Chemistry · Lesson

Substances, mixtures and physical or chemical changes

Use water, salt and an iron surface to separate two questions: what a sample contains, and whether a change forms different chemical substances.

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By the end, you can

Distinguish elements, compounds and mixtures.
Separate chemical composition from phase and visible uniformity.
Use substance identity rather than reversibility or a visible sign to distinguish kinds of change.

A sample and its composition

A sample is the portion of matter you are considering. A chemical substance has a characteristic chemical identity and composition. An actual sample can contain one substance or several. Purity is a claim about the sample; it is not guaranteed by a neat label or a clear appearance.

A mixture contains more than one substance, and the relative amounts can vary. Water containing a small amount of dissolved salt and water containing more dissolved salt are both mixtures. They need not have the same properties. A sample that looks uniform can still contain several substances.

Elements and compounds

An element is defined by the number of protons in its atoms; later atomic lessons explain this count. Ordinary chemical changes do not turn one element into another. A compound contains atoms of more than one element combined in a characteristic chemical arrangement. Water is a compound, with hydrogen and oxygen atoms in H₂O molecules.

Compounds do not simply keep the properties of their constituent elements. Nor do they all consist of separate molecules. Solid sodium chloride contains sodium and chloride ions in an extended crystal arrangement. NaCl records their simplest ratio; it does not depict an isolated molecule in that crystal. Keep the formula’s meaning attached to the kind of substance.

Uniformity is a separate question

A homogeneous mixture has a uniform composition at the scale being considered. A salt solution can be homogeneous even though it contains water and dissolved ions. A heterogeneous mixture has regions with different compositions, such as a settled sample of sand and water.

Uniform does not mean pure, and a physical phase is not the same thing as a chemical substance. Pure water can contain both ice and liquid. The phases differ, while the substance remains water. Conversely, a single clear liquid phase can contain multiple dissolved substances. To classify a sample, ask about both composition and phase.

Composition and phase answer separate questions

Compare composition with phase

Sample	Substances present	Phase or phases	Composition
Pure liquid water	Water	Liquid	One substance
Pure ice + pure liquid water	Water	Solid and liquid	One substance
Uniform salt solution	Water and dissolved salt constituents	Liquid	Homogeneous mixture
Settled sand and water	Water and sand constituents	Solid and liquid regions	Heterogeneous mixture

The table compares pure liquid water, pure ice plus liquid water, uniform salt solution and sand in water. More than one phase need not mean more than one substance; a uniform phase need not be a pure substance.

What changes: arrangement or chemical identity?

A physical change alters a state or physical form without changing the chemical identities of the substances involved. Melting pure ice is an example. A chemical change produces different chemical substances. Formation of rust on iron is an example involving iron, oxygen and water; ordinary rust is not one uniquely specified pure compound.

Dissolving table salt in water is commonly introduced as a physical change: its ions become separated and surrounded by water, rather than salt being destroyed. This example does not establish that all dissolving is physical. Some substances react with a liquid as they dissolve. The deciding question is which chemical identities are present before and after, including any solvent that participates.

Clues are useful, but they are not the criterion

A colour change, bubbles or a temperature change may suggest that a reaction is occurring. Each can also have a physical cause: a coloured material can spread through a liquid, dissolved gas can escape, and heat can transfer between a sample and its surroundings. You need evidence appropriate to the proposed new substances.

Reversibility is also an unreliable shortcut. Breaking a glass changes its physical form, even if you cannot easily restore it. Some chemical reactions can proceed in both directions under suitable conditions. Use changes in chemical identity as the distinction. When evidence is insufficient, describe the observation and leave the classification open.

Worked examples

Two questions about one change

Pure ice melts. Separately, salt becomes distributed through water. How do you classify the samples and changes?

Pure ice and its resulting liquid are the same substance in different phases.
Melting retains water’s chemical identity: it is a physical change.
The salt solution contains water and dissolved salt constituents: it is a mixture.
For ordinary table-salt dissolution, the introductory account describes physical dispersal and hydration of ions, not destruction of matter.

The melted pure water is one substance; the salt solution is a mixture. Both described processes are ordinarily classified as physical, while the particle arrangements change.

Try it yourself

A liquid is transparent and uniform. Must it be a pure substance?
Show the answer and reasoning
No.
A homogeneous mixture can look uniform. Appearance alone does not establish composition.
A sample contains only pure ice and pure liquid water. Does having two phases make it a mixture of two substances?
Show the answer and reasoning
No. It contains one chemical substance in two phases.
Composition and phase are separate classifications. Both portions consist of water.
Bubbles appear in a liquid. Has formation of a new chemical substance been demonstrated?
Show the answer and reasoning
No.
A reaction is one possible explanation, but gas can also come out of solution or form during a phase change. Identification requires additional evidence.
A piece of glass is broken. Does the difficulty of putting it back together make the change chemical?
Show the answer and reasoning
No.
The change in shape does not by itself create different chemical substances. Reversibility is not the definition.
Why should a particle picture of a salt crystal avoid presenting it as a collection of isolated NaCl molecules?
Show the answer and reasoning
Its sodium and chloride ions form an extended crystal arrangement.
The formula expresses their ratio in the solid. It should not be mistaken for a drawing of separate molecules.

What to carry forward

Composition identifies substances; phase describes physical state.
Elements, compounds and mixtures are different kinds of chemical classification.
A uniform sample can be a mixture, and one substance can occupy multiple phases.
A visible clue or difficulty of reversal cannot alone establish a chemical reaction.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.