Physical and Chemical Changes

Sea air contains salt water moisture; salt increases the rate of rusting by acting as an electrolyte

Step 1: Rusting requires both oxygen (O₂) and water (H₂O). In coastal areas, humidity (moisture content in air) is much higher than in desert areas, so more water is available for rusting. Step 2: Additionally, sea air contains salt (sodium chloride) from ocean spray and sea breeze. Salt dissolved in water forms an electrolyte solution that significantly speeds up the rusting process. Step 3: Desert regions are very dry (low humidity), so very little water vapour is present in the air. Without sufficient water, rusting cannot occur rapidly even though oxygen is available. Step 4: Option A (sun

Coating iron with a layer of zinc; zinc prevents iron from coming into contact with oxygen and water

Step 1: Galvanisation specifically means depositing a thin layer of ZINC on iron. This is done by dipping iron in molten zinc or by electroplating. Step 2: Since rusting requires both oxygen (O₂) and water (H₂O) to contact the iron surface, the zinc coating acts as a physical barrier preventing both from reaching the iron. Step 3: Even if the zinc layer is scratched, zinc is more reactive than iron and preferentially reacts with oxygen and water, protecting the iron underneath. Step 4: Option A describes chromium coating (used in chrome plating), not galvanisation. Option C describes making st

The reaction is chemical because carbon dioxide (a new substance) is produced, which turns lime water milky

Step 1: The reaction is: Acetic acid + Sodium hydrogencarbonate → Carbon dioxide + other substances. CO₂ is a completely new substance not present in vinegar or baking soda. Step 2: This makes it a CHEMICAL change. The defining feature of a chemical change is the formation of new substances. Step 3: The gas produced (CO₂) turns lime water milky by forming calcium carbonate, which confirms it is CO₂ – a new substance. Step 4: Option A is wrong – the absence of a solid product does not make something a physical change. New gas = new substance = chemical change. Step 5: Option C is completely inc

Dissolving copper sulphate crystals in water and then allowing the water to evaporate slowly to get crystals back

Step 1: A physical change is one where NO new substance is formed, and the change is often REVERSIBLE – meaning the original substance can be recovered. Step 2: When copper sulphate crystals dissolve in water, the copper sulphate is still present in solution. No chemical reaction occurs. By evaporating the water slowly (crystallisation), the original copper sulphate crystals can be recovered – confirming it is physical and reversible. Step 3: Burning paper produces ash and gases (CO₂, H₂O vapour). Paper cannot be recovered from ash. This is a chemical change. Step 4: Setting curd involves bact