Question

Identify the following as physical or chemical change. [6 marks]

Answer 1

$\bullet$ Crystallisation: The liquid solution is the original substance and its solid crystals are the final substance. But the formation of crystals from a solution involves only a change in state from liquid to solid, there is no change in chemical composition. Hence, this is a physical change. [1 mark]
$\bullet$ Ripening of fruit: Raw fruit is the original substance and ripened fruit is the final substance. On ripening, the taste changes due to changes in chemical composition. Hence, this is a chemical change. [1 mark]
$\bullet$ Clay modelling: Clay piece(s) is the original substance and the clay model is the final substance. Since only the shape has changed, clay modelling does not involve any change in the chemical composition of clay. Hence, this is a physical change. [1 mark]
$\bullet$ Respiration: We inhale oxygen which is the original substance and releases carbon dioxide, which is the final substance. Thus, respiration involves changes in chemical composition. Hence, this is a chemical change. [1 mark]
$\bullet$ Action of antacid on the stomach: Antacids consist of bases, carbonates and bicarbonates like calcium carbonate, aluminium hydroxide, sodium bicarbonate. These reacts with the hydrochloric acid produced in the stomach neutralising it and aids in proper digestion. A reaction between an acid and base to form salt and water is called a neutralisation reaction. [1 mark]
$\bullet$ Burning of magnesium ribbon: Magnesium ribbon is the original substance and magnesium oxide is the final substance. Thus, on burning the chemical composition changes. Hence, this is a chemical change. [1 mark]