Mixing Up a Solution

Today you will learn how to compare a substance before and after it dissolves, and how to show that matter is never lost when you make a solution.

Think about making a pitcher of sweet tea for a family cookout. Before you stir, the sugar is white, dry, and grainy. The tea is a dark brown liquid. Both are easy to tell apart just by looking. You can even measure each one, because mass is the amount of matter in something.

Now stir the sugar into the tea. The grains seem to disappear. What really happened is that the sugar broke into pieces so tiny that your eyes cannot see them. The sugar spread evenly all through the tea. That even mix is called a solution, a mixture in which one substance dissolves completely into a liquid.

Some properties change and some stay the same. The sugar is no longer white or grainy, and you cannot see it anymore. But taste a sip, and you know the sugar is still there. The tea also looks a little different, because it is now sweet. So the way things look can change, even when nothing has left the pitcher.

Here is the big idea. Matter is conserved, which means the amount of matter stays the same before and after mixing. If the sugar has a mass of 20 grams and the tea has a mass of 500 grams, the sweet tea will have a mass of 520 grams. Nothing vanished. Scientists test this by weighing a covered cup of water, adding salt, stirring, and weighing it again. The mass matches every time, as long as nothing spills or evaporates.

Try it: If you stir 5 grams of salt into 100 grams of water in a closed jar, what will the jar's contents weigh after the salt disappears from sight?