E. Solutions and Solubility · Solutions and Solubility

Solutions, Solubility and Saturation

E2.1 A1.12About 65 minutes

Learning goals

I can use the terms solution, solute, solvent, aqueous, miscible and immiscible correctly.
I can give examples of solutions in all three states (solid, liquid, gas).
I can define solubility and use it to decide whether a solution is unsaturated, saturated or supersaturated.
I can tell the difference between how fast a solute dissolves and how much of it can dissolve.

Words to know

solution
A homogeneous mixture: it has the same composition throughout, and the dissolved particles are too small to see or to settle out.

solute
The substance that is dissolved, usually present in the smaller amount.

solvent
The substance that does the dissolving, usually present in the larger amount.

aqueous solution (aq)
A solution in which water is the solvent.

solubility
The maximum mass of solute that dissolves in a given amount of solvent at a stated temperature, e.g. 36.0 g NaCl per 100 mL water at 20 °C.

saturated / unsaturated
A saturated solution holds the maximum amount of solute at that temperature; an unsaturated one could still dissolve more.

supersaturated
A solution holding more dissolved solute than its solubility. It is unstable and crystallizes when disturbed.

miscible / immiscible
Two liquids are miscible if they mix in all proportions (ethanol and water); immiscible if they form separate layers (oil and water).

Mixtures you drink, breathe and wear

Maple syrup, the air in your lungs, the brass of a trumpet and the salty water of a pickle jar have something in common: each is a solution. A solution is a mixture that looks like one substance all the way through, because one substance has broken up into particles (molecules or ions) and spread evenly through another.

Most of the chemistry in this unit happens in aqueous solutions: solutions in which water is the solvent. When you see (aq) after a formula, as in NaCl(aq), it means "dissolved in water."

Remember from Grades 9 and 10

In Grade 9 you classified matter as pure substances (elements and compounds) or mixtures, and mixtures as homogeneous or heterogeneous. A solution is a homogeneous mixture. In Grade 10 you wrote state symbols in equations: (s), (l), (g) and (aq).

Solute and solvent

In a solution of sugar in water:

the solute is sugar (the substance dissolved),
the solvent is water (the substance doing the dissolving),
the solution is sugar water.

When two liquids mix, we usually call the one present in the larger amount the solvent. In vinegar, acetic acid (about 5%) is the solute and water is the solvent.

A true solution is clear (it can be coloured, like tea, but you can see through it), it does not settle on standing, and it passes through filter paper unchanged.

Sugar molecules (solute) spread evenly between water molecules (solvent). The result is a solution: the same in every part. Particles are drawn much larger than they really are.

Solute state	Solvent state	Example	Solute(s) / solvent
gas	gas	air	oxygen, argon, CO₂ / nitrogen
gas	liquid	carbonated water	carbon dioxide / water
liquid	liquid	vinegar; windshield washer fluid	acetic acid / water; methanol / water
solid	liquid	ocean water; maple syrup	salts / water; mostly sucrose / water
solid	solid	brass; steel; sterling silver	zinc / copper; carbon / iron; copper / silver

Solutions can be gases, liquids or solids. Solid solutions of metals are called alloys.

Miscible and immiscible liquids

Ethanol and water mix in any proportion: they are miscible. Vegetable oil and water separate into two layers no matter how hard you shake: they are immiscible. You will learn why in the next lesson ("like dissolves like").

Solubility

There is a limit to how much solute dissolves. Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a specific temperature. It is often given in grams of solute per 100 mL (or 100 g) of water.

For example, at 20 °C:

sodium chloride (table salt): 36.0 g/100 mL
potassium nitrate: 31.6 g/100 mL
sucrose (table sugar): about 204 g/100 mL
calcium carbonate (chalk, limestone): a tiny fraction of a gram per 100 mL. We call it slightly soluble or "insoluble".

Chemists sort compounds roughly as high solubility (more than about 1 g/100 mL) and low solubility (less than about 0.1 g/100 mL). No ionic compound is completely insoluble; "insoluble" just means so little dissolves that it is usually not noticed.

Unsaturated, saturated and supersaturated

An unsaturated solution contains less solute than its solubility. More can dissolve.
A saturated solution contains the maximum amount. Extra solute just sits on the bottom. (The dissolved and undissolved solute keep trading places, but the amount dissolved stays the same.)
A supersaturated solution contains more dissolved solute than the solubility. It is made by dissolving solute in hot water, then cooling it very carefully. It is unstable: one seed crystal or a jolt makes the extra solute crystallize out quickly.

At 20 °C, 100 mL of water can hold at most 36.0 g of sodium chloride. Below that the solution is unsaturated; at that point it is saturated and any extra solid stays undissolved; above it (made by careful cooling) the solution is supersaturated.

Rate of dissolving is not the same as solubility

Stirring, crushing the solute into smaller pieces, and warming the solvent all make a solute dissolve faster. But stirring and crushing do not change how much can dissolve. Only temperature (and, for gases, pressure) changes the solubility itself. You will study those two factors in the lesson on temperature and pressure.

Stirring or shaking: rate only.
Smaller pieces (more surface area): rate only.
Higher temperature: both the rate and (for most solids) the solubility.

Worked example

Example 1: Saturated or not?

The solubility of sodium chloride is 36.0 g/100 mL of water at 20 °C. At 20 °C, describe each mixture: (a) 25.0 g NaCl in 100 mL water, (b) 50.0 g NaCl in 100 mL water, (c) 80.0 g NaCl in 250 mL water.

(a) 25.0 g < 36.0 g, so all of it dissolves. The solution is unsaturated; it could dissolve 36.0 − 25.0 = 11.0 g more.
(b) Only 36.0 g can dissolve. The solution is saturated, and 50.0 − 36.0 = 14.0 g stays undissolved on the bottom.
(c) Scale the solubility to 250 mL: 36.0 g × (250 mL ÷ 100 mL) = 90.0 g can dissolve. 80.0 g < 90.0 g, so the solution is unsaturated.

(a) unsaturated; (b) saturated with 14.0 g undissolved; (c) unsaturated (it could hold 90.0 g).

Worked example

Example 2: Identify the parts

For each solution name the solute and the solvent and the state of each: (a) soda water, (b) brass (about 65% copper and 35% zinc), (c) a saline solution for contact lenses.

(a) Soda water: solute carbon dioxide (gas), solvent water (liquid).
(b) Brass: solute zinc (solid), solvent copper (solid), since copper is present in the larger amount. Brass is an alloy.
(c) Saline: solute sodium chloride (solid), solvent water (liquid). It is an aqueous solution, NaCl(aq).

See the steps: the solvent is the substance present in the larger amount.

Just for fun

Reusable hand warmers that you 'click' on a cold Canadian morning usually contain a supersaturated solution of sodium acetate. Bending the little metal disc starts crystallization, which releases heat. Heat the pouch in boiling water, following the package directions, to redissolve the crystals and it's ready for the next snow day.

Hands-on activity

Find the solubility of salt in your kitchen

You need: a measuring cup, a kitchen scale if you have one (otherwise level teaspoons), table salt, sugar, two clear glasses, a spoon and room-temperature water.

Measure 50 mL of water into a glass.
If you have a scale, weigh the salt container. Add salt one level teaspoon at a time, stirring for a full minute after each, until some salt stays on the bottom no matter how long you stir.
Weigh the container again: the difference is the mass you added. Subtract an estimate of the undissolved salt (or note the number of teaspoons; one level teaspoon of table salt is roughly 6 g).
Calculate your solubility in g/100 mL by doubling your result for 50 mL.
Repeat with sugar in a fresh 50 mL of water. (You may need many teaspoons!)

What to look for: salt saturates at roughly 18 g in 50 mL (36 g/100 mL), about 3 teaspoons. Sugar takes about 100 g in 50 mL. Your values will be rough; list two sources of error (e.g., undissolved salt hiding in the cloudy water, teaspoon sizes, water temperature). Do not taste anything made in a science activity.

Your turn

Name the solute and solvent in a solution of iodine dissolved in ethanol (tincture of iodine).

What does (aq) mean after a formula?

Give an example of a solid solution and name its parts.

The solubility of potassium nitrate is 31.6 g/100 mL at 20 °C. Is 25 g in 100 mL at 20 °C saturated or unsaturated?

How much potassium nitrate can dissolve in 50 mL of water at 20 °C?

Name two ways to make sugar dissolve faster that do not change its solubility.

Oil and vinegar salad dressing separates into layers. What word describes the two liquids?

More practice: warm-up

Is a solution homogeneous or heterogeneous?

In salt water, which is the solvent?

What is the solvent in air?

True or false: a supersaturated solution is stable.

What is an alloy?

More practice: on your own

A student stirs 40.0 g of NaCl into 100 mL of water at 20 °C (solubility 36.0 g/100 mL). How much remains undissolved? Is the solution saturated?

How many grams of sucrose can dissolve in 30 mL of water at 20 °C, if its solubility is 204 g/100 mL?

Explain the difference between 'dilute' and 'unsaturated'.

How could you test whether a clear salt solution is unsaturated, saturated or supersaturated, using one more crystal of salt?

A drink mix dissolves faster in hot water than cold. Does that prove its solubility is higher in hot water? Explain.

Challenge

At 20 °C, 100 mL of water dissolves 36.0 g of NaCl. A student has 1.00 kg of salt and wants all of it dissolved at 20 °C. What minimum volume of water is needed?

Explain, in terms of particles, why a saturated solution in contact with undissolved solid is described as dynamic.

Windshield washer fluid is mostly methanol and water. Methanol and water are miscible. Why does this mixture work better than plain water in a Canadian winter?

Check your understanding

Quiz

In a solution of carbon dioxide in water, the solute is

Which is a solid solution?

The solubility of NaCl at 20 °C is 36.0 g/100 mL. 20.0 g of NaCl in 50 mL of water at 20 °C gives

Which change increases the solubility of most solids in water?

Oil and water are

A solution holds more dissolved solute than its solubility. It is

For parents and teachers

Big idea (E2.1, E3.2): solutions are homogeneous mixtures, and every solute has a limit (its solubility) at each temperature.

How to help: At the kitchen counter, ask your teen to name the solute and solvent in things you use: tea, salad dressing (not a solution: it separates), vinegar, maple syrup, windshield washer fluid. Ask: "How could you tell if it's saturated?"

Common difficulties:

Confusing rate of dissolving with solubility. Stirring makes it faster, not more.
Forgetting that solubility depends on temperature, so a solubility value always needs a temperature.
Mixing up dilute/concentrated (how much is dissolved) with unsaturated/saturated (how much could dissolve).
Scaling: when the volume is not 100 mL, multiply the solubility by (volume ÷ 100 mL).

Safety: windshield washer fluid contains methanol and is poisonous; it is mentioned here only as an example, not for experiments.