Mixtures and Solutions

Mixing things up

Have you ever made a fruit salad, stirred sugar into a drink, or watched mud swirl in a puddle? Every time, you made a mixture. Mixtures are everywhere — in your kitchen, in the sea, even in the air you are breathing right now. Learning how they work is one of the first big ideas in chemistry, the science of what things are made of.

A mixture is two or more substances mixed together but not joined into a new substance. This is the key idea: in a mixture, each part keeps being itself. If you mix dried fruit and nuts, the fruit is still fruit and the nuts are still nuts. You could even pick them apart again. Nothing brand new was made.

Solutions: when things dissolve

Some mixtures are special. Stir a spoonful of sugar into a glass of warm water and watch carefully. The sugar seems to disappear! But it has not vanished — it has dissolved. The sugar has broken into pieces far too tiny to see and spread out evenly through the water. A mixture like this is called a solution.

Scientists use three useful words for solutions:

• The solute is the thing that dissolves (the sugar).
• The solvent is the thing it dissolves into (the water).
• Together they make a solution (the sweet water).

Water is the most common solvent on Earth. So much dissolves in it that it is sometimes called the "universal solvent." Salt water in the sea is a solution. The fizzy bubbles in a drink are gas dissolved in water. Even the cup of tea a grown-up drinks is a solution.

How can you tell if something has truly dissolved? A true solution is see-through (even if it has a colour, like orange squash) and it does not settle at the bottom. The solute is spread out so evenly that it stays mixed.

Mixtures that don't dissolve

Not everything dissolves. Stir sand into water and the sand does not disappear — it sinks to the bottom. Stir flour into water and you get a cloudy mixture that slowly settles. These are still mixtures, but they are not solutions, because the parts do not spread out evenly. A cloudy mixture where bits float around and then settle is called a suspension.

So here is a neat way to sort them out:

Mixture	Does it dissolve?	What you see
Sugar in water	Yes	Clear, sweet solution
Salt in water	Yes	Clear, salty solution
Sand in water	No	Cloudy, then sinks
Oil in water	No	Floats in separate layer

Pulling mixtures apart again

Because the parts of a mixture are not truly joined, we can separate them again. Scientists have clever methods for this, and the one you choose depends on the mixture.

Filtering uses a filter, like paper with tiny holes, to catch solid bits while liquid passes through. This separates sand from water — the sand stays on the paper.

Evaporating is how we get a dissolved solute back. If you leave salt water in a warm place, the water slowly turns into vapour and floats away, but the salt cannot evaporate, so it is left behind as dry crystals. This is exactly how sea salt is made. It also connects to what you may know about water changing state, as in The Three States of Water.

Using a magnet works when one part is magnetic. To separate iron filings from sand, you simply run a magnet over the mixture and the iron jumps to it.

Sieving uses a mesh to separate bigger pieces from smaller ones, like pebbles from sand.

Why mixtures matter

Understanding mixtures and solutions is useful in real life. Cooks dissolve sugar and salt to flavour food. Water companies filter and clean the water that reaches your tap. Factories separate useful materials from rock and waste. And in nature, rivers carry dissolved minerals to the sea, which is partly why the ocean is salty. You can see water doing this work in The Water Cycle.

Try this — make and un-make a solution

With a grown-up's help, half-fill a clear cup with warm water and stir in a teaspoon of salt until it dissolves and the water looks clear again. You have made a solution! Now pour a little of the salty water onto a small plate and leave it on a warm, sunny windowsill for a few days. As the water evaporates, watch tiny white salt crystals appear on the plate. You made a solution, then separated it — just like a real scientist. Do not eat the crystals, and wash everything afterwards.

Mixtures, solutions, dissolving and separating — once you spot them, you will see chemistry happening all around you, every single day.