Science Revision (R1+R2)

Mixtures ( R2 )

Pure substances

In chemistry, a pure substance is a single substance made of only one type of element or one type of cmpound.

Impurities

Changes the temperature at which a substance melts and boils.

For example, salt is a pure substance made only of sodium chloride.

A substance made from more than one element or one compound is impure, meaning it is a mixture.

For example, it is challenging to make pure water. This mineral water contains small amounts of impurities such as sodium and nitrate.

Fixed Boiling Points

A pure substance has a fixed melting point and boiling point, which means they will always melt or boil at exactly the same temperature. For example, the melting point of pure water is 0 °C and its boiling point is 100 °C.

Dissolving

Solution

A solution is made when a substance dissolves into a liquid. The liquid is called the solvent. The substance that has been dissolved is called the solute. A solution can also be called a mixture. When a solid dissolves, it breaks down into smaller particles that spread out through the solvent.

For example, salt dissolves when it is stirred into water. In sea water, the water is the solvent and the salt is the solute.

Solute

A substance which dissolves in a solvent as the solvent breaks up the solute which therefore dissolves it.

For example, Salt, sugar and instant coffee.

Solvent

A substance which dissolves a solute as it break up The solute.

For example, water, milk and ethanol.

Solubility

Is how well something dissolves

Soluble

Insoluble

This is a term used to show if something can dissolve.

This is a term used to show if something can not dissolve.

Examples

Wax is insoluble in water.

Chalk is insoluble in water.

Salt is insoluble in propanone.

Polystyrene is insoluble in water, but soluble in propanone.

Examples

Salt is soluble in water

Sugar is soluble in water

Instant coffee is soluble in water

Solubility of gas

Solubility measures how much solute can dissolve in a volume of solvent at a specific temperature. For example, sugar has a higher solubility in water than salt. Therefore, more sugar than salt can dissolve in a volume of water at a specific temperature. The solubility of a gas usually decreases when the temperature increases.

You can speed up dissolving a solid by:

Heating up the solvent. The solubility of a solid solute usually increases when the temperature increases.

Stirring.

Using fine powder rather than large pieces of solute.

For example, fizzy drinks have lots of carbon dioxide dissolved in them when they are cold. Fizzy drinks warm up quickly in your mouth and release bubbles of carbon dioxide gas.

Coloured solutions

When a coloured solid dissolves, it makes a coloured solution, for example, copper sulphate dissolves to makes a blue solution.

Colourless solutions

When a white solid dissolves, it makes a colourless solution, for example, salt or sugar in water.

Saturation

There is a limit to the mass of solute that will dissolve in a particular volume of the solvent. When no more solute dissolves, the solution is saturated.

Filtration

Filtration is used to separate an insoluble solid from a pure liquid or a solution.

Filtration usually involves a circle of filter paper folded to make a cone and placed into a filter funnel.

The filtrate is the liquid which passes through the filter paper and the residue is the solid left on the filter paper.

Filtration is the process of separating solids from liquids using filter paper.

Filteration

Crystallisation

Evaporation occurs when a liquid slowly turns into a gas below its boiling point.

Crystallisation is a separation technique used to obtain crystals of a solid solute.

When a solution is heated, the solvent evaporates and crystals of the solute are left behind.

Evaporation

Evaporation describes the process of a liquid turning into a gas. Evaporation is a slower process than boiling because it occurs at temperatures below the boiling point of the liquid.

Crystilisation

Steps for crystallisation

Rather than heating the evaporating basin directly with a Bunsen burner, it is often better to warm it gently and slowly over a beaker of boiling water.

Heat the solution until half the solvent (water) has evaporated, which makes the solution more saturated.

Leave the solution to cool and large crystals will form slowly.

Put the evaporating basin in a warm place so the rest of the water evaporates.

Chromatography

Chromatography is a separation technique used to separate mixtures of soluble substances.

These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Chromatograms can be used to match known pigments with those in a mixture.

On a chromatogram, one spot means that the substance is pure. An impure substance produces two or more spots.

Distillation

Distillation is a separation technique used to separate a solvent from a mixture. For example, water can be separated from salt solution by distillation.

Distillation involves boiling the mixture and then condensing the gas to produce a liquid.

Alloys

An alloy is a mixture of two or more elements, where at least one element is a metal. Many alloys are mixtures of two or more metals.

Water

Potable water

Water that is safe for humans to drink is called potable water. Potable water is not pure water because it almost always contains dissolved impurities.

For water to be potable, it must have sufficiently low levels of dissolved salts and microbes. This is because:

dissolved salts can sometimes be harmful for humans

microbes can cause illnesses

In the UK

Rain provides water with low levels of dissolved substances. In the UK, rain provides sufficient water to meet the needs of the population, although sometimes it needs to be stored and transported. Sometimes during the summer months in some areas of the UK, water reserves run low and hosepipe bans are used to encourage people to conserve water. Most potable water in the UK is produced from naturally occurring fresh water by:

choosing an appropriate source of fresh water

passing the water through filter beds to remove insoluble particles

sterilising the water to kill microbes

The methods used for sterilisation include chlorine, ozone and ultraviolet light.