Rocket is making a fruit drink for a team picnic. He pours sugar into a pitcher of water and stirs fast.
"Done," he says. "The sugar is gone, so the pitcher weighs the same as plain water."
Raven tilts her head. "Is the sugar gone, or just hiding?" she asks. "Remember the salt crystals last week?"
Nova hovers beside the pitcher. "Here is a pattern I like," she says. "Before plus before equals after."
Rocket grabs a pencil. "So I should add the weights," he says. "Let's build the math!"
When something dissolves, the total weight of the mixture equals the weight of the parts added together.
Here is an example with a pretend cup. The water weighs 200 grams. The sugar weighs 10 grams.
After the sugar dissolves, the sweet water weighs 210 grams. Nothing was lost.
| Before (part 1) | Before (part 2) | After |
|---|---|---|
| Water: 200 grams | Sugar: 10 grams | Sugar water: 210 grams |
| Water: 150 grams | Salt: 5 grams | Salt water: 155 grams |
| Ice in a sealed bag: 300 grams | Nothing added | Melted water in the bag: 300 grams |
Scientists check results by adding up the before weights.
If the after weight matches, the rule holds.
If it does not match, they look for something that got in or out.
| Statement | True or false? |
|---|---|
| 200 grams of water plus 10 grams of sugar makes 210 grams of sugar water. | ? |
| Dissolved sugar adds no weight. | ? |
| A sealed bag of ice weighs the same after melting. | ? |
| After weight should equal the before weights added together. | ? |
| Statement | True or false? |
|---|---|
| 150 grams of water plus 5 grams of salt makes 155 grams of salt water. | ? |
| When sugar dissolves, its weight disappears. | ? |
| Before plus before equals after. | ? |
Strong math work today. Tomorrow you will test this rule with a real scale.