What is it made of, and what changed?
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Video lessonMatter classification · physical vs chemical change
Video coming soonFour rule cards
Pure substances vs mixtures
Pure = one substance, fixed composition. Mixture = two or more substances, variable composition (homogeneous or heterogeneous).
Pure = one substance, fixed composition. Mixture = two or more substances, variable composition (homogeneous or heterogeneous).
Elements vs compounds
Element = one kind of atom. Compound = two or more elements chemically bonded in a fixed ratio.
Element = one kind of atom. Compound = two or more elements chemically bonded in a fixed ratio.
Physical vs chemical change
Physical: no new substance (state, shape, dissolution). Chemical: new substance(s) form — colour change, gas released, precipitate, or a temperature change unrelated to the surroundings.
Physical: no new substance (state, shape, dissolution). Chemical: new substance(s) form — colour change, gas released, precipitate, or a temperature change unrelated to the surroundings.
Conservation of mass
In a closed system, total mass before a reaction equals total mass after. Atoms are rearranged, not created or destroyed.
In a closed system, total mass before a reaction equals total mass after. Atoms are rearranged, not created or destroyed.
Worked example
Classify each of the following as a physical or chemical change, and state whether mass is conserved: (a) ice melting at 0 °C, (b) iron rusting in moist air, (c) sugar dissolving in water, (d) magnesium burning in air.
Look for the formation of a new substance, then check whether mass is conserved overall.
Show solution
(a) Physical change — state change; mass conserved. (b) Chemical change — new substance (rust) forms; mass conserved overall (including oxygen from air). (c) Physical change — dissolution, no new substance; mass conserved. (d) Chemical change — new substance (MgO) forms; mass conserved (including $\mathrm{O_2}$ from air).