Because the parts of a mixture keep their own properties, you can separate them using physical methods. Filtering works by passing a mixture through a material with tiny holes — the liquid goes through, but larger solid pieces get caught. Evaporating works by heating a solution until the liquid turns into gas and escapes, leaving the dissolved solid behind. These methods prove that mixing does not create new substances — the original materials are still there and can be recovered.
Have students make salt water and sand water. Filter the sand water through a coffee filter to recover the sand. Pour the salt water into a shallow dish and let it sit by a sunny window (or on a warm plate) for a day — students return to find salt crystals left behind. Discuss why filtering cannot separate salt water.
One of the most useful things about mixtures is that you can undo them. Because the parts of a mixture keep their own properties, you can use those different properties to pull them apart. Two of the most common separation methods are filtering and evaporating, and they work in different situations.
Filtering means pouring a mixture through a material with tiny holes — like a coffee filter, a paper towel, or even a fine mesh screen. The liquid and any dissolved materials pass through the holes, but solid particles that are too big get caught. This is perfect for separating things like sand from water, coffee grounds from coffee, or dirt from muddy water. After filtering, the solid is sitting on the filter and the liquid is in the cup below.
But what if the solid is dissolved, like salt in salt water? Filtering will not help, because dissolved particles are incredibly tiny — they slip right through the filter with the water. For this, you need a different method: evaporation. Pour salt water into a shallow dish and leave it in a warm place. Over time, the water evaporates — it turns to gas and floats away into the air. The salt cannot evaporate (it would need a wildly high temperature for that), so it stays behind. When all the water is gone, you are left with salt crystals sitting in the dish.
These two methods can even be combined. Imagine you have a messy mixture of sand, salt, and water. First, filter it — the sand gets trapped, and the salt water passes through. Now you have separated the sand. Next, evaporate the salt water — the water leaves as gas, and the salt stays behind. Now you have all three components separated: sand on the filter, salt in the dish, and the water is in the air.
The reason these methods work is that they take advantage of differences in properties. Sand particles are big, so they get caught by a filter. Salt is dissolved, so it needs evaporation. Water turns to gas easily, so it is the first to leave. Every separation method in science works the same way — find a property that makes the materials different, and use that difference to pull them apart.
This is the same idea behind how cities clean drinking water. River water is a messy mixture of dirt, dissolved minerals, and germs. Water treatment plants use filtering to remove the solid particles and other methods to handle the dissolved materials, sending clean water to your faucet.