Mixtures & combining materials
The idea
When you stir two or more materials together, you make a mixture. In a mixture, each material is still itself, even when it is hard to see. Trail mix is an easy one: you can still pick out every raisin. Salt water is sneakier: the salt seems to vanish into the water. Mixing happens all the time in cooking, cleaning, and painting, so it pays to understand what really goes on.
Some solids spread out so evenly in a liquid that they seem to disappear. That is called dissolving. Salt and sugar dissolve in water: the grains break into pieces far too small to see, and those pieces spread through the whole drink. Other materials do not dissolve. Sand stirred into water just swirls around for a moment and then settles to the bottom, still plainly sand.
Dissolved is not the same as gone. Stir sugar into water and taste it: every sip is sweet, so the sugar is still in there. A scale agrees — the water with its dissolved sugar weighs as much as the water and the sugar did separately. You can even get the sugar back by letting the water evaporate, which leaves the solid grains behind.
Worked example
You have two clear cups of warm water. You stir a spoonful of salt into the first cup and a spoonful of sand into the second. Before stirring, the first cup with its salt weighed 210 g in total. What do you see happen in each cup, and what does the first cup weigh after stirring?
- Watch the salt cup as you stir: the grains get smaller and smaller until you cannot see them at all, and the water turns clear again — the salt has dissolved.
- Prove the salt is still there: one tiny taste of the water is salty, so the salt spread through the water instead of vanishing.
- Because dissolving only spreads the salt out and nothing left the cup, the scale still reads 210 g after stirring.
- Now watch the sand cup: the water goes cloudy while you stir, then the sand sinks and piles up at the bottom, still easy to see — sand does not dissolve in water.
- Compare the two cups: both hold mixtures, but only in the salt cup did the solid dissolve and disappear from view.
Answer. The salt dissolves and its cup still weighs 210 g; the sand never dissolves and settles in a pile at the bottom of its cup.
Check your understanding
- How could you get the salt back out of salt water, and why does that plan work?
- What evidence shows that dissolved sugar is still in the water even though you cannot see it?
- Why does stirring sand into water never make the sand disappear the way salt does?
- If you mixed raisins, oats, and seeds for a snack, what makes that a mixture, and how is it different from salt water?