Changes Around Us: Physical and Chemical — Important Questions

Rusting of iron, Burning of paper, Curdling of milk

Rusting forms iron oxide, burning creates ash and gases, and curdling produces new substances. Folding cloth and boiling water are physical changes as no new substances form.

Combustible substance (fuel), Oxygen, Heat to reach ignition temperature

The fire triangle shows that combustion requires fuel, oxygen, and heat. Remove any one of these and the fire will extinguish. CO2 and water are not required for combustion.

False

Not all physical changes are reversible. For example, cutting paper or breaking glass are physical changes that cannot be easily reversed to get the original form back.

Melting of ice, Folding paper, Inflating a balloon

Ice can be refrozen, paper can be unfolded, and balloon can be deflated. Burning wood and cooking food create new substances and cannot be reversed.