Pure substances and mixtures

A sample of water freezes without anything being added or removed. Which statement describes its substance identity?

Recall what stays the same during a change of state

Count substances, not objects

A sample is the portion of material being considered. One object or container can hold one substance or several substances

A pure sample contains only one chemical substance. A mixture contains two or more substances together

Several pieces of the same substance do not make a mixture. Different states of the same substance do not make a mixture either

Use the stated composition. Colour, clarity, natural origin and an everyday pure label do not establish chemical purity

Separate pieces, unchanged composition

A block contains only copper. It is cut into three pieces without contamination or chemical reaction, and all three pieces are kept together

Explain whether cutting this copper changes the sample from one substance to a mixture

Cutting changes the size and number of pieces, while the record excludes any new substance

All retained material is still copper, so the collection remains a pure sample

The bars count portions of two different known substances, A and B. Each portion contains many particles; it is not one atom or molecule. No reaction occurs. Keep B at zero and change A, then add B and compare

Starting with A only, which change makes a mixture in this model?

Two states, one identity

A model needs a key

A mark or token means what its model key says it means. Repeated marks can show several portions of the same substance

In the next model, A and B label different known substances. The letters do not identify elements or chemical formulas

A portion contains many particles. This portion model does not show atoms, bonding, particle spacing or whether a substance is an element or a compound

Interpret a keyed model

Uniform does not mean pure

Composition describes which substances are present and their relative amounts

A homogeneous mixture has uniform composition throughout the sample. A heterogeneous mixture has composition that differs between regions

A solution is a homogeneous mixture. When sugar dissolves in water without reacting, both sugar and water remain even though the sugar is no longer visible as separate grains

A sample can contain two substances and still look uniform. Classify the number of substances first, then consider their distribution

Classify a solution

Sugar dissolves in water without reacting. Both remain present and their proportions are uniform throughout the resulting liquid

Classify this sugar solution using its constituents and their distribution

Sugar and water both remain, so the liquid is a mixture

Their proportions are uniform throughout, so this is a homogeneous mixture, or solution

Use the rule for gases

Compare regions of a solid sample

Keep the claim within the evidence

A description may state all the substances present, or it may supply only an observation such as colour or clarity. These are not the same amount of evidence

If a record does not establish the composition, do not invent it. Pure liquids and solutions can both be clear and colourless

A result for one small region need not describe an entire object. Say which sample was tested and identify what remains unknown

Recognise insufficient information

A chip is not the whole rock

Compare two uniform mixtures

Review composition and evidence

First identify the sample and the substances known to be present. One substance means a pure sample; two or more means a mixture

For a mixture, compare composition within the sample. Uniform composition is homogeneous; differing composition between regions is heterogeneous

Use the supplied model key and observations. A visible boundary, an everyday label or a result from one small region does not by itself establish the composition of the whole sample