Grade by Grade
C. Matter and Energy · Pure Substances and Mixtures

Solutions and Solubility

C2.4 C2.3 A1.2 A1.4About 45 minutes
Learning goals
  • I can name the solute and the solvent in a solution.
  • I can use the particle theory to explain how a solute dissolves.
  • I can explain what solubility means, and describe how temperature, pressure and the type of substance affect it.

Words to know

solute
The substance that dissolves. In salt water, salt is the solute.
solvent
The substance that does the dissolving. It is usually the substance there is more of. In salt water, water is the solvent.
dissolve
To break apart into separate particles that spread evenly through a solvent.
soluble
Able to dissolve in a certain solvent. Sugar is soluble in water.
insoluble
Not able to dissolve in a certain solvent. Sand is insoluble in water.
solubility
The greatest amount of a solute that can dissolve in a certain amount of solvent at a certain temperature.

Where did the sugar go?

Stir a spoonful of sugar into a cup of warm tea. In a minute, you can't see it anymore. But the tea tastes sweet, so the sugar must still be there. In Grade 5 you learned that dissolving is a physical change: the sugar is still sugar.

Today you'll use the particle theory to explain exactly what happens.

Sugar falling from a spoon into a cup of tea, with a sugar bowl beside it.
The sugar is the solute and the tea water is the solvent. Once dissolved, the sugar is still there even though you can't see it. AI-generated image

The parts of a solution

  • The solute is the substance that dissolves (the sugar).
  • The solvent is the substance that does the dissolving (the water in the tea).
  • Solute + solvent = solution.

The solvent is usually the substance there is more of. In air, nitrogen is the solvent (about 78% of air) and oxygen and other gases are solutes.

Dissolving with the particle theory

  1. Solvent particles are always moving (postulate 4). They bump into the solid solute.
  2. Solvent particles and solute particles attract each other (postulate 6). If the solvent's pull is strong enough, it pulls solute particles off the outside of the solid, one layer at a time.
  3. The freed solute particles spread out into the spaces between the solvent particles (postulate 3), until they are evenly mixed.

If the solvent particles don't attract the solute particles strongly enough, the solute stays together. It is insoluble. Sand in water and oil in water are examples.

1. water particles bumpinto the sugar crystal2. attraction pullsparticles off the surface3. sugar particles spreadout evenly: dissolved(square = sugar particle, circle = water particle)
Dissolving, step by step. Moving water particles hit the sugar, their attraction pulls sugar particles off the surface, and the freed particles spread into the spaces between the water particles.

Solubility and what changes it

The solubility of a substance is the most of it that can dissolve in a certain amount of solvent at a certain temperature. For example, about 36 g of table salt can dissolve in 100 mL of water at room temperature. Sugar is much more soluble: about 200 g dissolves in 100 mL of room-temperature water.

Three things affect solubility:

  • The type of solute and solvent. Salt and sugar are soluble in water, but they don't dissolve in cooking oil. Grease doesn't dissolve in water, but it does dissolve in some other solvents.
  • Temperature. For most solids, solubility goes up as the water gets warmer: faster solvent particles hit the solid harder and more often. For gases, solubility goes down as the liquid gets warmer: gas particles move faster and escape from the liquid. (Table salt is an exception among solids: its solubility barely changes with temperature.)
  • Pressure (for gases). More pressure pushes more gas into a liquid. Soft drinks are bottled under pressure to dissolve lots of carbon dioxide. Open the cap, the pressure drops, and bubbles come out.
050100150200table saltabout 36 gsugarabout 200 ggrams that dissolve in 100 mL of room-temperature water
Solubility drawn to scale. About 36 g of salt but about 200 g of sugar can dissolve in 100 mL of room-temperature water: sugar is more than 5 times as soluble.
ChangeEffect on a solid (like sugar) in waterEffect on a gas (like carbon dioxide) in water
Warmer waterUsually more dissolvesLess dissolves (it fizzes out)
Colder waterUsually less dissolvesMore dissolves
More pressureLittle or no effectMore dissolves
How temperature and pressure affect solubility.
Worked example

Let's explain a flat soft drink

A can of pop left open in a hot car tastes flat much sooner than one left open in the fridge. Explain using solubility and the particle theory.

  1. Identify the solute, solvent and solution: carbon dioxide gas (solute) is dissolved in water (solvent).
  2. Opening the can lowers the pressure, so the gas starts to escape.
  3. In the hot car, the particles move faster. The carbon dioxide particles have more energy and escape from the liquid more easily. The solubility of a gas goes down as temperature goes up.
  4. In the cold fridge, particles move more slowly, so more gas stays dissolved for longer.

Gases are less soluble in warm liquids. In the hot car, the fast-moving carbon dioxide particles escape quickly, so the pop goes flat sooner.

Just for fun

Why won't sugar dissolve in cooking oil?

There's just no attraction between them. Sorry, sugar, it's not you, it's your particles.

Why this matters for fish

Fish breathe oxygen dissolved in water. Warm water holds less dissolved oxygen than cold water. In a very hot summer, shallow ponds and streams can warm up so much that fish struggle to get enough oxygen. Trout, which live in many Ontario streams, need cool water with plenty of oxygen. Shade from trees along a stream helps keep the water cool.

Hands-on activity

Soluble or insoluble?

You need: 6 clear cups, room-temperature water, a measuring spoon, a spoon for stirring, and these food items: salt, sugar, flour, ground black pepper, cooking oil and drink crystals (or instant coffee). Labels or sticky notes.

  1. Put 100 mL of water in each cup and label it.
  2. Predict: which items will dissolve?
  3. Add ½ teaspoon of one item to each cup. Stir 20 times. Use a clean spoon (or rinse it) between cups.
  4. Wait 2 minutes, then look closely. Is the mixture clear and the same throughout? Is anything floating or settled at the bottom?
  5. Make a table: item | soluble or insoluble | evidence.

Part 2: fizzy water and temperature (with a grown-up). Take two unopened bottles of the same plain soda water: one from the fridge, one that has been at room temperature for a few hours. Over a sink, a grown-up opens each one slowly. Which fizzes more? Pour some of each into two glasses and compare the bubbles.

What to look for: salt, sugar and drink crystals dissolve (soluble). Flour makes the water cloudy and settles (insoluble). Pepper floats or sinks (insoluble). Oil floats on top (insoluble). In Part 2, the warmer bottle fizzes more, because gases are less soluble in warm water.

Safety: don't taste the test cups. Opening fizzy drinks can spray, so a grown-up opens them slowly, pointed away from faces.

Your turn

1

In lemonade made from water and lemonade crystals, name the solute and the solvent.

2

What does insoluble mean? Give an example.

3

Use the particle theory to explain how sugar dissolves in water.

4

Define solubility.

5

For most solids, what happens to solubility when the water gets warmer?

6

For gases, what happens to solubility when the water gets warmer?

7

Why are soft drinks bottled under pressure?

More practice: warm-up

1

Solute + solvent = ?

2

Is salt soluble or insoluble in water?

3

Is cooking oil soluble or insoluble in water?

4

In air, which gas is the solvent?

5

About how many grams of salt dissolve in 100 mL of room-temperature water?

More practice: on your own

1

Why do you think sugar dissolves easily in water but not in cooking oil?

2

Sugar's solubility is about 200 g per 100 mL of room-temperature water. Salt's is about 36 g. Which is more soluble? By about how many times?

3

Why is a shaded stream better for trout than an unshaded one?

4

A cold glass of tap water left on the counter slowly forms tiny bubbles on the inside of the glass. Explain.

5

In the soluble or insoluble test, why did we use the same amount of water and the same number of stirs for every item?

Challenge

1

A recipe says to dissolve a lot of sugar in water for a syrup and tells you to heat the water first. Use solubility to explain why.

2

Some factories use river water to cool their machines and then return it to the river warmer. Predict an effect on the river and explain with solubility.

3

Why does a can of pop fizz up more if you open it right after shaking it, even though no gas was added?

Check your understanding

Quiz

1.

In salt water, the salt is the...

2.

Which is insoluble in water?

3.

For most solids, raising the water temperature...

4.

A warm soft drink loses its fizz faster than a cold one because...

5.

Solubility is...

6.

According to the particle theory, a solute dissolves because...

For parents and teachers

Big idea: Ontario expectation C2.4 asks students to use the particle theory to describe how different factors affect solubility (how much dissolves) and the rate of dissolving (how fast). This lesson covers solubility; the next covers rate. The tests practise fair testing (A1.2) and safe handling (A1.4).

How to help: In the kitchen, ask "What is the solute? What is the solvent?" (drink crystals and water, sugar and tea). Talk about why pop goes flat on a hot day.

Common difficulties:

  • Thinking dissolved substances disappear or melt. Dissolving is not melting; the solute is still there, spread out.
  • Mixing up solubility (how much) with rate of dissolving (how fast).
  • Assuming warmth always increases solubility. True for most solids, but the opposite for gases.

Safety: you open the fizzy drinks. If you make sugar syrup, you do all the heating; hot syrup sticks to skin and burns.