Drop a sugar cube into water and stir, and the sugar gets smaller and smaller until you cannot see it, yet the water still tastes sweet.
The sugar has not disappeared; it has split into tiny particles spread evenly through the water. This is called dissolving: the sugar that dissolves is the solute, the water that dissolves it is the solvent, and the sugar water they make together is the solution.
But there is a limit to how much sugar a given amount of water can dissolve, and if you add more than that, undissolved sugar stays at the bottom.
In the first picture, add more and more sugar to 40 mL of water; in the second, compare how strong a solution is depending on how much sugar is dissolved.
What does dissolving mean?
When sugar in water can no longer be seen, it is easy to say the sugar has disappeared. But the water still tastes sweet, and if you evaporate the water, the sugar is left behind again. We name the dissolving substance the solute, the liquid that dissolves it the solvent, and the mixture the solution so that we can tell "can no longer be seen" apart from "is gone". Also, in the same amount of water, a different amount of sugar makes a different strength, and if you add too much, it stops dissolving and stays at the bottom.
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Sugar dissolving in 40 mL of water: it spreads evenly between the water
The first picture is a beaker holding 40 mL of water at 20 °C. Moving the sugar slider from 0 g to 100 g in 5 g steps makes the water look darker and adds more yellow dots spread through it. Once the sugar goes past 80 g, sugar that cannot dissolve piles up on the bottom of the beaker and the title changes to say it will not dissolve any more. The second picture shows three cups side by side holding a weak, a medium, and a strong solution, and changing the amount of sugar dissolved in 100 mL of water darkens the outline of the cup that matches that strength.
40 g
🧊 What is dissolving?
①Dissolving: a substance mixing evenly into a liquid and disappearing into it
②The solute is the substance that dissolves, the solvent is the liquid that dissolves it, and the solution is the liquid with the solute dissolved in it
③Sugar water = sugar (solute) + water (solvent)
④There is a limit to how much a given amount of water can dissolve, and a solution that cannot dissolve any more is called a saturated solution
⑤The 40 mL of water in the picture can dissolve up to about 80 g of sugar at 20 °C
How strong is a solution?
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How the strength of a solution changes with the amount of sugar dissolved
Medium solution
🔬 Strong and weak solutions
①The more sugar dissolved in the same amount of water, the stronger the solution
②Dissolving less sugar makes a weak solution
③The stronger the sugar water, the sweeter it is, and the heavier it is for the same volume
④An egg can float in strong sugar water and sink in weak sugar water, so you can use it to compare strengths
Ways to dissolve faster
When sugar disappears from view, it is easy to think it is gone from the water. Dissolving means the sugar has split into tiny particles spread between the water, so the water tastes sweet and the sugar comes back when the water evaporates. Swapping the names solute and solvent turns the explanation of what dissolves and what does the dissolving upside down. And sugar left on the bottom means the water has already dissolved as much as it can, which is different from a substance that does not dissolve at all. Adding more water or warming it makes the leftover sugar dissolve too.
Dissolving faster
Stir + crush into small pieces + warm the water
All three help the solute meet the solvent more
💡 Why does it dissolve faster?
①Stirring keeps changing the water around the sugar, so it meets fresh water
②Crushing it into small pieces gives more surface touching the water
③Warming the water makes the particles move more actively, so it dissolves faster
④That is why granulated sugar dissolves faster than a sugar cube, and faster in warm water than in cold
Summary
The essentials of dissolving
Solution = solute (substance that dissolves) + solvent (liquid that dissolves it)
Dissolving is the solute spreading evenly through the solvent
🎯 Key points
①The solute is the substance that dissolves (sugar, salt, and so on)
②The solvent is the liquid that dissolves it (water, and so on)
③A solution is a liquid with the solute evenly mixed into the solvent
④A saturated solution is one that cannot dissolve any more
⑤Stirring, crushing into small pieces, and warming the water make things dissolve faster
Examples and Unit Test
Example 1
In sugar water, what is the substance that dissolves (solute) and what is the liquid that dissolves it (solvent)?
1
Dissolving is a substance mixing evenly into a liquid.
2
In sugar water, the sugar that dissolves is the solute, and the water that dissolves it is the solvent.
▸ Solute: sugar / solvent: water
In salt water, the salt is the solute and the water is the solvent.
Example 2
What are three ways to dissolve a sugar cube in water faster?
1
Stirring changes the water around the sugar, so it meets fresh water more often and dissolves faster.
2
Crushing it gives more surface touching the water, and warming the water makes the particles more active, so it dissolves faster.
▸ Stirring, crushing into small pieces, warming the water
All three change how fast it dissolves, but only warming the water raises how much sugar can dissolve.
Unit-test style
Which statement about dissolving and solutions is correct?
①The solute is the liquid that dissolves other substances
②Warming the water makes sugar dissolve faster
③Crushing the solute into small pieces makes it dissolve more slowly
④A strong solution has less solute than a weak one
⑤More solute can still dissolve in a saturated solution
▸ ② Warming the water makes sugar dissolve faster
1
The higher the water temperature, the more actively the particles move and the faster it dissolves.
2
The solute is the substance that dissolves (①), crushing it makes it dissolve faster (③), a strong solution has more solute (④), and no more dissolves in a saturated solution (⑤).