Physical and Chemical Changes
Types of changes
- Physical change
- Chemical change
- Change in a substance’s physical properties such as shape, colour, and size
- In such a change, no new substance is formed
- Eg: Crystallisation
- Crystals of pure substances can be formed from their solutions. The is called crystallisation
- Example: copper sulphate crystals from copper sulphate solution
- A change in which one or more substances are formed is called as chemical change.
- It is also called as chemical reaction
- Burning of a ribbon of magnesium
- Magnesium (Mg) + Oxygen (O2) à Magnesium oxide (MgO)
- Magnesium oxide (MgO) + Water (H2O) à Magnesium hydroxide [Mg(OH)2]
- Copper sulphate solution (blue) + Iron à Iron sulphate solution (green) + Copper
- Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) à Carbon dioxide + other substances
- Carbon dioxide (CO2) + Lime water [Ca(OH)2] à Calcium Carbonate (CaCO3) + Water (H2O) (Lime water turns milky)
- Turning lime water milky is the test for ‘carbon dioxide’
Properties of a chemical reaction
- New products are formed
- Heat, light or any other radiation may be given off
- Sound may be produced.
- A change in smell may take place or
- A new smell may be given off.
- A colour change may take place.
- A gas may be formed.
Rusting of iron
Iron (Fe) + Oxygen (O2, from the air) + water (H2O) à Rust (iron oxide Fe2O3)
- For rusting, the presence of both oxygen and water (or water vapour) is essential
- High moisture content makes rusting faster
- Saltwater makes rusting faster
- Prevent iron articles from coming in contact with oxygen, or water, or both.
- Apply a coat of paint or grease on iron items
- Deposit a layer of a metal like chromium or zinc on iron
- Process of depositing a layer of zinc on iron is called galvanisation