Chapter 5: Physical and Chemical Changes
5.1 Physical Changes
Introduction to Changes
- Everyday changes involve one or more substances.
- Changes are broadly classified into two kinds: physical and chemical.
Activity 5.1: Paper Cutting
- Cutting paper into pieces is a change in size.
- No new substance is formed; the property of the paper remains unchanged.
Activity 5.2: Chalk Dust
- Crushing chalk into dust and reforming it into chalk.
- This change is reversible; chalk can be recovered from dust.
Activity 5.3: Ice and Water
- Melting ice into water and then freezing water back into ice.
- This demonstrates a change in state (solid to liquid and back), which is reversible.
Activity 5.4: Boiling Water
- Boiling water produces steam.
- Steam condenses into water droplets on a cooler surface.
- This is a change in state (liquid to gas and back).
Activity 5.5: Heating a Blade
- Heating a hack-saw blade changes its color.
- The blade regains its original color after cooling.
- This is a temporary change in color.
Definition of Physical Change
- Physical properties: Characteristics such as shape, size, colour, and state of a substance.
- Physical change: A change in which a substance undergoes a change in its physical properties.
- Key characteristics:
- Generally reversible.
- No new substance is formed.
5.2 Chemical Change
Introduction to Chemical Change
- Rusting of iron is a common example of a chemical change.
- Rusting slowly destroys iron articles, causing significant monetary loss.
Activity 5.6: Burning Magnesium Ribbon
- Magnesium ribbon burns with a brilliant white light, leaving a powdery ash.
- Equation: Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)
- The ash (magnesium oxide) is a new substance.
- Dissolving ash in water forms a solution that turns red litmus blue, indicating it is basic.
- Equation: Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide [Mg(OH)₂]
- Magnesium hydroxide is also a new substance and a base.
Activity 5.7: Copper Sulphate and Iron Reaction
- Adding an iron nail to a blue copper sulphate solution.
- The solution's color changes from blue to green due to the formation of iron sulphate (a new substance).
- A brown deposit of copper (another new substance) forms on the iron nail.
- Equation: Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)
Activity 5.8: Vinegar and Baking Soda Reaction
- Mixing vinegar (acetic acid) and baking soda (sodium hydrogencarbonate) produces a hissing sound and gas bubbles.
- Equation: Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances
- Passing the evolved gas (carbon dioxide) through lime water.
- The lime water turns milky due to the formation of calcium carbonate (a new substance).
- Equation: Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O)
- Turning lime water milky is a standard test for carbon dioxide.
Definition of Chemical Change
- Chemical change (or chemical reaction): A change in which one or more new substances are formed.
- Importance: All new substances are formed through chemical changes (e.g., digestion, fruit ripening, medicine, plastics, detergents).
- Accompaniments of a chemical change:
- Heat, light, or other radiation (e.g., ultraviolet) may be given off or absorbed.
- Sound may be produced.
- A change in smell or a new smell may be given off.
- A color change may take place.
- A gas may be formed.
- Examples: Burning of magnesium ribbon, coal, wood, leaves (always accompanied by heat production).
- Other examples: Explosion of fireworks, spoiling of food, browning of cut fruits/vegetables, neutralization.
- Ozone layer: Ozone absorbs harmful ultraviolet radiation and breaks down into oxygen, which is a chemical change.
5.3 Rusting of Iron
Process of Rusting
- Rusting: A chemical change that affects iron articles, forming a brownish substance called rust.
- Equation: Iron (Fe) + Oxygen (O₂, from air) + Water (H₂O) → Rust (Iron oxide Fe₂O₃)
- Essential conditions for rusting: Presence of both oxygen and water (or water vapour).
- Rusting is faster in humid conditions (high moisture content in air).
Prevention of Rusting
- Prevent iron articles from coming in contact with oxygen or water (or both).
- Methods:
- Applying a coat of paint or grease regularly.
- Depositing a layer of another metal (like chromium or zinc) on iron.
- Galvanisation: The process of depositing a layer of zinc on iron to prevent rusting (e.g., galvanised iron pipes).
- Rusting of ships is faster due to salt water.
- Stainless steel: An alloy of iron with carbon, chromium, nickel, and manganese, which does not rust.
5.4 Crystallisation
Definition and Process
- Salt obtained by evaporation of sea water is often impure and has unclear crystals.
- Crystallisation: The process of obtaining large crystals of pure substances from their solutions.
- Crystallisation is an example of a physical change.
Activity 5.9: Preparing Copper Sulphate Crystals
- Dissolve copper sulphate powder in hot water containing a few drops of dilute sulphuric acid until no more powder dissolves.
- Filter the solution and allow it to cool undisturbed.
- Large, pure crystals of copper sulphate will form.
What You Have Learnt
- Changes are of two types: physical and chemical.
- Physical changes alter physical properties, form no new substances, and may be reversible.
- Chemical changes produce new substances.
- Pure substances can be obtained from their solutions by crystallisation.