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

How Fast Does It Dissolve? Rate of Dissolving

C2.4 A1.2 A1.4 A1.5 A3.2About 50 minutes
Learning goals
  • I can name three factors that change how fast a solute dissolves: temperature, stirring and particle size.
  • I can use the particle theory to explain each factor.
  • I can tell the difference between solubility (how much) and rate of dissolving (how fast).
  • I can plan and carry out a fair test with an independent, dependent and controlled variables.

Words to know

rate of dissolving
How fast a solute dissolves in a solvent.
surface area
The total area on the outside of an object. Breaking a solid into smaller pieces gives it more surface area.
agitation
Stirring or shaking.
independent variable
The one thing you change on purpose in a fair test.
dependent variable
The thing you measure to see the effect of the change.
controlled variables
All the things you keep the same so the test is fair.
trial
One run of an experiment. Doing several trials makes results more reliable.

Hurry up, sugar!

You're making hot chocolate and iced tea from powder on the same afternoon. The hot chocolate powder disappears almost right away, while the iced tea powder sits at the bottom of the cold jug until you stir and stir.

Last lesson was about how much dissolves (solubility). This lesson is about how fast something dissolves: the rate of dissolving.

A mug of hot chocolate topped with cream next to a glass of iced tea with ice and lemon.
Powder dissolves faster in the hot drink because the hot water particles move faster and bump into the solute more often. AI-generated image

Three factors that speed up dissolving

Remember: dissolving happens when moving solvent particles bump into the solute and pull particles off its surface. Anything that makes those collisions happen more often speeds up dissolving.

  1. Temperature. In hot water, solvent particles move faster. They hit the solute more often and harder, so they pull solute particles off faster.
  2. Stirring (agitation). Stirring carries the dissolved particles away from the solid and brings fresh solvent to its surface. Without stirring, the water right next to the solid fills up with dissolved particles and dissolving slows down.
  3. Particle size (surface area). A crushed solid has much more surface area than one big lump. More surface means more places for solvent particles to hit at the same time. Granulated sugar dissolves faster than a sugar cube; icing sugar dissolves faster still.
1 big cubesame cube cut into 4
Cutting a solid into pieces exposes new surfaces. The same amount of solute now has more surface area for the solvent to attack, so it dissolves faster. (Not to scale.)
FactorChange that speeds up dissolvingParticle theory reason
TemperatureWarmer solventFaster solvent particles collide more often and harder
StirringStir or shakeMoves dissolved particles away; brings fresh solvent to the surface
Particle sizeCrush or grind the soluteMore surface area, so more particles exposed to collisions at once
Three ways to speed up dissolving.

How much vs how fast

Stirring and crushing make sugar dissolve faster, but they don't change how much sugar can dissolve (its solubility). Temperature is special: it changes both the rate and (for most solids) the solubility.

Worked example

Let's plan a fair test

Plan a fair test to find out whether particle size affects how fast sugar dissolves.

  1. Question: Does a sugar cube dissolve faster or slower than the same mass of granulated sugar?
  2. Hypothesis: If the sugar is in small grains, then it will dissolve faster, because it has more surface area for water particles to hit.
  3. Independent variable (what I change): the size of the sugar pieces (one cube vs loose granulated sugar).
  4. Dependent variable (what I measure): the time, in seconds, until no sugar can be seen.
  5. Controlled variables (what I keep the same): mass of sugar (weigh one cube on a kitchen scale and weigh out the same mass of granulated sugar), amount of water (200 mL), water temperature (both from the same jug), stirring (none, or the same stirs for both), same cups.
  6. Trials: do the test 3 times and find the average time to make the result more reliable.

A fair test changes only the particle size, measures the dissolving time, and keeps everything else the same, with 3 trials for each.

Worked example

Let's analyze results

Here are sample results from one student's test (no stirring, room-temperature water). Find the average time for each and write a conclusion.

  • Sugar cube: 312 s, 290 s, 305 s
  • Granulated sugar: 95 s, 104 s, 99 s
  1. Average for the cube: (312 + 290 + 305) ÷ 3 = 907 ÷ 3 ≈ 302 s (about 5 minutes).
  2. Average for granulated sugar: (95 + 104 + 99) ÷ 3 = 298 ÷ 3 ≈ 99 s.
  3. Compare: 302 ÷ 99 ≈ 3. The cube took about 3 times as long.
  4. Conclusion: smaller particles dissolved faster. The hypothesis was supported. Reason: more surface area, so more sugar particles were exposed to the water at once.

The granulated sugar dissolved about 3 times faster (99 s vs 302 s on average), which supports the hypothesis.

050100150200250300sugar cube302 sgranulated99 saverage time to dissolve (seconds), no stirring
The sample averages drawn to scale. The granulated sugar bar is about one third as long as the cube's bar: more surface area, faster dissolving.
Just for fun

Icing sugar is the sprinter of the sugar family. It has so much surface area that it dissolves before the water has even finished saying hello.

Hands-on activity

The dissolving race

A grown-up handles the hot water. Use hot tap water, not boiling water.

You need: 6 identical clear cups (or rinse and reuse 2), 6 sugar cubes, cold water from the fridge, hot tap water, a spoon, a measuring cup, a timer and paper.

Test each factor, changing only one thing at a time:

  1. Temperature: cup A = 200 mL cold water, cup B = 200 mL hot tap water. Drop one sugar cube into each at the same time. Don't stir. Time until each cube has completely disappeared (or stop at 10 minutes and describe how much is left).
  2. Stirring: two cups of 200 mL room-temperature water. One cube each. Stir one cup gently and steadily; don't stir the other. Time both.
  3. Particle size: two cups of 200 mL room-temperature water. One whole cube; one cube crushed in a folded piece of paper with the back of a spoon. Time both, no stirring.
  4. Record the times in a table. If you have time, repeat each test and find the average.
  5. Share it: make a bar graph of your times and write a conclusion for each factor, using the particle theory.

What to look for: the cube dissolves faster in hot water, with stirring, and when crushed. Exact times will depend on your sugar and water, but each change should make a clear difference.

Safety: hot tap water can scald; keep the cups on a stable table. Don't eat sugar that has been on the work surface.

Your turn

1

Name three factors that affect the rate of dissolving.

2

Why does sugar dissolve faster in hot tea than in iced tea?

3

How does stirring speed up dissolving?

4

Why does crushed sugar dissolve faster than a sugar cube?

5

Does stirring change the solubility of sugar? Explain.

6

In a fair test of temperature, name two variables you must keep the same.

7

Why do scientists repeat a test 3 times?

More practice: warm-up

1

Which dissolves faster: a whole bouillon cube or a crumbled one?

2

Which dissolves faster: drink crystals in cold or warm water?

3

What is another word for stirring or shaking?

4

In a fair test, what do we call the one thing you change on purpose?

5

What do we call the thing you measure?

More practice: on your own

1

Find the average of these dissolving times: 48 s, 52 s, 56 s.

2

Max tests whether stirring speeds up dissolving. He stirs one cup of hot water and does not stir one cup of cold water. Is this a fair test? Explain.

3

Why do instant drink mixes come as a fine powder instead of one solid block?

4

Make a hypothesis about stirring, using an if... then... because sentence.

5

Explain why temperature is the only one of the three factors that also changes solubility.

Challenge

1

Sample results: a cube in hot water took 45 s, 51 s and 48 s. In cold water it took 240 s, 228 s and 252 s. Find both averages and how many times faster the hot water was.

2

A camper wants to dissolve a powdered drink mix quickly in cold lake water, with no way to heat it. Suggest two things she can do and explain each with the particle theory.

3

Design a fair test to find out whether the type of solute affects the rate of dissolving. Name your independent, dependent and controlled variables.

Check your understanding

Quiz

1.

Which change will make sugar dissolve faster in water?

2.

Crushing a solid speeds up dissolving because it...

3.

The rate of dissolving tells you...

4.

In a fair test about water temperature, the independent variable is...

5.

Which factor changes both the rate of dissolving and the solubility of most solids?

6.

Times: 30 s, 34 s, 32 s. The average is...

For parents and teachers

Big idea: Ontario expectation C2.4 also asks students to use the particle theory to explain the factors that affect the rate at which a substance dissolves. This lesson builds the fair-test skills of the scientific experimentation process (A1.2): variables, trials and averages, which also connects to math (A3.2).

How to help: Let your child run the dissolving race and make a bar graph. Ask: "What did you change? What did you measure? What did you keep the same?"

Common difficulties:

  • Mixing up rate (how fast) and solubility (how much).
  • Changing more than one variable at once.
  • Saying hot water "melts" sugar. The sugar dissolves; it does not melt.

Safety: you pour the hot tap water. Never use boiling water for this test.