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Melting, freezing, dissolving and mixing keep the total mass; an open cup loses only the gas that escapes.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will find the total mass after melting, freezing, dissolving or mixing, explain why an open fizzing cup seems to lose mass, and work out how much gas escaped.
You can weigh things in grams and kilograms, add masses together, and you know from the energy lessons that the total stays the same even when stores change. Matter follows a rule just like that one.
You have also seen things change: ice melting in a drink, sugar vanishing into tea, puddles freezing in winter. This lesson asks what happens to the mass in each case.
| Term | What it means |
|---|---|
| Matter | Anything that has mass and takes up space: solids, liquids and gases. |
| Conserved | Kept the same: the total mass before equals the total after. |
| Dissolve | Break into pieces too small to see, spread through a liquid. |
| Melt | Change from a solid to a liquid. |
| Freeze | Change from a liquid to a solid. |
| Sealed | Closed so that nothing can get in or out, not even a gas. |
| Gas | Matter that spreads out to fill any space, like air or the fizz in soda. |
When matter changes, the total mass stays the same. Scientists say mass is conserved. It does not matter what kind of change it is:
That last part matters. If a change makes a gas and the container is open, the gas escapes and the scale reads less. The mass has not been destroyed; it has left. Seal the container and the scale reads exactly the same before and after.
Another way: action
Weigh a glass of water and a spoonful of salt separately. Stir the salt in until it vanishes. Weigh the glass again. The reading equals the two added together.
Another way: steps
Stir sugar into water and it seems to disappear. It has not gone anywhere. It has broken into pieces so tiny that you cannot see them, and those pieces have spread out through the water.
You can prove the sugar is still there in two ways. First, taste the water: it is sweet everywhere. Second, weigh it: the glass weighs exactly as much as the water plus the sugar. If you leave the glass somewhere warm until the water dries up, the sugar appears again as crystals at the bottom. Out of sight is not out of the glass.
Drop a fizzing tablet into an open cup of water and weigh it after the fizzing stops. The reading is a few grams less than the cup and tablet were before. Has mass been destroyed?
Try it again with the cup sealed in a bottle with the cap on tight. Now the reading after is exactly the same as before. The difference is the gas in the bubbles. In the open cup it floated off into the room, taking its grams with it. In the sealed bottle it was trapped, and its grams stayed on the scale.
This is the best evidence for the rule. The only time mass seemed to disappear, a gas had escaped, and catching the gas made the missing mass come back.
Sometimes a change makes something take up more space without adding any matter. Bread dough rises to twice its size, because tiny bubbles of gas form inside it. Popcorn puffs up many times bigger when it pops. Water gets a little bigger when it freezes, which is why a full bottle can crack in a freezer.
In every case the mass barely changes, because nothing was added. The same matter just takes up more room. So a change in size is never, by itself, evidence of a change in mass. The only way to know the mass is to weigh it.
Why should the mass stay the same? Because every bit of matter is made of pieces far too small to see, and a change only moves those pieces around. It does not make new ones or destroy old ones.
When ice melts, the same pieces that were locked in place in the solid start sliding past each other in the liquid. There are exactly as many pieces as before, so there is exactly as much mass. When sugar dissolves, its pieces drift apart and mix in among the pieces of water. None of them leave the glass, so the glass still holds all of their mass.
Even when a change makes something completely new, like the gas from a fizzing tablet, the new substance is built from the same pieces that were there before, just joined up in a new way. That is why a sealed bottle weighs the same before and after a fizz. The pieces are still all inside; some of them are now a gas.
So the rule is not a lucky pattern. It follows from what matter is: count the pieces, and nothing in an ordinary change can add to them or take them away.
Every question in this lesson can be answered with the same short method. Take this one: a student puts $150$ g of vinegar in a bottle and adds $10$ g of baking soda. It fizzes wildly. What does the bottle read afterward?
Notice that the method never asks you to guess. You add what went in, you decide whether anything could get out, and a subtraction tells you how much did. A scientist who does these five things carefully will never be fooled into thinking matter was destroyed.
The same five steps work for any change you meet. Melting chocolate in a covered pan, mixing paint in a sealed can, stirring cocoa powder into warm milk: add up what went in, ask whether anything could get out, and weigh again. If the container was closed, the two readings will match to within one mark of the scale. If it was open and you saw bubbles or steam, the second reading will be a little lower, and the difference is the mass of the gas that left. Either way, you now know exactly where every gram went, which is what a scientist means by saying mass is conserved.
Checking that mass is conserved needs careful weighing.
Even then, readings might differ by a gram, because every scale has a smallest mark. A difference of one mark is the scale's wobble, not matter disappearing. A difference of many grams in an open container means a gas escaped.
Run three checks.
The first check catches the most common mistake: leaving out the sugar or salt because it can no longer be seen.
A recipe lists $250$ g of flour, $200$ g of sugar, $200$ g of butter and $4$ eggs of about $50$ g each. The mixed batter should weigh $250 + 200 + 200 + 200 = 850$ g, plus the bowl. Bakers use this to check they have not forgotten an ingredient.
After baking, the cake weighs a little less, perhaps $780$ g. The oven's heat turned some of the water in the batter into steam, and the steam escaped from the open pan. No matter was destroyed; about $70$ g left as water vapor that you can smell as the kitchen fills with the scent of cake.
Professional bakers weigh their batter before and after to check how much moisture their ovens drive off, so every batch comes out the same.
At a recycling center, trucks are weighed on a giant scale as they drive in full and again as they drive out empty. The difference is the mass of material they delivered, say $4{,}000$ kg of glass bottles.
The glass is crushed, melted and made into new bottles. However many times its shape changes, the mass of glass stays the same: $4{,}000$ kg of old bottles become about $4{,}000$ kg of new glass, minus any dirt and labels that were cleaned off.
Recycling works because matter is conserved. Nothing we throw away truly disappears. It goes somewhere, and weighing it is how cities keep track of where.
The most common idea is that dissolved sugar or salt no longer counts, because it cannot be seen. It still counts, every gram. Weigh the glass and see.
A second mistake is thinking a change of form changes mass: that water is lighter than ice, or steam lighter than water. The form changes; the mass does not.
A third is trusting an open container. When bubbles form and the cup is open, some mass floats away as gas, and the scale drops. That is not matter being destroyed. Seal the container and the lost mass is there after all.
Weigh the water.
$250\ \text{g}$
Glass and water together.
Weigh the salt.
$15\ \text{g}$
On the same scale.
Add them for the total before.
$250 + 15 = 265\ \text{g}$
Everything that will take part.
Stir until the salt vanishes.
$\text{the salt dissolves}$
It breaks into pieces too small to see.
Weigh it again after.
$265\ \text{g}$
The same as before: nothing was lost.
Weigh the jar of ice.
$420\ \text{g}$
Lid on tight.
Name the change.
$\text{solid} \to \text{liquid}$
Melting in the warm room.
Check nothing can escape.
$\text{sealed}$
No water or air in or out.
Weigh after melting.
$420\ \text{g}$
Exactly the same.
Notice what did change.
$\text{shape and form, not mass}$
The water fills the bottom of the jar now.
State the rule.
$\text{mass is conserved in melting}$
The same matter in a new form.
Weigh cup and tablet before.
$300 + 5 = 305\ \text{g}$
Everything that will take part.
Let it fizz in an open cup.
$\text{after: } 301\ \text{g}$
The reading drops.
Find the difference.
$305 - 301 = 4\ \text{g}$
Four grams are no longer on the scale.
Repeat in a sealed bottle.
$\text{before } 305, \ \text{after } 305\ \text{g}$
Nothing missing this time.
Explain the difference.
$\text{the gas stayed in the bottle}$
In the open cup it escaped.
Name the missing grams.
$4\ \text{g of gas}$
Matter that left, not matter destroyed.
State the rule.
$\text{sealed: mass conserved}$
Catch everything and the total never changes.
Name the change.
$\text{liquid} \to \text{solid}$
Freezing.
Check nothing escapes.
Apply the rule.
A glass holds $390$ g of water. You stir in $9$ g of sugar until you cannot see it any more. What is the mass of the sweet water?
Complete the worked solution: a sealed jar holds $300$ g of water. $19$ g of salt is dissolved in it, and then the jar is frozen solid. Find the mass after dissolving and after freezing.
Add the salt.
$300 + 19 =$ a g
Every gram of salt is still in the water.
Freeze the salty water.
$\text{unchanged: }$ b g
Freezing changes the form, not the mass.
Check against the start.
$\text{water} + \text{salt} =$ c g
The total at the end is the total that went in.
Match each change to what happens to the total mass.
| the total mass stays the same | the reading drops, because gas escaped | |
|---|---|---|
| an ice cube melting in a closed jar | ||
| salt dissolving in a glass of water | ||
| a tablet fizzing in an open cup | ||
| water freezing in a sealed bottle |
A sealed plastic bag holds $275$ g of ice. It is left in the sun until all the ice has melted. What is the mass of the water in the bag?
Fill in the mass after each change. A jar holds $100$ g of water and $3$ g of salt is stirred in. A sealed bottle holds $90$ g of water that freezes. A bowl holds $100$ g of water and $90$ g of sand that are mixed.
| mass after (g) | |
|---|---|
| salt stirred into water | |
| water frozen in a sealed bottle | |
| sand mixed into water |
A student mixes $166$ g of water, $34$ g of lemon juice and $10$ g of sugar in a jug. What is the mass of the lemonade?
Answer: unit: g / kg
A cup of water has a mass of $350$ g, and a fizzing tablet $5$ g. The tablet is dropped in and fizzes, with no lid on the cup. Afterward the cup and everything in it has a mass of $352$ g. How many grams of gas escaped into the air?
Answer: unit: g / kg
A baker weighs a lump of bread dough: 800 g. It rises to twice its size under a cloth. What should the baker expect it to weigh now?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Fill in the mass after each change. A jar holds $200$ g of water and $6$ g of salt is stirred in. A sealed bottle holds $110$ g of water that freezes. A bowl holds $200$ g of water and $110$ g of sand that are mixed.
| mass after (g) | |
|---|---|
| salt stirred into water | |
| water frozen in a sealed bottle | |
| sand mixed into water |
You can show that mass is conserved. Tell someone why sweet tea weighs as much as the water plus the sugar.
18. Your turn: 200 g of water freezes in a sealed bottle. What is the mass of the ice?, step 2
$\text{sealed bottle}$
No water in or out.
18. Your turn: 200 g of water freezes in a sealed bottle. What is the mass of the ice?, step 3
$200\ \text{g}$
Freezing keeps the mass the same.