Where does the salt go when I stir it into water, and does the cup weigh less?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Use the words solute, solvent and solution correctly
Test how warm water and stirring change how fast sugar dissolves
Explain why mass stays the same when salt dissolves
Sugar disappears when you stir it into tea. Salt disappears in soup. Where do they go? Does the cup weigh less afterwards?
Draw it first
Draw sugar mixing into water
1 Draw a cup of water with open circles. Put the sugar, dark dots, in a lump at the bottom.
2 Draw it again a little later. The water particles pull some dark dots away from the lump.
3 Draw it later still. The dark dots are mixed through the water.
Water particles pull on the sugar particles and drag them away from the lump. The sugar particles spread out among the water particles. They are too small to see, so the water stays clear.
When a solid mixes into a liquid and can no longer be seen, we say it dissolves. The solid that dissolves is the solute. The liquid that does the dissolving is the solvent. Together they make a solution.
Salt in water. The water is the solvent. The salt is the solute. The mixture is the solution.
In salty water, the salt is the solute and the water is the solvent. The salty water is the solution. A solution is clear. You can see through it, though it may have a colour.
Not everything dissolves in water. Sand sinks to the bottom and stays as grains you can see, however long you stir. Sugar is soluble in water, which means it can dissolve. Sand is insoluble, which means it cannot.
Sugar spreads through the water as particles too small to see. Sand stays as grains at the bottom.
Dissolving is not melting. Melting is a solid turning into a liquid when it is heated. Dissolving is a solid mixing into a liquid, and it needs no heat.
Nothing is lost when a solid dissolves. The sugar particles are still in the cup. So the mass of the solution is the mass of the water plus the mass of the solute. Mass is how much matter something has, and we measure it in grams.
See it three ways
With things
Put a cup of water and a heap of salt on a kitchen scale. Read the scale. Then stir the salt into the water, and read the scale again.
As a picture
In words
The salt is still in the cup, spread through the water. Nothing has been lost, so the scale shows the same mass.
In symbols
200 g + 10 g = 210 g
Three things make a solid dissolve faster. Warm the water, so the particles move faster. Stir, so fresh water reaches the solid. Use smaller pieces, such as grains instead of a lump, so more of the solid touches the water.
Water can only hold so much. Keep adding salt, and at last some stays on the bottom.
Try it
Race the sugar
You need: 3 clear cups, sugar, a spoon, cold water, warm water from a grown up, a clock, or a way to count seconds.
Be safe: A grown up must pour the warm water. Use warm water, never boiling. Do not taste anything.
Pour the same amount of water into three cups. Cups A and B get cold water. Cup C gets warm water poured by a grown up.
Add one level spoon of sugar to each cup at the same moment. Stir cup B only, gently and steadily.
Count the seconds until no sugar can be seen in each cup. Cup A can take several minutes. Write the three times down.
What you will see: The sugar in cups B and C disappears much sooner than in cup A. Your times will differ, but the pattern will be the same. Warmth and stirring both speed up dissolving.
If you cannot try it, this is what happens.
A wrong idea many people have
Dissolving is the same as melting.
Why it feels right: Both turn a solid into something runny, and warmth helps both.
The test that breaks it: Salt melts at 801 °C, far hotter than any cup of water. Yet salt dissolves in cold water. Leave salty water in the sun. The water goes into the air, and the salt is left behind.
What is true: Melting is a solid turning into a liquid when it is heated. Dissolving is a solid mixing into a liquid, with its particles spread among the liquid particles.
Key words
dissolves
mixes into a liquid so completely that it cannot be seen.
solute
the solid that dissolves.
solvent
the liquid that does the dissolving.
solution
the clear mixture of a solute dissolved in a solvent.
soluble
able to dissolve in a liquid.
Back to your guess
The answer is A: The salt is still there, spread through the water, so the total weighs the same.
The salt particles are still in the cup, mixed among the water particles. Nothing is lost, so the mass is the same.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Salt is stirred into water until it has dissolved. Which names are right?
See the answer
The salt is the solute. The water is the solvent. The salty water is the solution.
The solute is the solid that dissolves. The solvent is the liquid that does the dissolving. Together they make the solution.
If you chose A: The solute is the solid that dissolves, however little of it there is. The liquid that does the dissolving is the solvent. Go to S3.5.7.
If you chose C: The water alone is the solvent. The solution is the whole mixture of salt and water together. Go to S3.5.7.
A cup of water has a mass of 200 g. You stir in 10 g of salt until it dissolves. What mass does the scale show now?
See the answer
210 g
Nothing is lost when salt dissolves. Add the two masses. 200 + 10 = 210.
If you chose B: The salt is not gone. Its particles are still in the cup, so their mass counts. Go to S3.5.7.
If you chose C: Dissolving does not take mass away. The salt is still there, spread through the water. Go to S3.5.7.
A cup of salty water has a mass of 250 g. It holds 20 g of dissolved salt. What was the mass of the cup of water before the salt went in?
See the answer
230 g
The mass of the solution is the water plus the salt. So take the salt away from the whole mass. 250 − 20 = 230.
If you chose A: The 20 g of salt is part of the total. Take it away to find the water alone. Go to S3.5.7.
If you chose C: You added the salt. The total already holds the salt, so take it away. Go to S3.5.7.
The chart shows the seconds the sugar took to disappear. Cup A was cold and not stirred. Cup B was cold and stirred. How many seconds sooner did the sugar disappear in cup B?
See the answer
80 seconds sooner
Stirring brings fresh water to the sugar. Take the shorter time from the longer time. 120 − 40 = 80.
If you chose A: That is the time for cup B on its own. The question asks how much sooner than cup A, so take it away from the time for A. Go to S3.5.7.
If you chose C: That is the time for cup A on its own. Compare the two cups: take the shorter time from the longer. Go to S3.5.7.
Sand is stirred into water. It sinks to the bottom and stays as grains you can see, however long you stir. Is sand soluble or insoluble in water?
See the answer
Insoluble. The grains stay as grains and sink.
The water particles cannot pull sand apart. The grains stay as they are, and sink.
If you chose A: Not every solid dissolves. A soluble solid disappears and leaves a clear solution. Sand does not. Go to S3.5.7.
If you chose B: Stirring only moves the grains about. They settle again, so they have not dissolved. Go to S3.5.7.
From an earlier chapter
Why does sugar dissolve faster in warm water than in cold water?
See the answer
Warm particles move faster, so they pull sugar apart and spread it sooner.
Faster particles hit the sugar harder and more often, and they carry it away sooner.
If you chose B: Warm and cold water in a cup hold about the same number of particles. Warm particles move faster. Go to S3.5.4.
If you chose C: Sugar is not melting. It is dissolving, and warmth only makes the particles move faster. Go to S3.5.2.
Keep it
Copy this into your exercise book.
A solute dissolves in a solvent to make a solution. Its particles spread among the solvent particles. Nothing is lost, so the mass stays the same. Warm water and stirring dissolve faster.
Draw: Draw a cup of water with sugar in a lump. Draw it again with the dark dots spread through the water.
