Adding More Sugar Eventually Stops Making It Sweeter
Learn why water dissolves so much, how to tell a saturated solution from an unsaturated or supersaturated one, how to calculate solubility per hundred grams of water, and why hot water dissolves more sugar but less gas.
Why does sugar stop dissolving however hard you stir?
Because at a given temperature, a fixed quantity of water can hold only a fixed quantity of sugar — and once that limit is reached, stirring achieves nothing.
The water has not run out of space in any simple sense. It has reached the point where sugar is leaving the solution and returning to the crystals at exactly the same rate as it is dissolving, so nothing appears to change. The solution is saturated.
Warm the same water and it will take more, which tells you the limit is not fixed by the water alone. This page covers the second part of the ICSE Class 8 Chemistry chapter on water: solutions, the three kinds, how to measure solubility, and what shifts it.
The water has not run out of space in any simple sense. It has reached the point where sugar is leaving the solution and returning to the crystals at exactly the same rate as it is dissolving, so nothing appears to change. The solution is saturated.
Warm the same water and it will take more, which tells you the limit is not fixed by the water alone. This page covers the second part of the ICSE Class 8 Chemistry chapter on water: solutions, the three kinds, how to measure solubility, and what shifts it.
Why is water called the universal solvent?
Because it dissolves more substances than any other liquid — not because it dissolves everything.
First, the three words that describe any solution:
- Solute — the substance that dissolves, usually present in the smaller amount. The sugar, the salt.
- Solvent — the substance that does the dissolving, usually present in the larger amount. The water.
- Solution — the homogeneous mixture that results, uniform throughout with no visible boundary.
So in salt water, salt is the solute, water is the solvent, and the clear liquid is the solution. A solution is a mixture, not a compound: the proportions can be varied at will and the solute can be recovered by evaporation.
What water dissolves. Common salt, sugar, copper sulphate, sodium hydroxide, most acids and alkalis, and gases such as oxygen, carbon dioxide and ammonia.
What it does not. Oil, ghee, wax, kerosene, sand, chalk, sulphur. The name universal solvent is a comparison, not a claim of completeness — and a question asking whether water dissolves everything should be answered no.
Why this property matters so much. It explains almost everything else about water in this chapter. Natural water is never pure because it dissolves the rocks it flows over and the gases it meets. Blood and sap can transport nutrients because those nutrients dissolve. Fish can breathe because oxygen dissolves. And the hardness of water in the fourth part is entirely a matter of what the water has dissolved on its way to you.
A boundary case worth knowing. Other liquids act as solvents for what water cannot take. Petrol and kerosene dissolve grease, and alcohol and acetone dissolve varnish and nail polish. This is why a stain that water will not shift often yields to a different solvent — the rule is that like dissolves like, and water is only universal within its own kind.
First, the three words that describe any solution:
- Solute — the substance that dissolves, usually present in the smaller amount. The sugar, the salt.
- Solvent — the substance that does the dissolving, usually present in the larger amount. The water.
- Solution — the homogeneous mixture that results, uniform throughout with no visible boundary.
So in salt water, salt is the solute, water is the solvent, and the clear liquid is the solution. A solution is a mixture, not a compound: the proportions can be varied at will and the solute can be recovered by evaporation.
What water dissolves. Common salt, sugar, copper sulphate, sodium hydroxide, most acids and alkalis, and gases such as oxygen, carbon dioxide and ammonia.
What it does not. Oil, ghee, wax, kerosene, sand, chalk, sulphur. The name universal solvent is a comparison, not a claim of completeness — and a question asking whether water dissolves everything should be answered no.
Why this property matters so much. It explains almost everything else about water in this chapter. Natural water is never pure because it dissolves the rocks it flows over and the gases it meets. Blood and sap can transport nutrients because those nutrients dissolve. Fish can breathe because oxygen dissolves. And the hardness of water in the fourth part is entirely a matter of what the water has dissolved on its way to you.
A boundary case worth knowing. Other liquids act as solvents for what water cannot take. Petrol and kerosene dissolve grease, and alcohol and acetone dissolve varnish and nail polish. This is why a stain that water will not shift often yields to a different solvent — the rule is that like dissolves like, and water is only universal within its own kind.
How do you tell a saturated solution from a supersaturated one?
By whether it can still take more solute at that temperature, and by what happens when you disturb it.
An unsaturated solution can dissolve more solute at the same temperature. Add a pinch and it disappears.
A saturated solution can dissolve no more at that temperature. Add a pinch and it settles at the bottom, undissolved, however long you stir.
A supersaturated solution contains more dissolved solute than a saturated one at that temperature. It is made by saturating the solution while hot and then cooling it slowly and undisturbed, so the excess stays dissolved even though it should have crystallised out.
How to test which one you have. Add a small further quantity of the solute and watch.
- It dissolves — the solution was unsaturated.
- It settles undissolved — the solution was saturated.
- It triggers sudden crystallisation of far more solid than you added — the solution was supersaturated.
That third result is the striking one. A supersaturated solution is unstable, and the single added crystal acts as a seed the excess solute can build on. A scratch on the inside of the beaker or a sharp jolt does the same.
Why a saturated solution is not full in any ordinary sense. At saturation, solute is still dissolving all the time — but solute is also crystallising back out at exactly the same rate. The two opposite processes are in balance, so nothing seems to happen. This is why a saturated solution left with excess crystals slowly grows larger, better-formed crystals while the total mass dissolved stays the same.
And saturation is always stated with a temperature. A saturated solution of potassium nitrate means nothing on its own; *saturated at * means something precise. Warm the same solution and it becomes unsaturated without anything being added to it, which is the next section's business.
An unsaturated solution can dissolve more solute at the same temperature. Add a pinch and it disappears.
A saturated solution can dissolve no more at that temperature. Add a pinch and it settles at the bottom, undissolved, however long you stir.
A supersaturated solution contains more dissolved solute than a saturated one at that temperature. It is made by saturating the solution while hot and then cooling it slowly and undisturbed, so the excess stays dissolved even though it should have crystallised out.
How to test which one you have. Add a small further quantity of the solute and watch.
- It dissolves — the solution was unsaturated.
- It settles undissolved — the solution was saturated.
- It triggers sudden crystallisation of far more solid than you added — the solution was supersaturated.
That third result is the striking one. A supersaturated solution is unstable, and the single added crystal acts as a seed the excess solute can build on. A scratch on the inside of the beaker or a sharp jolt does the same.
Why a saturated solution is not full in any ordinary sense. At saturation, solute is still dissolving all the time — but solute is also crystallising back out at exactly the same rate. The two opposite processes are in balance, so nothing seems to happen. This is why a saturated solution left with excess crystals slowly grows larger, better-formed crystals while the total mass dissolved stays the same.
And saturation is always stated with a temperature. A saturated solution of potassium nitrate means nothing on its own; *saturated at * means something precise. Warm the same solution and it becomes unsaturated without anything being added to it, which is the next section's business.
Formula
How do you calculate the solubility of a solute?
Solubility is the mass of solute in grams that saturates ** of solvent at a stated temperature**.
The fixed reference of is what makes different solutes comparable, and the temperature must always be quoted.
Worked example 1. of a salt saturates of water at . Its solubility is
Worked example 2. of a solute saturates of water:
Worked example 3 — working backwards from a solution's total mass. A solution saturated at , where the solubility is , weighs in total. How much solute does it contain?
At saturation, of water holds of solute, so the solution weighs . Scaling up:
Checking: , and . Both consistent.
Worked example 4 — crystals from cooling. A solute has solubility at and at . If of water is saturated at and then cooled to , the mass of crystals separating out is
This is exactly the process of crystallisation from the chapter on separation, now with a number attached to it.
The error to avoid. Solubility is per of solvent, not per of solution. In example 3 the answer would have been — wrong, because the water alone weighs , not . Read the question for which mass it has given you.
The fixed reference of is what makes different solutes comparable, and the temperature must always be quoted.
Worked example 1. of a salt saturates of water at . Its solubility is
Worked example 2. of a solute saturates of water:
Worked example 3 — working backwards from a solution's total mass. A solution saturated at , where the solubility is , weighs in total. How much solute does it contain?
At saturation, of water holds of solute, so the solution weighs . Scaling up:
Checking: , and . Both consistent.
Worked example 4 — crystals from cooling. A solute has solubility at and at . If of water is saturated at and then cooled to , the mass of crystals separating out is
This is exactly the process of crystallisation from the chapter on separation, now with a number attached to it.
The error to avoid. Solubility is per of solvent, not per of solution. In example 3 the answer would have been — wrong, because the water alone weighs , not . Read the question for which mass it has given you.
How do temperature and pressure change solubility?
Temperature affects solids and gases in opposite directions. Pressure matters greatly for gases and almost not at all for solids and liquids.
Temperature and solids — solubility usually increases. More sugar dissolves in hot water than in cold, and this is why crystals separate out when a hot saturated solution is cooled. Heating gives the solvent molecules more energy to break the solute's crystal apart.
Sugar, potassium nitrate and copper sulphate all behave this way. A few solids are exceptions and become less soluble when heated — calcium sulphate among them, which is why boiler scale forms on hot surfaces.
Temperature and gases — solubility decreases. Heating a solution drives dissolved gas out of it. Heat water in a pan and bubbles appear on the base well before boiling; those are dissolved air escaping, not steam.
The molecular reason is straightforward: a gas molecule already has enough freedom to leave, and heating gives it more. So warming helps a solid in and drives a gas out.
Pressure and gases — solubility increases. Raising the pressure above a liquid forces more gas into it. This is exactly how aerated drinks are made: carbon dioxide is dissolved under high pressure and sealed in. Open the bottle, the pressure falls to atmospheric, the gas can no longer stay dissolved, and it rushes out as fizz.
Pressure and solids — almost no effect, because solids and liquids are practically incompressible, so pressing on them changes nothing about the space available.
Putting the two together. A warm fizzy drink goes flat faster than a cold one, and both go flat once opened. Higher temperature and lower pressure each reduce gas solubility, and an open bottle in the sun suffers both at once.
Why this matters beyond the laboratory. Aquatic life depends on dissolved oxygen in water. Warm water holds less of it, so a river warmed by an industrial discharge can suffocate its fish without a single toxic substance being added — which is a genuine form of water pollution and the reason thermal discharge is regulated.
Temperature and solids — solubility usually increases. More sugar dissolves in hot water than in cold, and this is why crystals separate out when a hot saturated solution is cooled. Heating gives the solvent molecules more energy to break the solute's crystal apart.
Sugar, potassium nitrate and copper sulphate all behave this way. A few solids are exceptions and become less soluble when heated — calcium sulphate among them, which is why boiler scale forms on hot surfaces.
Temperature and gases — solubility decreases. Heating a solution drives dissolved gas out of it. Heat water in a pan and bubbles appear on the base well before boiling; those are dissolved air escaping, not steam.
The molecular reason is straightforward: a gas molecule already has enough freedom to leave, and heating gives it more. So warming helps a solid in and drives a gas out.
Pressure and gases — solubility increases. Raising the pressure above a liquid forces more gas into it. This is exactly how aerated drinks are made: carbon dioxide is dissolved under high pressure and sealed in. Open the bottle, the pressure falls to atmospheric, the gas can no longer stay dissolved, and it rushes out as fizz.
Pressure and solids — almost no effect, because solids and liquids are practically incompressible, so pressing on them changes nothing about the space available.
Putting the two together. A warm fizzy drink goes flat faster than a cold one, and both go flat once opened. Higher temperature and lower pressure each reduce gas solubility, and an open bottle in the sun suffers both at once.
Why this matters beyond the laboratory. Aquatic life depends on dissolved oxygen in water. Warm water holds less of it, so a river warmed by an industrial discharge can suffocate its fish without a single toxic substance being added — which is a genuine form of water pollution and the reason thermal discharge is regulated.
Exam tip
Exam tip: state the temperature with every solubility
A solubility figure is meaningless without a temperature. Write * per of water at , and the same for saturated* — a solution is saturated at a stated temperature.
Use ** of solvent**, never of solution. When a question gives the mass of the whole solution, subtract to find the water first.
Show the formula with the substitution: . Method marks live in that line.
Remember the opposite directions: solids more soluble when hot, gases less soluble when hot. Writing a single rule for both is the commonest error here.
Say pressure affects gases strongly and solids negligibly, and give the aerated-drink example both ways — dissolved under pressure, released when opened.
For the three kinds of solution, give the test: added solute dissolves (unsaturated), settles undissolved (saturated), or triggers sudden crystallisation (supersaturated).
Say a supersaturated solution is unstable and needs to be made by cooling a hot saturated solution slowly and undisturbed.
And answer does water dissolve everything? with no — name oil, wax, sand and chalk, and say universal solvent is a comparison.
Use ** of solvent**, never of solution. When a question gives the mass of the whole solution, subtract to find the water first.
Show the formula with the substitution: . Method marks live in that line.
Remember the opposite directions: solids more soluble when hot, gases less soluble when hot. Writing a single rule for both is the commonest error here.
Say pressure affects gases strongly and solids negligibly, and give the aerated-drink example both ways — dissolved under pressure, released when opened.
For the three kinds of solution, give the test: added solute dissolves (unsaturated), settles undissolved (saturated), or triggers sudden crystallisation (supersaturated).
Say a supersaturated solution is unstable and needs to be made by cooling a hot saturated solution slowly and undisturbed.
And answer does water dissolve everything? with no — name oil, wax, sand and chalk, and say universal solvent is a comparison.
Did you know
Why do bubbles form on a pan long before the water boils?
Put a pan of tap water on a low flame and small bubbles cling to the base within a minute or two, while a thermometer still reads well below .
Those bubbles are not steam. They are dissolved air — chiefly nitrogen and oxygen — coming out of solution because gas solubility falls as temperature rises. The water is warm enough to drive the air out and nowhere near hot enough to boil.
You can prove which it is by watching where the bubbles go. Early bubbles rise and reach the surface intact, because they are air. Bubbles of genuine steam formed before boiling point collapse on the way up, as they pass into cooler water and condense — which is the faint rumbling sound a kettle makes shortly before it boils.
It also explains why boiled and cooled water tastes flat. The dissolved air that gave it a slight liveliness has been driven off, and it takes time standing in contact with the atmosphere to dissolve back in.
Those bubbles are not steam. They are dissolved air — chiefly nitrogen and oxygen — coming out of solution because gas solubility falls as temperature rises. The water is warm enough to drive the air out and nowhere near hot enough to boil.
You can prove which it is by watching where the bubbles go. Early bubbles rise and reach the surface intact, because they are air. Bubbles of genuine steam formed before boiling point collapse on the way up, as they pass into cooler water and condense — which is the faint rumbling sound a kettle makes shortly before it boils.
It also explains why boiled and cooled water tastes flat. The dissolved air that gave it a slight liveliness has been driven off, and it takes time standing in contact with the atmosphere to dissolve back in.
Key takeaways
Solutions and solubility: quick revision
- Solute dissolves, solvent does the dissolving, and the solution is the homogeneous mixture — so the solute can be recovered by evaporation.
- Water is the universal solvent because it dissolves more substances than any other liquid — but not everything: oil, wax, sand, chalk and sulphur resist it.
- Unsaturated — can dissolve more. Saturated — can dissolve no more at that temperature. Supersaturated — holds more than saturation, made by cooling a hot saturated solution slowly, and is unstable.
- Test by adding a little solute: it dissolves (unsaturated), settles undissolved (saturated), or triggers sudden crystallisation (supersaturated).
- At saturation, dissolving and crystallising are in balance — nothing appears to change, but both continue.
- Solubility , in grams per of solvent at a stated temperature.
- in of water gives ; in gives .
- From a total mass: at solubility , a solution weighs per of water, so of solution holds of solute and of water.
- Crystals on cooling: solubility at falling to at gives of crystals from of water.
- Solubility is per of solvent, never of solution.
- Temperature: solids usually more soluble when hot (calcium sulphate is an exception); gases less soluble when hot.
- Pressure: raises the solubility of gases strongly (aerated drinks) and affects solids negligibly.
- Warm water holds less dissolved oxygen, so thermal discharge alone can harm aquatic life.
Work through a set of solubility calculations, including one that gives you the mass of the whole solution — spotting which mass you have been handed is what these questions actually test.
- Water is the universal solvent because it dissolves more substances than any other liquid — but not everything: oil, wax, sand, chalk and sulphur resist it.
- Unsaturated — can dissolve more. Saturated — can dissolve no more at that temperature. Supersaturated — holds more than saturation, made by cooling a hot saturated solution slowly, and is unstable.
- Test by adding a little solute: it dissolves (unsaturated), settles undissolved (saturated), or triggers sudden crystallisation (supersaturated).
- At saturation, dissolving and crystallising are in balance — nothing appears to change, but both continue.
- Solubility , in grams per of solvent at a stated temperature.
- in of water gives ; in gives .
- From a total mass: at solubility , a solution weighs per of water, so of solution holds of solute and of water.
- Crystals on cooling: solubility at falling to at gives of crystals from of water.
- Solubility is per of solvent, never of solution.
- Temperature: solids usually more soluble when hot (calcium sulphate is an exception); gases less soluble when hot.
- Pressure: raises the solubility of gases strongly (aerated drinks) and affects solids negligibly.
- Warm water holds less dissolved oxygen, so thermal discharge alone can harm aquatic life.
Work through a set of solubility calculations, including one that gives you the mass of the whole solution — spotting which mass you have been handed is what these questions actually test.