing magnesium oxide with water.
- Change in colour of the copper sulphate solution due to reaction with iron.
- A change in which one or more new substances are formed is called a chemical change.
- A chemical change is also called a chemical reaction.
- Vinegar and baking soda together produced carbon dioxide, which turned lime water milky.
- For rusting, the presence of both oxygen and water (or water vapour) is essential.
- Prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a coat of paint or grease.
- In fact, these coats should be applied regularly to prevent rusting. Another way is to deposit a layer of a metal like chromium or zinc on iron. This process of depositing a layer of zinc on iron is called galvanisation.
- Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.
- Large crystals of pure substances can be formed from their solutions. The process is called crystallisation. It is an example of a physical change.