step 1 of 6
You know dissolving hides a solid without destroying it. Priya sets the scale to zero with an empty cup on it. She pours in 100 grams of water, then stirs in 20 grams of salt until no grains show.
What does the scale read now?
Conservation of Mass
Measure and provide evidence that the total weight of matter is conserved regardless of the type of change (heating, cooling, or mixing)
What a learner can do afterwards
- State the principle that matter is neither created nor destroyed during physical or chemical changes
- Describe an investigation weighing materials before and after a change to show mass is conserved
- Explain why dissolved sugar still contributes to the total weight even though it can't be seen
1 · Read
You already know that heating, cooling, and mixing can change how something looks. Ice can melt into water. Sugar can dissolve and seem to disappear into water. Here is the surprise: even when matter changes shape or seems to vanish, none of it is created or destroyed. If you weighed every bit before and after, the total weight would stay exactly the same.
Try it together
Priya sets a kitchen scale to zero with an empty cup on it, so the scale weighs only what goes in. She pours in 100g of water, then stirs in 20g of salt until it disappears from sight. The scale still reads 120g. The salt did not leave. It just spread out into pieces too small to see.
Good to know
Whenever weight seems to disappear during a change, ask where the matter actually went. It may have turned into a gas, dissolved into a liquid, or just spread out, but it is still there somewhere.
When a weight seems to drop, ask where the matter went. A puddle dries up because its water turned into invisible vapor drifting in the air. A leaking beaker reads less because some water dripped out and never got weighed. Burning and fizzing make new materials, but the total still stays the same once you count every gas that forms or escapes. If the scale shows less, some matter left the scale; it was never destroyed.
Matter never disappears when it heats, cools, or mixes. It only changes form, so the total weight stays the same.
Readings 'Weighing before and after' and 'Where did the matter go?' are original to this dataset.
2 · Watch
3 · Play
Take it off screen
Print a worksheetA4 with an answer key page for grown-ups. No screen, no internet.
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
Then practise
24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.