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

Concentration: Saturated and Unsaturated Solutions

C2.5 A2.1 A3.2 A1.2About 50 minutes
Learning goals
  • I can describe a solution as dilute or concentrated (qualitative).
  • I can calculate concentration in grams per 100 mL or grams per litre (quantitative).
  • I can explain the difference between saturated and unsaturated solutions.
  • I can follow and plan a simple program that calculates concentration and checks for saturation.

Words to know

concentration
The amount of solute in a certain amount of solution.
dilute
Having a small amount of solute in the solution. A weak solution.
concentrated
Having a large amount of solute in the solution. A strong solution.
qualitative
Describing with words, not numbers (for example, "very concentrated").
quantitative
Describing with numbers and units (for example, "25 g/L").
unsaturated solution
A solution that can still dissolve more solute at that temperature.
saturated solution
A solution that holds as much solute as it can at that temperature. Extra solute stays undissolved.
supersaturated solution
A solution holding more solute than it normally could at that temperature. It is unstable, and crystals can form suddenly.

Too strong, too weak

You make orange drink from frozen concentrate. With too little water, it's super sweet. With too much water, it tastes watery. The difference is the concentration: how much solute is in a certain amount of solution.

Describing concentration with words (qualitative)

  • A dilute solution has a little solute: weak lemonade, slightly salty water.
  • A concentrated solution has a lot of solute: strong syrup, very salty water.
  • To dilute a solution, you add more solvent. The amount of solute stays the same, but it's spread through more solution.

These words are useful, but they're not exact. Concentrated to one person might be dilute to another.

diluteconcentrateda little solutea lot of soluteadd water10 solute particlesstill 10, more spread
Left: the same volume with a little solute (dilute) or a lot (concentrated). Right: diluting adds solvent; the amount of solute stays the same but is spread through more solution.

Describing concentration with numbers (quantitative)

Scientists use numbers:

concentration = mass of solute ÷ volume of solution

Common units are grams per litre (g/L) and grams per 100 mL (g/100 mL).

For small amounts of solute, the volume of the solution is about the same as the volume of water you started with, so we'll use that volume in our calculations.

Remember: 1 L = 1000 mL, so to change g/100 mL to g/L, multiply by 10.

Worked example

Let's calculate concentration

Ella dissolves 30 g of sugar to make 500 mL of lemonade. Find the concentration in g/L and in g/100 mL.

  1. g/L: 500 mL = 0.5 L. 30 g ÷ 0.5 L = 60 g/L.
  2. g/100 mL: 500 mL is 5 groups of 100 mL. 30 g ÷ 5 = 6 g/100 mL.
  3. Check: 6 g/100 mL × 10 = 60 g/L. ✓

The concentration is 60 g/L, which is the same as 6 g/100 mL.

Saturated and unsaturated

Keep adding salt to water, one spoonful at a time, stirring well.

  • At first, each spoonful dissolves. The solution is unsaturated: it can dissolve more.
  • Eventually, no matter how much you stir, some salt stays at the bottom. The solution is saturated: it holds as much solute as it can at that temperature.

In particle terms: there's a limit to how many solute particles the solvent particles can surround and hold apart. Once that limit is reached, any extra solute particles stay together as a solid.

The concentration of a saturated solution is the solubility. For table salt at room temperature, a saturated solution has about 36 g in 100 mL of water.

Supersaturated solutions: if a grown-up dissolves lots of sugar in very hot water, then lets it cool slowly and undisturbed, the cooled solution can hold more sugar than it normally could. It's unstable: a string or a seed crystal makes sugar crystals grow out of it. That's how rock candy is made.

Mass of salt in 100 mL of room-temperature waterUnsaturated or saturated?What you see
10 gUnsaturatedAll dissolves; clear
30 gUnsaturatedAll dissolves; clear
36 gSaturated (just)All dissolves, but no more will
50 gSaturatedAbout 14 g stays undissolved at the bottom
Table salt in 100 mL of water at room temperature (solubility about 36 g/100 mL).
0102030405010 gunsaturated30 gunsaturated36 gsaturated (just)14 g left50 gsaturatedsolubility ≈ 36 gsalt in 100 mL water (g)
The salt table as a graph. Below the dashed line (about 36 g) all the salt dissolves. In the 50 g glass, the extra 14 g (dashed part) can't dissolve and stays at the bottom.
Worked example

Let's decide: saturated or unsaturated?

At room temperature, sugar's solubility is about 200 g/100 mL of water. Jamal stirs 90 g of sugar into 50 mL of water. Is the solution saturated or unsaturated?

  1. Find the concentration per 100 mL: 50 mL is half of 100 mL, so double both: 90 g in 50 mL = 180 g in 100 mL.
  2. Compare with the solubility: 180 g/100 mL is less than 200 g/100 mL.
  3. So more sugar could still dissolve. How much more? 200 − 180 = 20 g per 100 mL, so about 10 g more in his 50 mL.

The solution is unsaturated. About 10 g more sugar could dissolve in his 50 mL of water.

Just for fun

Rock candy is what happens when a supersaturated sugar solution gets too crowded: the extra sugar particles move out and build crystal towers on a string. Tasty science!

# Concentration checker for table salt
SET solubility TO 36 # grams per 100 mL at room temperature
ASK "Grams of salt?" and STORE in mass
ASK "Millilitres of water?" and STORE in volume
SET concentration TO mass / volume * 100 # grams per 100 mL
DISPLAY "Concentration: ", concentration, " g/100 mL"
IF concentration < solubility THEN
DISPLAY "Unsaturated: more salt can dissolve."
ELSE IF concentration = solubility THEN
DISPLAY "Saturated: no more salt can dissolve."
ELSE
DISPLAY "Saturated, with extra salt left at the bottom."
END IF

A program plan (pseudocode). It turns any amounts into g/100 mL, then uses an IF... ELSE IF... ELSE decision to classify the solution. You could build it in Scratch with "ask" and "if/else" blocks.

Hands-on activity

Make a saturated salt solution

You need: table salt, a ½-teaspoon measure, a clear glass, a measuring cup, room-temperature water, a spoon, paper.

  1. Measure 50 mL of water into the glass.
  2. Add ½ teaspoon of salt. Stir for 1 minute. Did it all dissolve?
  3. Keep adding ½ teaspoon at a time, stirring 1 minute each time, and tally the spoonfuls.
  4. Stop when some salt stays at the bottom even after stirring. Your solution is now saturated.
  5. Calculate: a level ½ teaspoon of table salt is roughly 3 g. How many grams dissolved? Turn it into g/100 mL (double it, because you used 50 mL). Compare with the expected value of about 36 g/100 mL.
  6. Code it (optional): build the concentration checker in Scratch or another block-coding tool and test it with your own numbers.

What to look for: about 6 half-teaspoons (about 18 g) dissolve in 50 mL, which is about 36 g/100 mL. Your result may be a bit different, because spoonfuls aren't exact. Why might your answer differ? Talk about measurement error: heaped spoons, salt that's still dissolving, water temperature.

Save it: pour your saturated solution into a shallow dish and leave it on a windowsill for a few days. As the water evaporates, salt crystals appear. (Don't eat it; pour leftovers down the sink.)

Your turn

1

What is concentration?

2

Is "very sweet" a qualitative or quantitative description?

3

20 g of salt is dissolved to make 1 L of solution. What is the concentration in g/L?

4

15 g of sugar is dissolved to make 250 mL of solution. What is the concentration in g/L?

5

What is a saturated solution?

6

How can you make a concentrated juice more dilute?

7

A glass of salt water has salt crystals sitting on the bottom that won't dissolve even after long stirring. Saturated or unsaturated?

More practice: warm-up

1

Strong syrup: dilute or concentrated?

2

How many mL are in 1 L?

3

Change 4 g/100 mL to g/L.

4

If all the solute dissolves and more could dissolve, the solution is...

5

Is "25 g/L" qualitative or quantitative?

More practice: on your own

1

A sports drink has 30 g of sugar in 500 mL. A juice has 50 g of sugar in 500 mL. Which is more concentrated? Give both concentrations in g/L.

2

You add 12 g of salt to 50 mL of room-temperature water. Use the salt solubility (36 g/100 mL) to decide if it is saturated.

3

Explain, using particles, why there is a limit to how much solute can dissolve.

4

A saturated sugar solution at room temperature is heated (by a grown-up). Will it stay saturated? Explain.

5

In the pseudocode program, what does the program display for 9 g of salt in 25 mL of water?

Challenge

1

You need 2 L of a drink with a concentration of 40 g/L of sugar. How much sugar do you need? If you only have 50 g of sugar, how much drink can you make at that concentration?

2

25 g of salt is stirred into 50 mL of room-temperature water. How much salt stays undissolved? (Solubility: 36 g/100 mL.)

3

Add a line to the pseudocode so it also tells you how many more grams of salt could dissolve in the volume of water entered, when the solution is unsaturated.

Check your understanding

Quiz

1.

A solution with only a little solute is...

2.

40 g of sugar is dissolved to make 2 L of solution. The concentration is...

3.

A saturated solution...

4.

Which description is quantitative?

5.

To make a solution more dilute, you...

6.

Salt's solubility is about 36 g/100 mL at room temperature. Is 10 g of salt in 100 mL of water saturated?

7.

Rock candy is grown from a...

For parents and teachers

Big idea: Ontario expectation C2.5 asks students to describe the concentration of a saturated solution qualitatively (words) and quantitatively (numbers), and to tell saturated and unsaturated solutions apart. The pseudocode connects to coding (A2.1), and the calculations use rates and unit conversions from math (A3.2).

How to help: Read nutrition labels together and work out grams of sugar per 100 mL. Ask: "Is this drink more or less concentrated than that one?" If your child codes in Scratch, help them build the checker and test it with numbers from the activity.

Common difficulties:

  • Mixing up concentrated and saturated. A concentrated solution has a lot of solute but may still be unsaturated; a saturated one holds the most it can.
  • Forgetting to convert mL to L (divide mL by 1000).
  • Thinking a saturated solution can never dissolve more. Heating (for most solids) can raise the limit.

Safety: the salt activity uses room-temperature water. If you make rock candy, you do all the heating: hot sugar syrup causes serious burns.