Tap water, air and salt: which is a mixture?

Support map

Mixture and clear-water samples: appearance does not establish composition.

Situation and question

The air above a table appears uniform. Tap water is clear. Yet both can contain several components. Appearance alone does not reveal a complete composition.

New terms, fully explained

In our school model, a pure substance contains one chemical kind of particle and has a characteristic composition; a real sample with impurities is no longer strictly pure. A mixture is a physical combination of two or more substances whose proportions may vary. A component is one constituent. A homogeneous mixture looks the same in sufficiently large portions; a heterogeneous mixture has visibly different parts or phases. Here a ‘phase’ is a region of uniform observable state and composition. Looking uniform does not prove purity.

H₂O is shorthand for water; we will explain the formula in full after introducing atoms. Distilled water is water obtained by evaporating water and turning its vapour back into liquid; this reduces many impurities, but a label alone does not prove any sample is absolutely pure. A phase is a region of roughly uniform state and composition.

Predict first

Before the hint, classify a sealed sample of distilled water, room air, dry sand and tap water. Where is appearance insufficient for a final judgement? Why is one colour of sand not proof of one pure substance?

Investigate or analyse given data

List possible components without tasting or smelling. Air is mainly nitrogen and oxygen, with argon, water vapour and other gases. Tap water contains water and dissolved substances, in varying amounts. Sand may contain minerals of different composition. Mark which statements rely on known information and which need analysis of the particular sample.

Worked example

Salt and sand in a bowl form an obviously heterogeneous mixture: different grains are visible. When table salt dissolves completely in water, the result is a homogeneous solution. Salt remains a component even though we cannot see it. ‘No visible boundary’ does not mean ‘one substance’.

A common mistake

Mistake: ‘Clear tap water is pure H₂O.’ Clarity does not rule out dissolved salts. Another mistake is confusing ‘safe to drink’ in everyday speech with ‘chemically pure’; these answer different questions.

Transfer to a new case

An unlabelled closed bottle contains a colourless liquid. Can you identify it as pure water? Plan a safe way to gather information without opening, heating or tasting it. What conclusions remain impossible?

Answer and recall

Check: no; appearance cannot establish composition. You may investigate its origin and label, but without reliable evidence cannot call it water or safe. From memory draw ‘composition → one kind or several → visible or invisible parts’ and explain why one visual feature is not enough.

Investigator’s table

When to revisit

Close the page and, after 10 minutes, reconstruct the map, define the terms in your own words and solve the new case. Repeat without hints after a day and a week. If an error persists, return to the worked example and write down the missing step.

Glass, glassware and transparency: three different answers

Dissolving without vanishing: a particle model