Warm Water Dissolves More Sugar but Less Gas
Learn to name the solute and solvent in any solution, tell a saturated solution from an unsaturated one, see how temperature changes the solubility of solids, and why gases behave the opposite way.
Why does warming water dissolve more sugar but less gas?
Because solids and gases respond to heat in opposite ways. Heating gives sugar particles the energy to separate and spread among the water particles, so more dissolves — but it gives dissolved gas particles the energy to escape, so less stays in.
That single contrast is the heart of this chapter. This page covers everything in the CBSE Class 8 Science chapter's first part: solutes and solvents, saturated solutions, temperature and the solubility of solids, and the solubility of gases.
That single contrast is the heart of this chapter. This page covers everything in the CBSE Class 8 Science chapter's first part: solutes and solvents, saturated solutions, temperature and the solubility of solids, and the solubility of gases.
How do you identify the solute and the solvent?
A solution is a uniform mixture of two or more substances. The substance that dissolves is the solute, and the one that does the dissolving is the solvent.
The solvent is normally the component present in the larger amount.
Identifying them:
- Sugar in water — sugar is the solute, water the solvent
- Salt in water — salt the solute, water the solvent
- Lemon juice in water (sharbat) — lemon juice and sugar the solutes, water the solvent
- Carbon dioxide in a cold drink — the gas is the solute, water the solvent
- Air — nitrogen and the other gases are solutes, and the gas present in the largest amount acts as the solvent
- Brass — zinc is the solute, copper the solvent
Water is called the universal solvent because it dissolves such a large number of substances, which is why it appears as the solvent in nearly every example.
How a solution forms: the solute particles separate from one another and spread into the spaces between the solvent particles. That is why stirring sugar into a full glass of water raises the level far less than the sugar's own volume.
The test students find useful is the filter paper test. A true solution's particles are so small that they pass straight through filter paper and never settle on standing — which is what separates a solution from a suspension such as chalk in water.
The solvent is normally the component present in the larger amount.
Identifying them:
- Sugar in water — sugar is the solute, water the solvent
- Salt in water — salt the solute, water the solvent
- Lemon juice in water (sharbat) — lemon juice and sugar the solutes, water the solvent
- Carbon dioxide in a cold drink — the gas is the solute, water the solvent
- Air — nitrogen and the other gases are solutes, and the gas present in the largest amount acts as the solvent
- Brass — zinc is the solute, copper the solvent
Water is called the universal solvent because it dissolves such a large number of substances, which is why it appears as the solvent in nearly every example.
How a solution forms: the solute particles separate from one another and spread into the spaces between the solvent particles. That is why stirring sugar into a full glass of water raises the level far less than the sugar's own volume.
The test students find useful is the filter paper test. A true solution's particles are so small that they pass straight through filter paper and never settle on standing — which is what separates a solution from a suspension such as chalk in water.
What is the difference between a saturated and an unsaturated solution?
A saturated solution is one that has dissolved as much solute as it can at that temperature — add more and it settles undissolved at the bottom. An unsaturated solution can still dissolve more.
The experiment. To find how much solute a fixed amount of solvent can hold:
1. Take a measured amount of water, say 50 mL, in a beaker at room temperature.
2. Add one level spoon of salt and stir until it dissolves completely.
3. Add another spoon, stir, and continue.
4. Keep count of the spoons added.
5. At some point a spoonful refuses to dissolve however long you stir — it settles at the bottom. The solution is now saturated.
6. Record the number of spoons that did dissolve.
Repeating with sugar instead of salt, in the same 50 mL, shows that a different number of spoons dissolves — so different solutes have different solubilities in the same solvent.
The amount that dissolves to make a saturated solution at a given temperature is called that substance's solubility.
Two conditions must be stated when reporting the result, and leaving them out makes the figure meaningless: the amount of solvent and the temperature. "Six spoons of salt dissolve" says nothing without "in 50 mL of water at room temperature", because warming the water changes the answer — which is exactly what the next section examines.
The experiment. To find how much solute a fixed amount of solvent can hold:
1. Take a measured amount of water, say 50 mL, in a beaker at room temperature.
2. Add one level spoon of salt and stir until it dissolves completely.
3. Add another spoon, stir, and continue.
4. Keep count of the spoons added.
5. At some point a spoonful refuses to dissolve however long you stir — it settles at the bottom. The solution is now saturated.
6. Record the number of spoons that did dissolve.
Repeating with sugar instead of salt, in the same 50 mL, shows that a different number of spoons dissolves — so different solutes have different solubilities in the same solvent.
The amount that dissolves to make a saturated solution at a given temperature is called that substance's solubility.
Two conditions must be stated when reporting the result, and leaving them out makes the figure meaningless: the amount of solvent and the temperature. "Six spoons of salt dissolve" says nothing without "in 50 mL of water at room temperature", because warming the water changes the answer — which is exactly what the next section examines.
How does temperature change the solubility of a solid?
For most solids, solubility increases as temperature rises, and decreases as it falls.
The demonstration follows on from the last experiment. Take a saturated solution of sugar in water with a little undissolved sugar at the bottom, and warm it gently while stirring. The extra sugar dissolves — the water can now hold more than it could when cold.
Now cool that warm solution. As the temperature drops the water can hold less, so the excess sugar separates out again, often as crystals on the sides and base.
Everyday evidence is easy to find:
- Sugar dissolves quickly and completely in hot tea, while in a cold drink it needs long stirring
- Making sugar syrup for gulab jamun or jalebi requires heat, because a great deal of sugar must dissolve in a little water
- Jaggery dissolves far faster in warm water than in cold
The reason is that heating makes the particles move faster, so solute particles break away from each other more readily and spread among the solvent particles more quickly.
This is also the basis of crystallisation, which links back to purification. A hot saturated solution cooled slowly deposits the dissolved solid as pure crystals — which is how copper sulphate and alum crystals are grown, and how sugar is obtained from cane juice.
One caution about the wording: heating makes a solid dissolve both faster and in greater quantity, and those are two different effects. Stirring also speeds dissolving up, but it does not let more dissolve — a saturated solution stays saturated however long you stir it.
The demonstration follows on from the last experiment. Take a saturated solution of sugar in water with a little undissolved sugar at the bottom, and warm it gently while stirring. The extra sugar dissolves — the water can now hold more than it could when cold.
Now cool that warm solution. As the temperature drops the water can hold less, so the excess sugar separates out again, often as crystals on the sides and base.
Everyday evidence is easy to find:
- Sugar dissolves quickly and completely in hot tea, while in a cold drink it needs long stirring
- Making sugar syrup for gulab jamun or jalebi requires heat, because a great deal of sugar must dissolve in a little water
- Jaggery dissolves far faster in warm water than in cold
The reason is that heating makes the particles move faster, so solute particles break away from each other more readily and spread among the solvent particles more quickly.
This is also the basis of crystallisation, which links back to purification. A hot saturated solution cooled slowly deposits the dissolved solid as pure crystals — which is how copper sulphate and alum crystals are grown, and how sugar is obtained from cane juice.
One caution about the wording: heating makes a solid dissolve both faster and in greater quantity, and those are two different effects. Stirring also speeds dissolving up, but it does not let more dissolve — a saturated solution stays saturated however long you stir it.
Why do gases dissolve less in warm water?
Because the dissolved gas particles gain energy and escape from the liquid. Gases behave opposite to solids: their solubility decreases as temperature rises, and increases as temperature falls.
Gases also respond to pressure: solubility increases with pressure and decreases when pressure is released.
Aerated drinks show both effects at once. Carbon dioxide is dissolved into the drink under high pressure and the bottle is sealed to keep that pressure. Open the cap and the pressure falls, so the gas comes out as fizz. Leave the bottle open, or let the drink go warm, and it loses its gas entirely and tastes flat.
Dissolved oxygen matters even more. Fish and other aquatic animals breathe the oxygen dissolved in water through their gills. Because gas solubility falls as water warms, warm water holds less oxygen than cold — which is why fish may struggle in a pond during a heatwave, and why an aquarium is fitted with an air pump to keep bubbling oxygen in.
Boiling water also drives dissolved gases out, which is why boiled-and-cooled water tastes flat compared with fresh water.
So the two rules must be kept apart, and this is the distinction the chapter tests. For a solid, heating means more dissolves. For a gas, heating means less dissolves. Applying the solid rule to a gas is the standard error — and it would predict fizzy drinks improving in the sun, which anyone who has tried it knows is wrong.
Gases also respond to pressure: solubility increases with pressure and decreases when pressure is released.
Aerated drinks show both effects at once. Carbon dioxide is dissolved into the drink under high pressure and the bottle is sealed to keep that pressure. Open the cap and the pressure falls, so the gas comes out as fizz. Leave the bottle open, or let the drink go warm, and it loses its gas entirely and tastes flat.
Dissolved oxygen matters even more. Fish and other aquatic animals breathe the oxygen dissolved in water through their gills. Because gas solubility falls as water warms, warm water holds less oxygen than cold — which is why fish may struggle in a pond during a heatwave, and why an aquarium is fitted with an air pump to keep bubbling oxygen in.
Boiling water also drives dissolved gases out, which is why boiled-and-cooled water tastes flat compared with fresh water.
So the two rules must be kept apart, and this is the distinction the chapter tests. For a solid, heating means more dissolves. For a gas, heating means less dissolves. Applying the solid rule to a gas is the standard error — and it would predict fizzy drinks improving in the sun, which anyone who has tried it knows is wrong.
Exam tip
Exam tip: stating the temperature with every solubility
Solubility answers are incomplete without their conditions, and the marks reflect that.
Always give the amount of solvent and the temperature with a solubility figure — in 50 mL of water at room temperature. A bare number earns little.
Keep the two directions straight: solids dissolve more on heating, gases dissolve less. State which one you are talking about before giving the direction.
For gases, mention both factors — temperature and pressure — since questions on aerated drinks usually want both.
Distinguish faster from more. Stirring and powdering make a solid dissolve faster; only heating makes more of it dissolve.
And when describing the saturation experiment, include keeping count of the spoons and the moment a spoonful refuses to dissolve — that observation is what defines saturation.
Always give the amount of solvent and the temperature with a solubility figure — in 50 mL of water at room temperature. A bare number earns little.
Keep the two directions straight: solids dissolve more on heating, gases dissolve less. State which one you are talking about before giving the direction.
For gases, mention both factors — temperature and pressure — since questions on aerated drinks usually want both.
Distinguish faster from more. Stirring and powdering make a solid dissolve faster; only heating makes more of it dissolve.
And when describing the saturation experiment, include keeping count of the spoons and the moment a spoonful refuses to dissolve — that observation is what defines saturation.
Did you know
Why does a fizzy drink go flat in the sun?
Because both things keeping the gas in have been removed.
The carbon dioxide was forced into the drink under high pressure, and the sealed cap held that pressure. Opening the bottle releases it, so the gas immediately starts leaving as bubbles.
Warmth makes it worse. As the liquid heats, the dissolved gas particles gain enough energy to escape more readily, so solubility falls further. An open bottle left in the sun loses its gas on both counts at once — which is exactly why cold drinks are kept cold and capped, and why the last few sips of a warm one taste of nothing but sweet water.
The carbon dioxide was forced into the drink under high pressure, and the sealed cap held that pressure. Opening the bottle releases it, so the gas immediately starts leaving as bubbles.
Warmth makes it worse. As the liquid heats, the dissolved gas particles gain enough energy to escape more readily, so solubility falls further. An open bottle left in the sun loses its gas on both counts at once — which is exactly why cold drinks are kept cold and capped, and why the last few sips of a warm one taste of nothing but sweet water.
Key takeaways
Solutions and solubility: quick revision
- A solution is a uniform mixture; the solute dissolves and the solvent does the dissolving, usually the larger amount — water is the universal solvent.
- Solute particles spread into the spaces between solvent particles, and a true solution passes through filter paper without settling.
- A saturated solution can dissolve no more solute at that temperature; an unsaturated one can.
- Solubility must always be quoted with the amount of solvent and the temperature, since both change the answer.
- For most solids, solubility increases with temperature — the basis of making syrup and of growing crystals by slow cooling.
- For gases, solubility decreases with temperature and increases with pressure — so drinks go flat when warm or opened, and warm water holds less dissolved oxygen for fish.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Solute particles spread into the spaces between solvent particles, and a true solution passes through filter paper without settling.
- A saturated solution can dissolve no more solute at that temperature; an unsaturated one can.
- Solubility must always be quoted with the amount of solvent and the temperature, since both change the answer.
- For most solids, solubility increases with temperature — the basis of making syrup and of growing crystals by slow cooling.
- For gases, solubility decreases with temperature and increases with pressure — so drinks go flat when warm or opened, and warm water holds less dissolved oxygen for fish.
You will remember all of this far better after answering five questions on it than after reading it twice.