Physical and chemical changes MCQ - Practice Questions with Answers

Heat may be produced but never absorbed is the incorrect statement because chemical changes can be exothermic or endothermic. Burning releases heat, while processes such as thermal decomposition may absorb heat. Gas evolution is a common sign of a chemical reaction, as seen when acids react with carbonates. Sound may also be produced in some reactions, especially rapid or explosive ones. A colour change can occur when new substances with different properties are formed.

Dissolving sugar in water is a physical change because no new substance is formed; sugar molecules disperse in water and can be recovered by evaporation or crystallisation. Burning a candle includes chemical combustion of wax, so it is not purely a physical change. Making curd from milk forms new substances through bacterial action and is chemical. Blackening of silverware occurs due to formation of silver sulphide on the surface, which is also a chemical change.

Dissolution of salt in water is a physical change because the salt does not form a new substance; it disperses in water and can be recovered by evaporation. Cooking of food is a chemical change because new substances with different properties are formed. Ripening of fruit involves biochemical reactions, so it is chemical. Formation of curd from milk also involves chemical changes due to bacterial action and acid formation. Thus, among the choices, dissolving salt in water best fits a physical change.

Cooking a vegetable causes chemical changes in its constituents: cell walls soften, proteins and other compounds change, and the cooked material does not simply return to its raw state. A wet soil lamp becoming fragile after drying is mainly loss of water and a physical change. Dispersion of white light through a prism only separates colours because of refraction, without forming a new substance. Dissolving salt in water is generally treated as a physical change because the salt can be recovered by evaporation.