Conservation of matter (intro)
The idea
Matter does not pop out of existence, and it does not appear from nowhere. When you melt it, freeze it, cut it, squash it, or mix it, the amount of matter stays the same. Scientists call this conservation of matter. It is why a scale is such a powerful tool: if nothing got in and nothing got out, the weight before a change equals the weight after.
A good mental picture is a closed box. Imagine the change happening inside a sealed, see-through box sitting on a scale. Melt ice in the box: same reading. Dissolve salt in water in the box: the reading is the water's weight plus the salt's weight. Crush a cracker to crumbs in the box: same reading. Changing the state, shape, or look of matter never changes how much of it there is.
Sometimes a scale really does read less after a change — when a puddle dries, or soup simmers with the lid off. That does not break the rule. The missing matter escaped into the air as a gas: it left the box, it was not destroyed. Whenever weight seems to vanish, hunt for the escape route instead of deciding the matter is gone.
Worked example
An empty plastic bottle weighs 110 g. You drop in 90 g of ice cubes and screw the cap on tightly, so the whole thing weighs 200 g on the scale. You leave the bottle on the scale until all the ice has melted. What does the scale read then?
- Check the starting total: 110 g of bottle plus 90 g of ice makes 110 + 90 = 200 g, which matches what the scale shows.
- Name the change that happens: the ice melts, meaning the solid water turns into liquid water — the same stuff in a new state.
- Look for escape routes: the cap is screwed on tightly, so no water and no vapor can leave the bottle.
- Apply conservation of matter: nothing was added and nothing escaped, so the amount of matter inside is unchanged and the scale still reads 200 g.
- Sanity-check with your eyes: the puddle of melted water looks different from the pile of cubes, but a new look and shape do not mean a new weight.
Answer. The scale still reads 200 g, because melting changed the state of the ice but not the amount of matter inside the sealed bottle.
Check your understanding
- Why would the scale read less over time if you left the cap off the bottle on a warm day?
- How would you use a scale to show a friend that dissolving sugar does not destroy it?
- When wood burns in a campfire and only a little ash is left, where did the rest of the matter go?
- Why does crushing a cracker into crumbs change how it looks but not how much it weighs?