“Physical changes involve alterations in the shape, size, color, or state of a substance, such as melting ice or cutting paper, without forming new materials. Chemical changes, like rusting of iron or burning magnesium, result in the formation of new substances and are crucial in processes like digestion and photosynthesis. Rusting, a common chemical change, can be prevented through methods like galvanization. Crystallisation is a physical change used to obtain pure crystals from solutions. This article explains the differences between physical and chemical changes, provides key chemical equations in notes and free mind map form.
PHYSICAL CHANGES
- Physical changes involve a change in the physical properties of a substance, such as shape, size, color, or state.
- A physical change is generally reversible.
- No new substances are formed during a physical change.
- Examples of physical changes include:
- Cutting paper into pieces.
- Dissolving sugar in water.
- Melting ice.
- Crystallisation.
- Properties such as shape, size, color, and state of a substance are called its physical properties.
- Activity 5.5 mentions that a hack-saw blade changed colour on heating, but does not state explicitly that this is a physical change.
CHEMICAL CHANGE
- Chemical changes involve the formation of new substances. They are also called chemical reactions.
- New substances are formed.
- Chemical changes are very important in our lives; for example:
- Digestion of food.
- Ripening of fruit.
- Fermentation of grapes.
- New materials, like plastics and detergents, are produced by chemical reactions.
- A medicine is the end product of a chain of chemical reactions.
Signs of a Chemical Change
- Heat, light, or other radiation may be given off or absorbed.
- Sound may be produced.
- A change in smell may occur.
- A color change may occur.
- A gas may be formed.
Examples of Chemical Changes
- Rusting of iron: Iron + Oxygen + water → rust (iron oxide).
- Burning of magnesium ribbon: Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO).
- Reaction of copper sulphate with iron: Copper sulphate solution + Iron → Iron sulphate solution + Copper.
- Reaction between vinegar and baking soda: Vinegar and baking soda produce carbon dioxide, which turns lime water milky.
- Burning of coal, wood, or leaves.
- Explosion of fireworks.
- Spoiling of food.
- Ozone breaking down to oxygen.
- Photosynthesis.
- Neutralisation of an acid with a base.
RUSTING OF IRON
- Rusting of iron is a chemical change that affects iron articles and slowly destroys them.
- The monetary loss due to rusting is huge because iron is used in making bridges, ships, cars, truck bodies, and many other articles.
Key Points About Rusting of Iron
- Rust is not iron; it is different from iron on which it gets deposited.
- The equation for rusting is: Iron (Fe) + Oxygen (O2, from the air) + water (H2O) → rust (iron oxide Fe2O3).
- The presence of both oxygen and water (or water vapor) is essential for rusting.
- Rusting happens faster when the moisture content in the air is high (humid).
Preventing Rusting
- Applying a coat of paint or grease regularly can prevent rusting.
- Depositing a layer of a metal like chromium or zinc on iron.
- Depositing a layer of zinc on iron is called galvanization. Galvanized iron pipes are used to carry water to prevent rusting.
- Ships suffer a lot of damage from rusting because they are made of iron and a part of them remains underwater, with water drops clinging to the ship’s outer surface. Also, seawater contains many salts that make rust formation faster.
- Stainless steel, which does not rust, is made by mixing iron with carbon and metals like chromium, nickel, and manganese.
CRYSTALLISATION
- Crystallisation is a process by which large crystals of pure substances can be formed from their solutions.
- The salt obtained by evaporating sea water is not pure, and the shape of its crystals cannot be clearly seen; crystallisation is a means of obtaining pure crystals.
- Crystallisation is an example of a physical change.
Steps to Prepare Crystals of Copper Sulphate
- Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.
- Heat the water.
- When the water starts boiling, add copper sulphate powder slowly while stirring continuously.
- Continue adding copper sulphate powder until no more powder can be dissolved.
- Filter the solution.
- Allow the solution to cool, without disturbing it.
- Crystals of copper sulphate should be visible after some time.
EQUATIONS
- Rusting of iron: Iron (Fe) + Oxygen (O2, from the air) + water (H2O) → rust (iron oxide Fe2O3).
- Burning of magnesium: Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO).
- Dissolving magnesium oxide in water: Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2].
- Reaction of copper sulphate with iron: Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit).
- Reaction between vinegar and baking soda: Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances.
- Reaction between carbon dioxide and lime water: Carbon dioxide (CO2) + Lime water [Ca(OH)2] → Calcium Carbonate (CaCO3) + Water (H2O).
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