Chapter 5 – Physical and Chemical Changes
- What is Change? (What is Change?)
- Something changes around us all the time. These changes are called changes.
- There are two types of changes: physical changes and chemical changes.
- Physical Changes
- Definition: changes in the physical properties of a substance (such as size, shape, color, and state) but no new substance is formed.
- These changes are usually reversible, i.e. the original substance can be regained.
- Examples:
- Tearing the paper: The shape of the paper changes, but it remains the paper.
- Melting of ice and freezing of water: Water changes its state (from solid to liquid and from liquid to solid), but remains water.
- Chemical Changes
- Definition: Changes in which one or more new substances are formed.
- These changes are usually irreversible, i.e. the original substance cannot be easily regained.
- They are also called chemical reactions.
Signs of Chemical Change:
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- Emission or Absorption of Heat, Light or any other radiation, such as burns.
- Sound production: Such as the bursting of firecrackers.
- Change in odour: such as spoilage of food.
- Change in colour: Such as rusting of iron.
- Formation of gas: Such as bubbles coming out when Eno is put in water.
- Formation of a precipitate: The formation of a solid in a solution.
- Examples:
- Rusting of Iron:
- When iron comes in contact with oxygen and moisture (water) in the air, rust, a brown substance, is formed. It is a new substance (iron oxide).
- Necessary for rusting: oxygen and water (or water vapor).
- Prevention: By painting, applying grease, by galvanisation (zinc coating on iron).
- Burning of Magnesium Ribbon:
- Magnesium burns with a bright white flame when the ribbon is burnt and forms a white powder magnesium oxide. It’s a new substance.
- Burning of Coal or Wood:
- On burning, ash, carbon dioxide, etc. are formed, which are new substances.
- Burning of a Candle:
- It has both changes – melting of wax (physical) and burning of wax (chemical).
- Digestion of food: Complex food turns into simpler substances.
- Ripening of Fruit: Change in colour, taste, smell.
- Curdling of Milk: Lactic acid is formed.
- Fermentation of grapes: Wine is made.
- Rusting of Iron:
- Crystallisation
- Definition: It is a physical process that is used to obtain pure crystals of a substance.
- This is an example of a physical change because no new substance is formed.
- Example:
- Obtaining salt from sea water: Although it is not usually in the form of crystals, it is a related process.
- Making crystals of copper sulphate:
- Take water in a beaker and add a few drops of dilute sulphuric acid to it.
- Heat the water.
- When the water starts boiling, continue to add the copper sulphate powder and stir until the powder stops dissolving.
- Filter the solution and let it cool.
- When cooled, you’ll see beautiful blue crystals of copper sulfate.