Twenty Grams of Salt in a Hundred of Water Is Not a Twenty Percent Solution
Learn to classify mixtures as homogeneous or heterogeneous, identify solute and solvent in every state, calculate concentration by mass and by volume, and read a solubility curve.
Why is 20 g of salt in 100 g of water not a 20 percent solution?
Because the percentage is taken over the whole solution, not over the water.
Adding g of salt to g of water gives a solution weighing g, so
A genuine solution would need g of salt in a total of g — so g of salt and only g of water.
The difference looks small and it is the most penalised mistake in the whole chapter, because the formula's denominator is the mass of the solution:
Read the question carefully enough to know whether you were given the solvent or the solution, and the arithmetic is easy. This page covers the first part of the CBSE Class 9 Science chapter on mixtures and their separation.
Adding g of salt to g of water gives a solution weighing g, so
A genuine solution would need g of salt in a total of g — so g of salt and only g of water.
The difference looks small and it is the most penalised mistake in the whole chapter, because the formula's denominator is the mass of the solution:
Read the question carefully enough to know whether you were given the solvent or the solution, and the arithmetic is easy. This page covers the first part of the CBSE Class 9 Science chapter on mixtures and their separation.
How do you tell a homogeneous mixture from a heterogeneous one?
A homogeneous mixture has the same composition throughout; a heterogeneous one does not.
In a homogeneous mixture the components cannot be told apart, and any sample taken from any part has the same composition as any other.
- Salt dissolved in water
- Sugar dissolved in water
- Air
- Alloys such as brass
- Vinegar
In a heterogeneous mixture the components are distinguishable and the composition varies from place to place.
- Sand and water
- Oil and water
- Soil
- A mixture of iron filings and sulphur powder
- Muddy water
Everyday evidence. A cup of sugared tea tastes the same from the top as from the bottom — homogeneous. A plate of poha with peanuts scattered through it gives a different mouthful each time — heterogeneous. Leave muddy water standing and the mud settles at the bottom, which no homogeneous mixture ever does.
The distinction depends on the scale you look at. Milk appears perfectly uniform to the eye, and yet it is heterogeneous — it contains tiny droplets of fat suspended in water. Shine a narrow beam of light through it and the beam becomes visible, scattered by those droplets. A true solution such as salt water does not scatter light this way, and the beam passes through unseen.
That scattering is the Tyndall effect, and it is the test that separates a colloid such as milk from a true solution. A mixture can therefore look homogeneous and still be classified as heterogeneous, which is why looks uniform is not by itself a sufficient answer.
A mixture is not a compound. In a mixture the components keep their own properties and can be separated by physical means, and there is no fixed ratio. In a compound the elements are chemically combined in a fixed ratio and the properties are entirely new — which is why iron filings in sulphur can be separated with a magnet, while iron sulphide cannot.
In a homogeneous mixture the components cannot be told apart, and any sample taken from any part has the same composition as any other.
- Salt dissolved in water
- Sugar dissolved in water
- Air
- Alloys such as brass
- Vinegar
In a heterogeneous mixture the components are distinguishable and the composition varies from place to place.
- Sand and water
- Oil and water
- Soil
- A mixture of iron filings and sulphur powder
- Muddy water
Everyday evidence. A cup of sugared tea tastes the same from the top as from the bottom — homogeneous. A plate of poha with peanuts scattered through it gives a different mouthful each time — heterogeneous. Leave muddy water standing and the mud settles at the bottom, which no homogeneous mixture ever does.
The distinction depends on the scale you look at. Milk appears perfectly uniform to the eye, and yet it is heterogeneous — it contains tiny droplets of fat suspended in water. Shine a narrow beam of light through it and the beam becomes visible, scattered by those droplets. A true solution such as salt water does not scatter light this way, and the beam passes through unseen.
That scattering is the Tyndall effect, and it is the test that separates a colloid such as milk from a true solution. A mixture can therefore look homogeneous and still be classified as heterogeneous, which is why looks uniform is not by itself a sufficient answer.
A mixture is not a compound. In a mixture the components keep their own properties and can be separated by physical means, and there is no fixed ratio. In a compound the elements are chemically combined in a fixed ratio and the properties are entirely new — which is why iron filings in sulphur can be separated with a magnet, while iron sulphide cannot.
What are the solute and solvent, and can a solution be a solid?
The solvent is the component present in the greater amount; the solute is what dissolves in it. And yes — solutions exist in all three states.
A solution is a homogeneous mixture of two or more substances, and it has two parts:
- Solute — the substance that dissolves, present in the lesser amount
- Solvent — the substance that does the dissolving, present in the greater amount
Solutions in every state.
- Solid in liquid: salt in water. Solute salt, solvent water.
- Liquid in liquid: alcohol in water.
- Gas in liquid: carbon dioxide in water, which is soda; and oxygen dissolved in river water, which is what fish breathe.
- Solid in solid: alloys. Brass is zinc dissolved in copper — solvent copper, solute zinc.
- Gas in gas: air, with nitrogen as the solvent and oxygen and the other gases as solutes.
- Solid in gas: iodine vapour in air.
Everyday evidence. A brass tumbler is a solid solution — you cannot see the zinc separately anywhere in it. A bottle of soda fizzes when opened because dissolved carbon dioxide escapes once the pressure drops.
Which one is the solvent can change. Mix g of alcohol with g of water and the water is the solvent. Reverse the amounts — g of alcohol and g of water — and now alcohol is the solvent. The labels describe the proportions, not the substances themselves.
Water is called the universal solvent because it dissolves a very large number of substances, but the name is a description of its range rather than a claim that it dissolves everything. Oil, wax and sand do not dissolve in water at all.
The properties of a true solution, worth knowing as a set:
- The particles are so small they cannot be seen even with a microscope
- The solute particles do not settle on standing
- The solution is stable and does not separate
- It cannot be filtered — the solute passes through filter paper with the solvent
- It does not scatter a beam of light
That last property is what distinguished salt water from milk in the previous section, and the fourth explains why filtration is useless for separating a solution — which is the whole reason the second part of this chapter needs different techniques.
A solution is a homogeneous mixture of two or more substances, and it has two parts:
- Solute — the substance that dissolves, present in the lesser amount
- Solvent — the substance that does the dissolving, present in the greater amount
Solutions in every state.
- Solid in liquid: salt in water. Solute salt, solvent water.
- Liquid in liquid: alcohol in water.
- Gas in liquid: carbon dioxide in water, which is soda; and oxygen dissolved in river water, which is what fish breathe.
- Solid in solid: alloys. Brass is zinc dissolved in copper — solvent copper, solute zinc.
- Gas in gas: air, with nitrogen as the solvent and oxygen and the other gases as solutes.
- Solid in gas: iodine vapour in air.
Everyday evidence. A brass tumbler is a solid solution — you cannot see the zinc separately anywhere in it. A bottle of soda fizzes when opened because dissolved carbon dioxide escapes once the pressure drops.
Which one is the solvent can change. Mix g of alcohol with g of water and the water is the solvent. Reverse the amounts — g of alcohol and g of water — and now alcohol is the solvent. The labels describe the proportions, not the substances themselves.
Water is called the universal solvent because it dissolves a very large number of substances, but the name is a description of its range rather than a claim that it dissolves everything. Oil, wax and sand do not dissolve in water at all.
The properties of a true solution, worth knowing as a set:
- The particles are so small they cannot be seen even with a microscope
- The solute particles do not settle on standing
- The solution is stable and does not separate
- It cannot be filtered — the solute passes through filter paper with the solvent
- It does not scatter a beam of light
That last property is what distinguished salt water from milk in the previous section, and the fourth explains why filtration is useless for separating a solution — which is the whole reason the second part of this chapter needs different techniques.
Formula
How do you calculate the concentration of a solution?
Two formulas, and the denominator of each is the solution, never the solvent:
with
Worked example 1 — solvent mass given. g of salt is dissolved in g of water.
Worked example 2. g of sugar is dissolved in g of water.
Worked example 3 — solution mass given. A solution of total mass g contains g of solute.
and the solvent mass is g.
Worked example 4 — the trap, worked in full. g of salt is dissolved in g of water.
The solution now weighs g, so
not . Compare with worked example 1, where g was the mass of the solution. The two questions differ by one word and by nearly four percentage points.
Worked example 5 — mass by volume. g of a substance is dissolved to give mL of solution.
Worked example 6 — working backwards. How much solute is present in g of a solution?
so the solvent is g.
Worked example 7 — preparing a solution. To make g of a solution:
So you weigh out g of solute and add g of water — not g of water, which would give a solution of g and a concentration of .
State which percentage you have used. *A solution is ambiguous on its own; by mass or by mass by volume* is complete. The two are not equal unless the solution's density happens to be g per mL.
with
Worked example 1 — solvent mass given. g of salt is dissolved in g of water.
Worked example 2. g of sugar is dissolved in g of water.
Worked example 3 — solution mass given. A solution of total mass g contains g of solute.
and the solvent mass is g.
Worked example 4 — the trap, worked in full. g of salt is dissolved in g of water.
The solution now weighs g, so
not . Compare with worked example 1, where g was the mass of the solution. The two questions differ by one word and by nearly four percentage points.
Worked example 5 — mass by volume. g of a substance is dissolved to give mL of solution.
Worked example 6 — working backwards. How much solute is present in g of a solution?
so the solvent is g.
Worked example 7 — preparing a solution. To make g of a solution:
So you weigh out g of solute and add g of water — not g of water, which would give a solution of g and a concentration of .
State which percentage you have used. *A solution is ambiguous on its own; by mass or by mass by volume* is complete. The two are not equal unless the solution's density happens to be g per mL.
How does temperature change solubility, and what does a curve show?
For most solids, solubility rises with temperature. For gases, it falls.
Solubility is the maximum mass of solute that dissolves in ** g of solvent at a stated temperature. The temperature has to be stated, because solubility is not a single fixed number for a substance.
- A saturated solution holds as much solute as it can at that temperature, and no more will dissolve
- An unsaturated solution can still dissolve more
Worked example 1 — scaling up from a solubility figure.** The solubility of potassium nitrate is g per g of water at . How much dissolves in g of water at that temperature?
Worked example 2 — reading a curve and cooling a solution. A solubility curve shows a salt dissolving to the extent of g per g of water at , and g per g at .
A saturated solution made at holds g. Cool it to and the water can now hold only g, so
That surplus appears as solid crystals, and it is exactly the process the next part of this chapter uses to purify a substance.
Worked example 3 — is it saturated? g of a salt is added to g of water at , where its solubility is g per g.
Since , all of it dissolves and the solution is unsaturated. Adding a further g would just saturate it.
Gases behave the opposite way. The solubility of a gas in a liquid falls as temperature rises, and rises as pressure rises.
Everyday evidence for both. Sugar dissolves far more readily in hot tea than in cold water — solid solubility rising with temperature. A fizzy drink left in the sun goes flat much faster than one in the fridge, because warming drives the dissolved carbon dioxide out — gas solubility falling with temperature. And the drink fizzes on opening because releasing the pressure lowers the solubility further.
Why this matters beyond the laboratory. Because warm water holds less dissolved oxygen, fish are harder pressed in a heated pond than a cool one — the same physical rule, with a biological consequence.
Solubility is a property of a pair, not of a substance. It always describes a particular solute in a particular solvent at a particular temperature. Sugar is highly soluble in water and barely soluble in oil — so sugar is soluble is an incomplete statement, and a question that omits the temperature cannot be answered with a number.
Solubility is the maximum mass of solute that dissolves in ** g of solvent at a stated temperature. The temperature has to be stated, because solubility is not a single fixed number for a substance.
- A saturated solution holds as much solute as it can at that temperature, and no more will dissolve
- An unsaturated solution can still dissolve more
Worked example 1 — scaling up from a solubility figure.** The solubility of potassium nitrate is g per g of water at . How much dissolves in g of water at that temperature?
Worked example 2 — reading a curve and cooling a solution. A solubility curve shows a salt dissolving to the extent of g per g of water at , and g per g at .
A saturated solution made at holds g. Cool it to and the water can now hold only g, so
That surplus appears as solid crystals, and it is exactly the process the next part of this chapter uses to purify a substance.
Worked example 3 — is it saturated? g of a salt is added to g of water at , where its solubility is g per g.
Since , all of it dissolves and the solution is unsaturated. Adding a further g would just saturate it.
Gases behave the opposite way. The solubility of a gas in a liquid falls as temperature rises, and rises as pressure rises.
Everyday evidence for both. Sugar dissolves far more readily in hot tea than in cold water — solid solubility rising with temperature. A fizzy drink left in the sun goes flat much faster than one in the fridge, because warming drives the dissolved carbon dioxide out — gas solubility falling with temperature. And the drink fizzes on opening because releasing the pressure lowers the solubility further.
Why this matters beyond the laboratory. Because warm water holds less dissolved oxygen, fish are harder pressed in a heated pond than a cool one — the same physical rule, with a biological consequence.
Solubility is a property of a pair, not of a substance. It always describes a particular solute in a particular solvent at a particular temperature. Sugar is highly soluble in water and barely soluble in oil — so sugar is soluble is an incomplete statement, and a question that omits the temperature cannot be answered with a number.
Exam tip
Exam tip: divide by the solution, never by the solvent
**Mass of solution mass of solute mass of solvent. Put the solution in the denominator every time.
Read whether the question gave you the mass of water or of the solution**. g salt in g water is ; g salt in g water is .
To prepare a solution, subtract: g of a solution needs g solute and ** g water.
Say which percentage you mean — by mass, or by mass by volume.
Solubility is the maximum solute per g of solvent at a stated temperature**. Scale it: g per g means g in g of water.
For a cooling question, subtract the two solubilities: g crystallises out.
Solid solubility rises with temperature; gas solubility falls with temperature and rises with pressure.
Homogeneous means uniform throughout with indistinguishable components; heterogeneous does not. Milk looks uniform but is heterogeneous — it shows the Tyndall effect.
The solvent is whichever is present in the greater amount, so it can change when the proportions change.
Remember that a true solution cannot be filtered and does not settle — which rules filtration out for separating one.
And write units on every mass and the ** sign** on every concentration.
Read whether the question gave you the mass of water or of the solution**. g salt in g water is ; g salt in g water is .
To prepare a solution, subtract: g of a solution needs g solute and ** g water.
Say which percentage you mean — by mass, or by mass by volume.
Solubility is the maximum solute per g of solvent at a stated temperature**. Scale it: g per g means g in g of water.
For a cooling question, subtract the two solubilities: g crystallises out.
Solid solubility rises with temperature; gas solubility falls with temperature and rises with pressure.
Homogeneous means uniform throughout with indistinguishable components; heterogeneous does not. Milk looks uniform but is heterogeneous — it shows the Tyndall effect.
The solvent is whichever is present in the greater amount, so it can change when the proportions change.
Remember that a true solution cannot be filtered and does not settle — which rules filtration out for separating one.
And write units on every mass and the ** sign** on every concentration.
Did you know
Why a warm fizzy drink goes flat so quickly
Open a chilled bottle of soda and it hisses gently. Open a warm one and it foams over the top. The same drink, and two quite different results.
Both come from the same rule: a gas dissolves less in a liquid as the temperature rises, and more as the pressure rises. A soda bottle is filled with carbon dioxide under pressure and then sealed, which is what keeps the gas dissolved.
Opening the bottle removes the pressure, so the solubility drops at once and the surplus gas leaves solution as bubbles. In a cold bottle the solubility was high to begin with, so less gas is surplus and the escape is slow. In a warm bottle the solubility was already low, a great deal of gas is surplus, and it all tries to leave at the same moment.
This is also why the drink tastes flat after standing. The dissolved gas is not merely a source of bubbles; carbon dioxide in water makes the drink slightly acidic, and losing it changes the taste as well as the fizz.
The same rule explains something with much larger consequences. River and pond water hold dissolved oxygen, and fish depend on it. Warm the water and it holds less oxygen — so a pond in a hot spell or a river receiving warm water from a factory can starve its fish of oxygen without any pollutant being added at all.
And it runs the other way too. Water boiled and cooled tastes noticeably flat because boiling drove the dissolved air out of it, which is why laboratory water for some experiments is deliberately boiled first.
So gas solubility falls with temperature is not an isolated fact to memorise beside the opposite rule for solids. It is one rule, and it turns up wherever a gas is dissolved in a liquid — which is to say, in every drink, every river and every fish tank.
Both come from the same rule: a gas dissolves less in a liquid as the temperature rises, and more as the pressure rises. A soda bottle is filled with carbon dioxide under pressure and then sealed, which is what keeps the gas dissolved.
Opening the bottle removes the pressure, so the solubility drops at once and the surplus gas leaves solution as bubbles. In a cold bottle the solubility was high to begin with, so less gas is surplus and the escape is slow. In a warm bottle the solubility was already low, a great deal of gas is surplus, and it all tries to leave at the same moment.
This is also why the drink tastes flat after standing. The dissolved gas is not merely a source of bubbles; carbon dioxide in water makes the drink slightly acidic, and losing it changes the taste as well as the fizz.
The same rule explains something with much larger consequences. River and pond water hold dissolved oxygen, and fish depend on it. Warm the water and it holds less oxygen — so a pond in a hot spell or a river receiving warm water from a factory can starve its fish of oxygen without any pollutant being added at all.
And it runs the other way too. Water boiled and cooled tastes noticeably flat because boiling drove the dissolved air out of it, which is why laboratory water for some experiments is deliberately boiled first.
So gas solubility falls with temperature is not an isolated fact to memorise beside the opposite rule for solids. It is one rule, and it turns up wherever a gas is dissolved in a liquid — which is to say, in every drink, every river and every fish tank.
Exam relevance
Why does concentration keep reappearing in JEE and NEET Chemistry?
Because every quantitative statement in chemistry is a statement about how much of what, in how much of something else — and this page is where that idea is first put into a formula.
This is the foundation for the Class 11 Chemistry chapter Some Basic Concepts of Chemistry, examined in both JEE Main and NEET. That chapter introduces molarity, molality and mole fraction, and every one of them follows the same pattern as the percentage formulas here: an amount of solute divided by a fixed amount of solution or solvent. The Class 9 distinction between dividing by the solution and dividing by the solvent becomes the formal distinction between molarity (moles per litre of solution) and molality (moles per kilogram of solvent) — and students who never settled it here get the two confused there.
Solubility and temperature feed into the Class 12 chapter Solutions, a substantial part of both JEE Main and NEET Chemistry. There the qualitative rule on this page becomes Henry's law for gas solubility, with pressure and temperature handled quantitatively, and the same fizzy-drink reasoning is used as the standard illustration.
Homogeneous against heterogeneous reappears in Class 11 when States of Matter and later Equilibrium distinguish homogeneous from heterogeneous equilibria, and in Class 12 Surface Chemistry, where colloids and the Tyndall effect are treated in full.
Alloys as solid solutions return in Class 12 The p-Block Elements and Metallurgy.
What the questions look like. Numericals on concentration are common, and the trap is nearly always the denominator. Assertion-reason items favour gas solubility falling with temperature, and the Tyndall effect distinguishing a colloid from a true solution. Graph-reading questions give a solubility curve and ask how much crystallises on cooling — precisely the g calculation worked above. Match-the-column items pair a solution type with an example, where brass and air are the two that catch students out.
How board and competitive emphasis differ. A board paper asks you to define solubility or to give two differences between a solution and a colloid, then sets a one-step percentage sum. A competitive paper sets the same sum with the solvent mass given rather than the solution mass, or asks for the mass of water needed to prepare a stated solution — so the reasoning about the denominator, rather than the formula itself, is what is being tested.
The single trap that costs the most marks. Dividing by the mass of the solvent instead of the solution. Write the line before every calculation, and the error becomes impossible.
A second trap worth naming. Quoting a solubility without its temperature, or treating solubility as a fixed property of a substance. It is a property of a solute-solvent pair at a stated temperature, and a question that gives a temperature is giving it because the answer depends on it.
This is the foundation for the Class 11 Chemistry chapter Some Basic Concepts of Chemistry, examined in both JEE Main and NEET. That chapter introduces molarity, molality and mole fraction, and every one of them follows the same pattern as the percentage formulas here: an amount of solute divided by a fixed amount of solution or solvent. The Class 9 distinction between dividing by the solution and dividing by the solvent becomes the formal distinction between molarity (moles per litre of solution) and molality (moles per kilogram of solvent) — and students who never settled it here get the two confused there.
Solubility and temperature feed into the Class 12 chapter Solutions, a substantial part of both JEE Main and NEET Chemistry. There the qualitative rule on this page becomes Henry's law for gas solubility, with pressure and temperature handled quantitatively, and the same fizzy-drink reasoning is used as the standard illustration.
Homogeneous against heterogeneous reappears in Class 11 when States of Matter and later Equilibrium distinguish homogeneous from heterogeneous equilibria, and in Class 12 Surface Chemistry, where colloids and the Tyndall effect are treated in full.
Alloys as solid solutions return in Class 12 The p-Block Elements and Metallurgy.
What the questions look like. Numericals on concentration are common, and the trap is nearly always the denominator. Assertion-reason items favour gas solubility falling with temperature, and the Tyndall effect distinguishing a colloid from a true solution. Graph-reading questions give a solubility curve and ask how much crystallises on cooling — precisely the g calculation worked above. Match-the-column items pair a solution type with an example, where brass and air are the two that catch students out.
How board and competitive emphasis differ. A board paper asks you to define solubility or to give two differences between a solution and a colloid, then sets a one-step percentage sum. A competitive paper sets the same sum with the solvent mass given rather than the solution mass, or asks for the mass of water needed to prepare a stated solution — so the reasoning about the denominator, rather than the formula itself, is what is being tested.
The single trap that costs the most marks. Dividing by the mass of the solvent instead of the solution. Write the line before every calculation, and the error becomes impossible.
A second trap worth naming. Quoting a solubility without its temperature, or treating solubility as a fixed property of a substance. It is a property of a solute-solvent pair at a stated temperature, and a question that gives a temperature is giving it because the answer depends on it.
Key takeaways
Mixtures, solutions and concentration: quick revision
- Homogeneous mixtures are uniform throughout with indistinguishable components: salt water, sugar solution, air, brass, vinegar.
- Heterogeneous mixtures vary from place to place: sand and water, oil and water, soil, iron filings with sulphur, muddy water.
- Milk looks uniform but is heterogeneous — it scatters a light beam, the Tyndall effect, which a true solution does not.
- A mixture keeps its components' properties, has no fixed ratio, and can be separated physically; a compound cannot.
- Solvent is the component in the greater amount; solute the lesser. Swap the amounts and the labels swap too.
- Solutions exist in all states: salt in water, alcohol in water, carbon dioxide in water, brass (zinc in copper), air (gases in gas), iodine vapour in air.
- A true solution has invisible particles, does not settle, is stable, cannot be filtered, and does not scatter light.
- **Mass of solution solute solvent**, and this is the denominator in both formulas.
- ; .
- g salt in g water gives ****; g salt in g water gives ****. One word apart.
- g sugar in g water is ; g in a g solution is ; g in mL is by mass by volume.
- Backwards: g of a solution holds g solute and g water. To make g of a solution, take g solute and ** g water.
- Solubility is the maximum solute per g of solvent at a stated temperature. A saturated** solution can hold no more.
- g per g at means g dissolves in g of water.
- Cooling a saturated solution from ( g per g) to ( g per g) crystallises out ** g**.
- g added to g of water where solubility is g per g stays unsaturated, since the maximum is g.
- Solid solubility rises with temperature; gas solubility falls with temperature and rises with pressure — hence flat warm soda and less oxygen in warm water.
- Solubility describes a pair at a temperature, not a substance on its own.
Work one concentration sum where the water mass is given and one where the solution mass is given, side by side — seeing the two answers differ is the fastest way to make the denominator stick.
- Heterogeneous mixtures vary from place to place: sand and water, oil and water, soil, iron filings with sulphur, muddy water.
- Milk looks uniform but is heterogeneous — it scatters a light beam, the Tyndall effect, which a true solution does not.
- A mixture keeps its components' properties, has no fixed ratio, and can be separated physically; a compound cannot.
- Solvent is the component in the greater amount; solute the lesser. Swap the amounts and the labels swap too.
- Solutions exist in all states: salt in water, alcohol in water, carbon dioxide in water, brass (zinc in copper), air (gases in gas), iodine vapour in air.
- A true solution has invisible particles, does not settle, is stable, cannot be filtered, and does not scatter light.
- **Mass of solution solute solvent**, and this is the denominator in both formulas.
- ; .
- g salt in g water gives ****; g salt in g water gives ****. One word apart.
- g sugar in g water is ; g in a g solution is ; g in mL is by mass by volume.
- Backwards: g of a solution holds g solute and g water. To make g of a solution, take g solute and ** g water.
- Solubility is the maximum solute per g of solvent at a stated temperature. A saturated** solution can hold no more.
- g per g at means g dissolves in g of water.
- Cooling a saturated solution from ( g per g) to ( g per g) crystallises out ** g**.
- g added to g of water where solubility is g per g stays unsaturated, since the maximum is g.
- Solid solubility rises with temperature; gas solubility falls with temperature and rises with pressure — hence flat warm soda and less oxygen in warm water.
- Solubility describes a pair at a temperature, not a substance on its own.
Work one concentration sum where the water mass is given and one where the solution mass is given, side by side — seeing the two answers differ is the fastest way to make the denominator stick.