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Physical Changes
Every day, you see many changes around you. These changes can happen to one or more substances.
- Examples:
- Dissolving sugar in water to make a cold drink.
- Turning milk into curd.
- Milk souring.
- Stretching a rubber band.
In this chapter, we will do some activities to learn about these changes. There are two main types: physical and chemical changes.
Physical Changes
Activity 5.1: Cutting Paper
- Cut a piece of paper into smaller pieces.
- You can’t put the pieces back together, but the paper itself hasn’t changed.
Activity 5.2: Chalk Dust
- Crush chalk into dust and add water to make a paste.
- Shape it back into a piece of chalk and let it dry.
- You can’t get the original chalk back.
Activity 5.3: Melting and Freezing Ice
- Melt some ice in a tumbler by placing it in the sun.
- Put the tumbler in a freezing mixture to turn the water back into ice.
Activity 5.4: Boiling Water
- Boil water and observe the steam.
- Hold an inverted pan over the steam and see water droplets forming on the pan.
Activity 5.5: Heating a Blade
- Heat the tip of a hacksaw blade on a gas stove until it changes color.
- Let it cool down and see if it returns to its original color.
Understanding Physical Changes
- Examples from Activities:
- Paper and chalk changing in size.
- Water changing its state (solid to liquid, or gas to liquid).
- Blade changing color when heated.
- Physical Properties:
- Shape, size, color, and state of a substance.
- Physical Change:
- A change in physical properties.
- Generally reversible.
- No new substance is formed.
Now, let’s learn about the other type of change.
Chemical Change
Chemical changes are changes where new substances are formed. A common example is rusting of iron.
- Rusting of Iron:
- When iron is left in the open, it forms a brownish substance called rust.
- Examples: Iron gates, benches, shovels, and pans left exposed to the atmosphere.
Activity 5.6: Burning Magnesium Ribbon
- Warning: Do not stare at the burning ribbon.
- Burn a small piece of magnesium ribbon.
- It burns with a brilliant white light and leaves behind a powdery ash.
- This ash is magnesium oxide, which is different from magnesium.
- Equation: Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO)
- Mix the ash with water and test with litmus papers.
- The mixture turns red litmus blue, indicating it is basic.
- Equation: Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2]
Activity 5.7: Reaction of Copper Sulphate and Iron
- Dissolve copper sulphate in water to get a blue solution.
- Add a few drops of dilute sulphuric acid.
- Save a sample of the solution.
- Drop a nail or shaving blade into the remaining solution.
- Wait for half an hour and observe changes.
- The solution changes color from blue to green.
- A brown deposit forms on the iron nail.
- Reason: Reaction between copper sulphate and iron forms new substances.
- Equation: Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)
Understanding Chemical Changes
- Characteristics of Chemical Changes:
- Formation of new substances.
- Examples: Rusting, burning magnesium, and reaction of copper sulphate with iron.
Chemical changes are different from physical changes because they create new substances and are usually not reversible.
Activity 5.8: Vinegar and Baking Soda
- Take a teaspoon of vinegar in a test tube.
- Add a pinch of baking soda.
- You’ll hear a hissing sound and see bubbles.
- Pass this gas through freshly prepared lime water.
- Result: Lime water turns milky.
Explanation:
- Reaction in Test Tube: Vinegar (Acetic acid) + Baking soda (Sodium bicarbonate) → Carbon dioxide + other substances.
- Reaction with Lime Water: Carbon dioxide (CO2) + Lime water [Ca(OH)2] → Calcium Carbonate (CaCO3) + Water (H2O).
- The milky appearance confirms the presence of carbon dioxide.
Understanding Chemical Changes
Examples from Activities:
- Activity 5.6: Magnesium burns to form magnesium oxide.
- Activity 5.7: Copper sulphate reacts with iron to form iron sulphate and copper.
- Activity 5.8: Vinegar and baking soda produce carbon dioxide.
Characteristics of Chemical Changes:
- Formation of one or more new substances.
- Accompanied by:
- Heat, light, or radiation.
- Sound.
- Change in smell.
- Change in color.
- Formation of gas.
Everyday Examples of Chemical Changes
- Burning: Magnesium ribbon, coal, wood, leaves.
- Explosion of Fireworks: Produces heat, light, sound, and gases.
- Spoiled Food: Produces a foul smell.
- Browning of Fruits/Vegetables: Slices of apple, potato, or brinjal change color.
- Neutralisation: Acid reacting with a base.
Importance of Chemical Changes
Chemical changes are essential for:
- Digestion of food.
- Ripening of fruits.
- Fermentation of grapes.
- Production of medicines.
- Creating useful materials like plastics and detergents.
Do You Know?
Ozone Layer:
- Protects us from harmful ultraviolet radiation.
- Ozone (O3) absorbs UV radiation and breaks down into oxygen (O2).
- This breakdown is a chemical change.
- Ozone acts as a natural shield, protecting life on Earth.
Chemical changes are crucial in everyday life and the natural world, leading to the formation of new substances and various reactions.
Rusting of Iron
What is Rusting?
- Rusting is a process that slowly destroys iron articles.
- It affects bridges, ships, cars, truck bodies, and many other items made of iron.
- Rusting causes significant monetary loss.
Rusting Equation:
- Iron (Fe) + Oxygen (O₂ from the air) + Water (H₂O) → Rust (Iron Oxide, Fe₂O₃)
Conditions for Rusting:
- Both oxygen and water (or water vapor) are essential for rusting.
- High moisture content in the air (humid conditions) speeds up rusting.
Preventing Rusting:
- Prevent iron from contacting oxygen and water.
- Apply a coat of paint or grease regularly.
- Deposit a layer of a metal like chromium or zinc on iron (called galvanization).
- Iron pipes in homes are often galvanized to prevent rusting.
Rusting of Ships:
- Ships are made of iron and exposed to water.
- Sea water contains salts, making rusting faster.
- Ships are painted, but still suffer rust damage.
- Parts of ships’ iron need replacing every year, causing monetary loss.
Interesting Fact:
- Stainless steel, which does not rust, is made by mixing iron with carbon and metals like chromium, nickel, and manganese.
Crystallisation
What is Crystallisation?
- Salt obtained by evaporating sea water is not pure and has unclear crystals.
- Large, pure crystals can be formed from solutions through crystallisation.
- Crystallisation is a physical change.
Activity 5.9: Making Copper Sulphate Crystals
- Materials Needed:
- Cup of water
- Few drops of dilute sulphuric acid
- Copper sulphate powder
- Steps:
- Add dilute sulphuric acid to a cup of water in a beaker.
- Heat the water until it boils.
- Add copper sulphate powder slowly while stirring continuously.
- Keep adding until no more powder dissolves.
- Filter the solution and allow it to cool without disturbing.
- Observe the crystals forming after some time.
Learning Point:
- Identify changes around you as either physical or chemical changes.
Chapter Summary:
- Changes can be of two types: physical and chemical.
- Physical changes affect the physical properties of substances.
- No new substances are formed in physical changes.
- Physical changes may be reversible.
- In chemical changes, new substances are produced.
- Some substances can be obtained in pure state from their solutions by crystallisation.
Keywords
| Number | Keyword | Number | Keyword |
|---|---|---|---|
| 1 | Chemical change | 4 | Galvanisation |
| 2 | Chemical reaction | 5 | Physical change |
| 3 | Crystallisation | 6 | Rusting |
