Homogeneous and heterogeneous mixtures

There are mixtures all around us. Some look very different to others. We usually divide mixtures into two groups. Mixtures where we can distinguish the different substances... ... and mixtures that look like one single substance.

But where do we draw the line between the two categories? Let's perform an experiment. We have two beakers of water. We'll pour sand into the first one. And mix some sugar into the second.

And stir them a bit. Let's start by looking at the beaker with sand in it. The grains of sand are clearly visible. And if we shine a torch through it... ... the sand particles are large enough to be lit up by the beam of light.

When we stop stirring, most of the sand will settle to the bottom. It is not evenly distributed throughout the mixture. We call this a heterogeneous mixture. Compare this to the other beaker. The sugar water looks completely clear.

There's no sign of the sugar that we mixed in. And when we shine a torch on the beaker, the light shines right through it. It is transparent. The sugar has dissolved into tiny particles. They are so small that they are impossible to see, even using a powerful microscope.

They are separated down to the molecular level. And the sugar molecules are spread evenly throughout the mixture. We call this type of mixture a homogeneous mixture. Another common name for a homogeneous mixture is a solution. Solutions in water are always transparent, but they don't have to be colourless.

Even though the particles are invisible they can still give the solution a colour. Solutions are not only in liquid form. Air is a gaseous solution, containing mostly nitrogen and oxygen. Solutions may also be solid. Brass is a solid solution of copper and zinc.

Solutions of metals are called alloys. Since an alloy is a mixture, the metal atoms are only mixed with each other, they haven't reacted to form new compounds. It's easy to identify some mixtures as heterogeneous. But some are more difficult. Blood for instance, looks like a solution... ...

but when left for a while, it separates into a layer of blood cells, and a layer of plasma. The bloods cells are not dissolved in the plasma. And if you have a powerful microscope... ... you can see the individual blood cells. So what kind of mixture is milk?

Milk is mostly water, with some substances dissolved in it, but also a small amount of fat. The fat is present as tiny fat globules dispersed in the water. In fresh, unprocessed milk, some of the fat will rise to the top, forming a layer of cream. The milk you buy in the store is processed so it won't form different layers. This is done by splitting the fat droplets into smaller pieces, in a process called homogenisation.

But still, even if these clumps of fat are small enough to remain suspended in the milk... ... each fat globule still contains thousands of fat molecules. So the fat isn't dissolved on the molecular level. Which means that "homogenised" milk is still - in chemistry terms - a heterogeneous mixture. Wipe your mouth, Michael.