Changes to materials

Matter can change from one state to another when you heat it or cool it. These are called changes of state.

• Melting — a solid is heated and turns into a liquid (ice → water).
• Freezing — a liquid is cooled and turns into a solid (water → ice).
• Evaporating — a liquid is heated and turns into a gas (water → water vapour).
• Condensing — a gas is cooled and turns into a liquid (water vapour → water on a cold window).

The temperature at which a material melts is its melting point. Heating gives particles more energy so they break free; cooling takes energy away so they settle down again.

When a material changes, ask one question: can I get the original back?

A reversible change can be undone. The material may look different, but no new material has been made.

• Melting ice into water, then freezing it back, is reversible.
• Dissolving sugar in water is reversible — evaporate the water and the sugar returns.

An irreversible change cannot be undone. A new material is made, often with very different properties.

• Baking a cake is irreversible — you cannot turn it back into flour and eggs.
• Burning wood, frying an egg and metal rusting are all irreversible.

Useful clues that a change is irreversible: you see a colour change, bubbles of a new gas, a new smell, or heat and light given out. When you spot those, a new material has usually been made.

Changes of state are always reversible — only the arrangement of particles changes, never the material itself.

A mixture is two or more materials simply mixed together. The important point: the materials are not joined chemically, so each one keeps its own properties.

Because the materials are not chemically joined, a mixture can be separated back into its parts. Making a mixture and separating it again are both reversible changes.

Everyday mixtures include:

• Sand and water — bits of solid floating in a liquid.
• Salt and sand — two solids mixed together.
• Air — a mixture of several different gases.
• A bowl of cereal in milk — solids in a liquid.

Mixing is different from a chemical reaction. If you mix iron filings and sand, the iron is still iron and the sand is still sand — a magnet can pull the iron straight back out. No new material has been made.

Knowing a material is a mixture is the first clue that you can separate it.

Scientists separate mixtures by using the different properties of the materials. Here are four key methods.

• Filtering — pours a mixture through filter paper. The liquid passes through (the filtrate); the solid is trapped (the residue). Use it for sand and water.
• Evaporating — heats a solution so the liquid turns to gas and escapes, leaving the dissolved solid behind. Use it to get salt back from salt water.
• Sieving — uses a mesh with holes to separate solids of different sizes, such as small stones from sand.
• Using a magnet — pulls out a magnetic material. Use it to separate iron filings from sand.

Stir sugar into a cup of tea and it seems to vanish. It has not been destroyed — it has dissolved.

When a solid dissolves, its particles spread out evenly between the particles of the liquid. The result is a solution — and a solution is a special kind of mixture.

Three key words go together:

• Solute — the solid that dissolves (the sugar).
• Solvent — the liquid it dissolves into (the water).
• Solution — the clear mixture you end up with (the sweet tea).

A material that dissolves is called soluble; one that does not is insoluble. Sugar and salt are soluble in water; sand and chalk are insoluble.

Two clues that a solid has dissolved rather than just disappeared: the solution stays clear (you can see through it), and you can get the solid back by evaporating the solvent. Dissolving is a reversible change — the solute is always still there.

Understanding how materials change helps you in many topics ahead:

• Chemical reactions — irreversible changes are your first step towards understanding reactions that make new substances.
• The water cycle — evaporating and condensing are exactly how rain clouds form and water moves around the Earth.
• Acids and materials — you will use separating skills again when investigating new mixtures.
• Everyday life — cooking, recycling and cleaning water all rely on the ideas in this topic.

Whenever you see something change — ice melting, a drink going fizzy, a window misting up — try asking: is this reversible, and has a new material been made? That question turns the everyday world into chemistry.