Background reading · Science

Mixtures, solutions and brand-new substances

Some changes just mix particles or move them around. Others rearrange particles into something completely new.

Stir a spoon of salt into water and it seems to vanish. Stir sand into water and it swirls, then sinks. Both are mixtures: two or more substances mingled together without turning into anything new. But they behave very differently.

Two kinds of mixture

Heterogeneous mixture: you can see the different parts, and it is not the same all the way through. Sand in water, muesli and a fruit salad are examples.

Homogeneous mixture: it looks the same everywhere, because the particles are evenly spread. Salt water, air and fizzy drink are examples.

Solution: a homogeneous mixture where one substance (the solute, like salt) has dissolved in another (the solvent, like water).

Suspension: tiny bits float in a liquid for a while but settle out over time, like muddy river water after rain.

Getting things back out

Mixtures can be separated using the properties of each part. A sieve catches big pieces. A magnet pulls out iron. Filter paper lets water through but traps sand. Evaporation removes the water from a solution and leaves the solid behind. At Lake Grassmere in Marlborough, seawater is left in huge shallow ponds so the sun and the dry nor'west wind evaporate the water, leaving salt crystals to harvest.

When something new is made

Melting, freezing and dissolving are physical changes: the particles stay the same, they just move or spread differently, and you can usually reverse them. A chemical change is different. The particles are rearranged to make new substances. Signs to look for include a colour change, gas bubbles, a temperature change, light or a new solid appearing. Baking soda fizzing in vinegar, iron rusting, a cake baking and wood burning are all chemical changes, and most cannot be undone.

Many solids dissolve faster and more fully in warm water, because the water particles are moving faster. Gases do the opposite: a warm fizzy drink goes flat faster than a cold one.

Sources and further reading

Written for Kōkiri Learn students in our own words. Check facts against the sources.

Used in: Matter on the Move: Heat, Mixtures and Change