Dissolving, solutions & solubility
In this topic you will:
- correctly use the scientific terms associated with dissolving
- investigate the properties of solutions
- practise measuring mass and volume
- make solutions of different concentrations
- compare the number of solute particles in solutions of different concentrations
- investigate solubility
- compare the solubility of various solutes
Key words
- conserved
- dissolving
- opaque
- solute
- solution
- solvent
- transparent
- concentrated
- dilute
- insoluble
- saturated
- solubility
- soluble
What is a solution?
When you place a lump of sugar in water, the sugar seems gradually to disappear. The sugar is dissolving. You are left with a colourless solution. The substance that dissolves is called the solute. The substance that it dissolves into is called the solvent.
A solution is a mixture. So, in our example, the colourless solution is a mixture of sugar and water. Although the sugar seems to disappear, it is still there. The sugar particles have simply spread out among the water particles.
The diagrams below show what happens to the sugar particles when the cube dissolves.
The sugar crystal is visible because it is made of lots of groups of vibrating particles that are tightly packed together.
As the water particles vibrate and slide past one another, they bump into the vibrating sugar particles. The movement helps to separate the sugar particles and they get mixed up with the water particles.
Eventually, the water particles separate all the sugar particles. The sugar particles are no longer in groups and are too small to be seen.
Important Concept
A solution is a mixture in which a substance (solute) dissolves in another (solvent), and the particles spread evenly, becoming too small to see.
All solutions are transparent. This means you can see through them. Transparent doesn’t mean colourless. For example, if you dissolve a coloured salt, such as copper sulfate, the solution formed is blue. But you can still see through it. It is still transparent.
A liquid such as milk is not transparent. You cannot see through it. It is opaque. Because of this, you can tell that milk is not a solution.
It is easy to confuse melting with dissolving. Remember: dissolving needs two substances, a solute and a solvent.
| Examples of dissolving | Examples of melting |
|---|---|
| Sugar (solute) in black tea (solvent) | Butter in a frying pan |
| Instant coffee (solute) in hot water (solvent) | Ice cream on a warm day |
| Nail polish (solute) in nail polish remover (solvent) | Candle wax as the candle burns |
Common Mistake
Students often confuse melting with dissolving. Melting involves only one substance changing state, while dissolving always requires both a solute and a solvent.
Think Like a Scientist
Dissolving and Mass
You will need: top pan balance • filter paper • salt • stirring rod • beaker of water
Step 1. Place the filter paper on the top pan balance. Measure and record its mass.
Step 2. Add about 20–25 g of salt. This is the solute. Measure and record the mass of the salt.
Step 3. Remove the paper and salt from the balance.
Step 4. Place a beaker containing at least 100 cm³ of water on the top pan balance. Measure and record the mass of the water and the beaker together. The water is the solvent.
Step 5. Add the salt to the water. Stir until all the salt has dissolved. Measure the mass of the beaker and salt solution.
Questions
Show Answer
The mass of salt used was the difference between the mass of the paper with salt and the mass of the paper alone.
Show Answer
This value is recorded from the balance after placing the beaker with 100 cm³ water.
Show Answer
This is the final mass after the salt has been added and fully dissolved.
Show Answer
It shows that the salt has completely dissolved and the total mass is conserved in the solution.
Questions
Show Answer
Sugar is the solute, and water is the solvent.
Show Answer
Dissolving involves mixing a solute into a solvent to form a solution. Melting is when a solid changes into a liquid due to heat, with no second substance involved.
Show Answer
Mass of solution = mass of solute + mass of solvent
= 9 g + 50 g
= 59 g
Show Answer
No, a solution has not been formed. In a true solution, the solute dissolves completely. Since the water is cloudy and lumps remain, the powder has not fully dissolved.
Show Answer
Place the measuring cylinder on a flat surface and check the level at eye level, reading from the bottom of the meniscus.
Solutions
A solution is made when a solute is dissolved in a solvent. More particles of the solute are dissolved in a concentrated solution than in a dilute solution.
A dilute solution of sugar has fewer sugar particles (right)
Quick Fact
The more solute particles in a given volume of solvent, the more concentrated the solution. Fewer solute particles mean the solution is more dilute.
Think Like a Scientist
Making Different Concentrations of a Solution
You will need: safety glasses • test tubes • test tube rack • pipette • two measuring cylinders, suitable for measuring 10 cm³ • concentrated solution of food dye • beaker of water
Step 1. Carefully measure out 10 cm³ of the concentrated food dye solution. When you have added about the correct volume, you can use the pipette to add or remove the final amount drop-by-drop, so that your measurement is as accurate as possible. Place it in a test tube and leave it in the test tube rack. This is solution A.
Step 2. Carefully measure another 8 cm³ of the concentrated food dye solution. Pour it into a test tube.
Step 3. Measure out 2 cm³ water and add it to the 8 cm³ of food dye. Leave it in the test tube rack. This is solution B.
Step 4. Use the table below to make up food dye solutions C, D and E. Place the solutions in the test tube rack, in order, from A–E.
| Solution | Volume of concentrated food dye solution in cm³ | Volume of water in cm³ | Total volume in cm³ |
|---|---|---|---|
| A | 10 | 0 | 10 |
| B | 8 | 2 | 10 |
| C | 6 | 4 | 10 |
| D | 4 | 6 | 10 |
| E | 2 | 8 | 10 |
Step 5. Look carefully at the solutions you have made.
Questions
Show Answer
Each solution becomes lighter in color as the concentration of food dye decreases from A to E.
Show Answer
The least concentrated solution has the lightest color and the highest ratio of water to dye — this is solution E.
Show Answer
Not visually — salt and sugar solutions are usually colorless, so you would need to label or measure them to compare concentration.
Show Answer
Accurate measurements ensure that each solution has the intended concentration and allows fair comparison.
Show Answer
It may be less accurate for small volumes like 2 cm³ or 4 cm³, as the markings on a 100 cm³ cylinder may not be precise enough.
Show Answer
The most concentrated solution (A) contains the highest number of dye particles, while the most dilute (E) contains the fewest, due to increased water volume.
Solubility
A solid that dissolves in a solvent such as water is said to be soluble. Sodium chloride (common salt) and sugar are soluble.
A solid that will not dissolve in water is insoluble. Iron filings are insoluble in water.
If you keep adding a soluble solid to a beaker of water, there comes a point where no more of the solid will dissolve. You have made a saturated solution.
Some soluble substances are more soluble than others. If you have 100 cm³ of water, you would be able to dissolve a lot of sodium chloride in it, but only a tiny amount of lead chloride. Sodium chloride has greater solubility than lead chloride.
Important Concept
Solubility is a measure of how much of a substance can dissolve in a solvent before the solution becomes saturated. Substances with higher solubility dissolve in greater amounts.
Comparing solubility
To compare the solubility of different solutes you must measure how much of each solute will dissolve in a known amount of the solvent.
The table shows the solubility of different salts. It shows how much of each salt can be dissolved when it is added to 100 g of water at 20 °C.
| Solute | Solubility in grams of solute per 100 g of the solvent at 20 °C |
|---|---|
| sodium chloride | 36 |
| copper sulfate | 32 |
| calcium chloride | 74 |
| potassium chlorate | 7 |
| lead chloride | 1 |
Common Mistake
Students sometimes think a higher number in the table means the solute is better. Be careful — this number only tells you how much will dissolve, not whether the solute is safe or useful.
Questions
Show Answer
A saturated solution is one in which no more solute can dissolve at a given temperature — the maximum amount has been dissolved.
Show Answer
32 g of copper sulfate will dissolve in 100 g of water at 20 °C.
Show Answer
7 g dissolves in 100 g → for 200 g of water:
7 × 2 = 14 g of potassium chlorate will dissolve.
Show Answer
36 g dissolves in 100 g → for 50 g of water:
36 ÷ 2 = 18 g of sodium chloride will dissolve.
Show Answer
Create a bar chart with solutes on the x-axis and solubility (g/100 g water) on the y-axis:
- Calcium chloride: 74 g
- Sodium chloride: 36 g
- Copper sulfate: 32 g
- Potassium chlorate: 7 g
- Lead chloride: 1 g
Temperature and solubility
Most solutes will dissolve more quickly and easily in hot water than in cold water. Think about what happens to the particles when they have more energy. The more energy the particles have, the more they vibrate and move.
You can dissolve a greater mass of a solute in hot water than in the same volume of cold water. In other words, as the temperature increases, the solubility of most solutes also increases.
For example, if you have 100 g of water at 20 °C you can dissolve 204 g of sugar in it. If you heat the water to 80 °C, you can dissolve 362 g of sugar in it.
Worked Example: Sugar Solubility
204 g of sugar dissolves in 100 g of water at 20 °C.
Show Answer
200 g is twice as much as 100 g, so:
204 × 2 = 408 g of sugar will dissolve.
Show Answer
In 1 g water: 204 ÷ 100 = 2.04 g sugar dissolves
In 50 g water: 2.04 × 50 = 102 g sugar dissolves
Questions
Show Answer
In 100 g water → 204 g sugar dissolves
So in 250 g water:
\[ 204 \div 100 \times 250 = 510 \text{ g} \]
510 g of sugar will dissolve at 20 °C.
Show Answer
At 80 °C, 362 g sugar dissolves in 100 g water
So in 250 g water:
\[ 362 \div 100 \times 250 = 905 \text{ g} \]
Extra sugar that can dissolve compared to 20 °C:
\[ 905 - 510 = \boxed{395 \text{ g more}} \]
Comparing the solubility of different salts
This table and the graph below show the solubility of three salts at a range of temperatures. Study both carefully to compare how the solubility of each salt changes with temperature.
| Temperature in °C | Potassium nitrate (g per 100 g water) |
Sodium nitrate (g per 100 g water) |
Copper sulfate (g per 100 g water) |
|---|---|---|---|
| 0 | 14 | 73 | 13 |
| 10 | 21 | 81 | 15 |
| 20 | 32 | 88 | 17 |
| 30 | 45 | 96 | 20 |
| 40 | 63 | 105 | 30 |
| 50 | 84 | 114 | 35 |
| 60 | 108 | 124 | 40 |
| 70 | 136 | 134 | 47 |
| 80 | 168 | 148 | 56 |
Note: The graph shows that potassium nitrate’s solubility increases very rapidly with temperature, more than the other two salts. Sodium nitrate also increases steadily, while copper sulfate increases slowly and remains the least soluble across all temperatures shown.
Questions
Show Answer
The solubility of all three salts increases as the temperature increases. This means they dissolve more easily in hotter water.
Show Answer
From the table:
At 40 °C → 63 g
At 50 °C → 84 g
Estimate halfway between:
\[ (63 + 84) \div 2 = \boxed{73.5 \text{ g}} \]
Show Answer
From the table at 10 °C:
- Sodium nitrate = 81 g
- Potassium nitrate = 21 g
- Copper sulfate = 15 g
So the most soluble is sodium nitrate.
Show Answer
At 80 °C:
- Potassium nitrate = 168 g
- Sodium nitrate = 148 g
- Copper sulfate = 56 g
The most soluble is potassium nitrate.
Other solvents
Water is not the only solvent. Some substances that are insoluble in water will dissolve in other solvents.
For example, some types of oil paint are not soluble in water. So, if you need to clean your brushes after you’ve used oil paint, you will need to use a solvent that the paint will dissolve in, such as methanol (methylated spirits).
Nail polish does not dissolve in water but dissolves in nail polish remover. Most nail polish remover contains the solvent propanone (acetone).
