A pure substance is made of only one kind of material, like pure water or a piece of gold. A mixture is two or more different materials combined together, where each material keeps its own properties. Trail mix is a mixture — you can still see and pick out the nuts, raisins, and chocolate pieces. Mixtures can be separated back into their parts, but the parts themselves are not changed by being mixed together.
Give students samples to classify: a cup of salt water, a bag of trail mix, a glass of pure water, a handful of soil, a piece of aluminum foil, and a bowl of fruit salad. Have them sort each into "pure substance" or "mixture" and explain their reasoning. Discuss what clues helped them decide.
Look at a glass of lemonade. It might seem like a simple drink, but it is actually a combination of several different materials — water, lemon juice, sugar, maybe some pulp. That makes it a mixture. Now look at a glass of distilled water from the science supply shelf. It contains only one thing: water. That makes it a pure substance. Learning to tell the difference between pure substances and mixtures is one of the most useful classification skills in science.
A pure substance is made of only one kind of material. Pure gold is just gold. Pure water is just water. A block of ice made from distilled water is a pure substance. You cannot separate a pure substance into different parts using simple physical methods like filtering or picking — there is nothing to separate because it is all the same stuff.
A mixture is two or more different materials combined together. The important thing about mixtures is that each material keeps its own properties. In trail mix, the peanuts still taste like peanuts, the raisins are still sweet, and the chocolate is still chocolate. Mixing them together did not create a new substance — it just put different substances in the same bag. You can easily separate them by picking them apart.
Some mixtures are harder to spot than others. Salt water looks like plain water — clear, colorless, and smooth. But taste it and you know the salt is in there. Even though you cannot see the salt anymore, it is still present and still salty. Mixtures where the parts are evenly spread out and hard to see are called solutions, and you will learn more about those soon. The key test is always the same: can you identify more than one material in it, even if you have to use something other than your eyes?
The reason scientists care about this difference is that understanding what something is made of helps you predict how it will behave. A pure substance has one set of properties that never changes. A mixture's properties depend on what is in it and how much of each ingredient there is. Weak lemonade tastes different from strong lemonade, even though both are mixtures of the same ingredients.