Physical and Chemical Changes

• (a) Photosynthesis: Chemical change (New substances formed).
• (b) Dissolving sugar in water: Physical change (Sugar can be recovered).
• (c) Burning of coal: Chemical change (Ash and gases formed).
• (d) Melting of wax: Physical change (State change).
• (e) Beating aluminium to make aluminium foil: Physical change (Shape change).
• (f) Digestion of food: Chemical change (Complex food broken to simple substances).

• (a) Cutting a log of wood into pieces is a chemical change. (False) (It is a physical change).
• (b) Formation of manure from leaves is a physical change. (False) (It is a chemical change).
• (c) Iron pipes coated with zinc do not get rusted easily. (True).
• (d) Iron and rust are the same substances. (False) (Iron is metal, Rust is Iron Oxide).
• (e) Condensation of steam is not a chemical change. (True) (It is a physical change).

• (a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium Carbonate.
• (b) The chemical name of baking soda is Sodium Hydrogen Carbonate.
• (c) Two methods by which rusting of iron can be prevented are painting and galvanisation.
• (d) Changes in which only physical properties of a substance change are called physical changes.
• (e) Changes in which new substances are formed are called chemical changes.

It is a Chemical Change.

Reason: New substances are formed. Lemon juice (Citric acid) reacts with Baking soda (Sodium hydrogen carbonate) to produce Carbon dioxide gas (bubbles) and other substances. The reaction is irreversible.

In a burning candle:

• Physical Change: Melting of wax (State change solid to liquid).
• Chemical Change: Burning of wax (Combustion produces heat, light, CO2 and soot).

Another Example: LPG (Cooking Gas)

• Physical: Gas comes out of cylinder (Liquid to Gas).
• Chemical: Burning of gas (Combustion).

Setting of curd is a chemical change because:

1. A new substance (curd) is formed with different taste (sour) and properties than milk.
2. The change is irreversible (Milk cannot be obtained back from curd).

• Burning of wood: Chemical Change. New substances like ash and smoke are formed.
• Cutting of wood: Physical Change. Only the size and shape change. No new substance is formed (sawdust is still wood).

By the process of Crystallisation:

1. Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid. Heat the water.
2. When it starts boiling, add copper sulphate powder slowly while stirring continuously.
3. Continue adding powder till no more powder can be dissolved (Saturated solution).
4. Filter the solution and allow it to cool. Do not disturb it.
5. After some time, pure crystals of copper sulphate will separate out.

Rusting requires contact of Iron with both Oxygen (air) and Water (moisture). Painting applies a coat on the iron surface. This layer cuts off the contact of iron from air and moisture, thus preventing rusting.

Rusting depends on moisture content in the air (humidity). In coastal areas, the humidity is higher (more moisture) compared to deserts where air is dry. Hence, faster rusting occurs in coastal areas.

The process of depositing a layer of metal like Zinc on iron is called Galvanisation. It is done to prevent iron from rusting. Example: Iron water pipes are galvanised.

When Magnesium ribbon is burnt, it burns with a brilliant white light and leaves behind a powdery ash called Magnesium Oxide.

Reaction: Magnesium (Mg) + Oxygen (O2) -> Magnesium Oxide (MgO)

• A change in which a substance undergoes a change in its physical properties (Shape, Size, Colour, State).
• Generally reversible.
• No new substance is formed.
• Examples: Crushing a can, melting ice, boiling water, stretching a rubber band, dissolving sugar.

• A change in which one or more new substances are formed.
• Also called Chemical Reaction.
• Generally irreversible.
• May be accompanied by: Heat/Light, Sound, Colour change, Gas formation.
• Examples: Rusting iron, Burning wood, Digestion, Ripening fruits, Souring of milk.

• Process: Iron acquires a film of brownish substance called **Rust** (Iron Oxide).
• Requirement: Oxygen (from air) and Water (moisture).
• Equation: Fe (Iron) + O2 (Oxygen) + H2O (Water) -> Fe2O3 (Rust).
• Prevention:
  • Painting or Greasing.
  • Galvanisation (Coating Zinc on Iron).
  • Alloying (Mixing with Carbon, Chromium, Nickel to make Stainless Steel).

• It is a **Physical Change**.
• Method to obtain pure solid crystals from impure solutions.
• Example: Obtaining pure copper sulphate crystals from blue vitriol solution using evaporation and cooling.

• Burning Magnesium:
  Magnesium (Mg) + Oxygen (O2) -> Magnesium Oxide (MgO)
• Ash in Water (Basic):
  Magnesium Oxide (MgO) + Water (H2O) -> Magnesium Hydroxide [Mg(OH)2]
• Vinegar + Baking Soda:
  Acetic Acid + Sodium Hydrogen Carbonate -> Carbon Dioxide + Other substances
• Lime Water Test:
  Carbon Dioxide (CO2) + Lime Water [Ca(OH)2] -> Calcium Carbonate (CaCO3) + Water

• Ozone layer protects us from UV radiation by absorbing it. This breaking down of ozone is a **Chemical Change**.
• Stainless steel does not rust. It is made by mixing iron with carbon and metals like chromium, nickel and manganese.
• Burning is always a chemical change and is accompanied by production of heat.