Classify each of the following as a chemical change or a physical change: (a) Melting ice by spreading salt on an icy sidewalk (b) Striking a match (c) Breaking a piece of metal by bending it back and forth (d) Baking a cake (e) Rubbing your hands together to get them warm (f) Using a tea bag (g) Cooking a hot dog
The Correct Answer and Explanation is:
Here is the classification for each change:
(a) Melting ice by spreading salt on an icy sidewalk – Physical change
(b) Striking a match – Chemical change
(c) Breaking a piece of metal by bending it back and forth – Physical change
(d) Baking a cake – Chemical change
(e) Rubbing your hands together to get them warm – Physical change
(f) Using a tea bag – Physical change
(g) Cooking a hot dog – Chemical change
Explanation:
Changes in matter are either physical or chemical. A physical change alters the form or appearance of a substance but does not change its composition. No new substances are formed. Examples include changes in state, size, or shape. A chemical change involves a transformation at the molecular level, producing one or more new substances with different properties.
(a) When salt is used to melt ice, the process involves dissolving salt into water and lowering the freezing point. The water changes state but remains H₂O. This is a physical change.
(b) Striking a match causes a chemical reaction involving friction, heat, and oxygen. New substances like smoke, gases, and ash form. This is a chemical change.
(c) Bending and eventually breaking a metal piece only changes its shape or structure. Its composition stays the same. This is a physical change.
(d) Baking a cake causes ingredients to react under heat. Chemical bonds break and reform, producing gases and a new structure. This is a chemical change.
(e) Rubbing your hands generates heat due to friction. This only affects temperature, not substance identity. It is a physical change.
(f) Steeping a tea bag releases flavor and color through diffusion. The tea leaves remain chemically the same. This is a physical change.
(g) Cooking a hot dog involves heat-driven reactions that change proteins and other molecules. This is a chemical change.
