What are Mixtures? | O Level chemistry

 Mixtures

A mixture contains more than one substance. The substances are just

mixed together, and not chemically combined. For example:

air is a mixture of nitrogen, oxygen, and small amounts of other gases

shampoo is a mixture of several chemicals and water.

Solutions

When you mix sugar with water, the sugar seems to disappear. That is because its particles spread all through the water particles, like this:

The sugar has dissolved in the water, giving a mixture called a solution.

Sugar is the solute, and water is the solvent:

solute + solvent = solution

You can’t get the sugar out again by filtering.

Not everything dissolves so easily

Now think about chalk. If you mix chalk powder with water, most of the powder eventually sinks to the bottom. You can get it out again by filtering.

Why is it so different for sugar and chalk? Because their particles are very different! How easily a substance dissolves depends on the particles in it.

Look at the examples in this table:

So silver nitrate is much more soluble than sugar – but potassium nitrate is a lot less soluble than sugar. It all depends on the particles.

Look at calcium hydroxide. It is only very slightly or sparingly soluble

compared with the compounds above it. Its solution is called limewater. Now look at the last two substances in the table. They are usually called insoluble since so very little dissolves.

Helping a solute dissolve

Sugar dissolves quite slowly in water at room temperature. If you stir the liquid, that helps. But if you keep on adding sugar …

… eventually no more of it will dissolve, no matter how hard you stir. The extra sinks to the bottom. The solution is now saturated.

But look what happens if you heat the solution. The extra sugar dissolves. Add more sugar and it will dissolve too, as the temperature rises.

So sugar is more soluble in hot water than in cold water.

A soluble solid usually gets more soluble as the temperature rises.

A solution is called saturated when it can dissolve no more solute, at

that temperature.

Water is not the only solvent

Water is the world’s most common solvent. A solution in water is called an

aqueous solution (from aqua, the Latin word for water).

But many other solvents are used in industry and about the house, to

dissolve substances that are insoluble in water. for example, white spirit dissolves gloss paint, propanone (acetone) dissolves grease and nail polish and ethanol can dissolve glues, inks etc.

All three of these solvents evaporate easily at room temperature – they are

volatile. This means that glues and paints dry easily. Aftershave feels cool

because ethanol cools the skin when it evaporates.