E. Solutions and Solubility · Solutions and SolubilityHow Temperature and Pressure Affect Solubility
E3.3 E2.1 A1.6 A1.8About 70 minutes
Learning goals- I can read and interpret solubility curves.
- I can explain how temperature affects the solubility of most solids and of gases in water.
- I can explain how pressure affects the solubility of gases, with everyday examples.
- I can use solubility data to calculate how much solute crystallizes when a saturated solution is cooled.
Words to know
solubility curve
A graph of the solubility of a substance (y-axis) against temperature (x-axis).
crystallization
The formation of solid crystals from a solution, for example when a saturated solution cools.
thermal pollution
Warming of a natural body of water by human activity, such as cooling water released by industry; it lowers dissolved oxygen.
dissolved oxygen (DO)
The concentration of O₂ gas dissolved in water, often in mg/L; aquatic animals need it to survive.
partial pressure
The share of a gas mixture's total pressure that comes from one gas, such as CO₂ above a soft drink.
A warm soda goes flat faster
Open a cold can of sparkling water and a warm one, and the warm one fizzes harder and goes flat sooner. Hot tea dissolves much more sugar than iced tea. And on a hot August day, fish in a shallow pond may gasp at the surface. All three come down to how temperature and pressure change solubility.
Remember from the last lesson
Stirring and crushing change only the rate of dissolving. Temperature changes the solubility (the maximum amount). Pressure matters for gases only.
Solids in water: usually more soluble when hot
For most solid solutes, solubility increases as temperature increases. Warmer water molecules move faster and hit the crystal harder and more often, and the dissolving process for many salts absorbs energy, so adding heat helps.
How much it increases varies a lot:
- Potassium nitrate (KNO₃): from 13.3 g at 0 °C to 246 g per 100 g water at 100 °C, a huge change.
- Sodium chloride (NaCl): from 35.7 g to only 39.2 g: almost flat.
A few ionic solids (for example, lithium sulfate) actually become slightly less soluble when heated, so always check the data.
Scientists display solubility data on a solubility curve. Each point on the curve is a saturated solution. A point below the curve is an unsaturated solution; a point above is supersaturated (or, more often, a saturated solution with extra solid on the bottom).
Units on solubility curves
Many tables give solubility per 100 g of water. Because 100 g of water is very close to 100 mL at room temperature, chemistry courses often treat the two as the same. This lesson uses the values as given.
Worked exampleExample 1: Reading a curve and cooling a solution
Use the KNO₃ data: 31.6 g/100 g water at 20 °C and 110 g/100 g water at 60 °C.
(a) A solution of 50 g KNO₃ in 100 g of water is at 40 °C (solubility 63.9 g). Saturated or unsaturated? (b) A saturated solution of KNO₃ in 100 g of water at 60 °C is cooled to 20 °C. What mass of crystals forms? (c) Repeat (b) for 250 g of water.
- (a) 50 g < 63.9 g, so it is unsaturated (the point is below the curve).
- (b) At 60 °C the water holds 110 g. At 20 °C it can hold only 31.6 g. The rest must crystallize: 110 − 31.6 = 78.4 g of crystals.
- (c) Scale by 250 g ÷ 100 g = 2.5: crystals = 78.4 g × 2.5 = 196 g.
(a) unsaturated; (b) 78.4 g; (c) 196 g. This is how chemists purify solids by recrystallization.
Gases in water: less soluble when hot
Gases behave the opposite way. As water warms, dissolved gas molecules gain energy and escape more easily, so the solubility of a gas decreases as temperature increases.
This matters for living things. Fish and other aquatic animals breathe dissolved oxygen. Cold water holds about 14.6 mg/L of O₂ (at 0 °C, in contact with air), but water at 30 °C holds only about 7.6 mg/L. Cold-water fish such as brook trout need cool, oxygen-rich streams.
When factories or power plants return warmed cooling water to a lake or river, the warmer water holds less oxygen. This is called thermal pollution. Removing shade trees along a stream can have a similar warming effect.
Pressure and gas solubility
The solubility of a gas increases when the pressure of that gas above the liquid increases. More gas molecules hit the liquid surface each second, so more dissolve. (For solids and liquids, pressure has almost no effect.)
- Carbonated drinks are sealed with CO₂ at a pressure several times higher than air pressure, which forces extra CO₂ into solution. When you open the bottle, the CO₂ pressure above the liquid drops, the drink is now supersaturated with CO₂, and bubbles form.
- Scuba divers breathe air at high pressure deep underwater, so extra nitrogen dissolves in their blood. If they rise too quickly, the pressure drops and the nitrogen can form bubbles in the body (decompression sickness, "the bends"). Divers ascend slowly to let the gas leave gradually.
You will study gas pressure in detail in the Gases and Atmospheric Chemistry unit.
| Type of solute | Higher temperature | Higher pressure |
|---|
| most solids | solubility increases | almost no effect |
| gases | solubility decreases | solubility increases |
Worked exampleExample 2: Explaining an observation
On a hot, still July afternoon, an angler notices fish crowding near a cold spring that feeds into a shallow pond. Use solubility to explain.
- Identify the factor: temperature. The pond is warm; the spring water is cold.
- Apply the rule for gases: oxygen is less soluble in warm water, so the warm pond water holds less dissolved oxygen.
- Link to the fish: the cold spring water holds more dissolved oxygen, so fish gather there to breathe more easily.
Warm water holds less dissolved O₂; the cold spring water holds more, so the fish move there.
Just for funA chemistry student puts two bottles of ginger ale in the fridge and one on a sunny windowsill. A week later the windowsill bottle sprays when opened. The student's lab report title: "Under Pressure: A Gas Solubility Story." (Always open warm fizzy drinks over a sink, pointed away from faces.)
Hands-on activityFizz and temperature
You need: two small unopened bottles of the same carbonated water (or soda), a fridge, two identical clear glasses, a sink, and a grown-up nearby.
- Put one bottle in the fridge for several hours. Leave the other at room temperature.
- Over a sink, with the caps pointed away from everyone, open both bottles slowly. Note which hisses more.
- Pour equal amounts into the two glasses at the same moment. Observe bubbling for 2 minutes.
- Bonus (sugar): in two cups put 100 mL of cold tap water and 100 mL of hot tap water (not boiling; a grown-up handles the hot water). Add sugar a level teaspoon at a time to each until no more dissolves, stirring well. Record how many teaspoons each holds.
What to look for: the warm drink hisses and bubbles more, because CO₂ is less soluble at higher temperature. More sugar dissolves in the hot water, because sucrose is more soluble at higher temperature. Write one sentence for each result linking the observation to solubility. (Do not drink anything used in the activity if it has been sitting out.)
Your turn
1Using the data in this lesson, what is the solubility of KCl at 60 °C?
2Which of KNO₃, KCl and NaCl has a solubility that changes least with temperature?
NaCl (35.7 g to 39.2 g from 0 °C to 100 °C).
3A solution contains 40 g KCl in 100 g of water at 60 °C. Saturated or unsaturated?
Unsaturated, because 40 g < 45.8 g.
4A saturated solution of KCl in 100 g water at 80 °C (51.3 g) is cooled to 20 °C (34.2 g). What mass crystallizes?
5How does the solubility of oxygen change as water warms?
It decreases (about 9.1 mg/L at 20 °C vs 7.6 mg/L at 30 °C).
6Why do soft drinks fizz when opened?
The pressure of CO₂ above the liquid drops, so CO₂ becomes less soluble and comes out of solution as bubbles.
7What is thermal pollution and why does it harm fish?
Warming of natural water by human activity (e.g., industrial cooling water). Warm water holds less dissolved oxygen, so fish can suffocate.
More practice: warm-up
1What does every point on a solubility curve represent?
A saturated solution at that temperature.
2True or false: increasing pressure greatly increases the solubility of salt in water.
False. Pressure has almost no effect on solids.
3What is the solubility of KNO₃ at 20 °C?
4Does a gas become more or less soluble in water when it is cooled?
5Name the gas that dissolves in a diver's blood and causes the bends on a fast ascent.
More practice: on your own
1Using the curve, estimate the temperature at which KNO₃ and KCl have the same solubility.
About 22 to 25 °C (where the curves cross, at roughly 34 to 36 g per 100 g water). Any answer in this range read from the graph is fine.
2A saturated KNO₃ solution in 50 g of water at 40 °C (63.9 g/100 g) is cooled to 0 °C (13.3 g/100 g). What mass of crystals forms?
Per 100 g: 63.9 − 13.3 = 50.6 g. For 50 g of water: 50.6 × 0.5 = 25.3 g.
3Why is NaCl a poor choice for purifying by cooling a hot saturated solution?
Its solubility barely changes with temperature (39.2 g at 100 °C vs 35.7 g at 0 °C), so very little crystallizes on cooling: only about 3.5 g per 100 g of water.
4A stream runs through a farm field. Explain how planting trees along its banks could help trout.
Trees shade the water, keeping it cooler. Cooler water holds more dissolved oxygen, which trout need.
5Explain, in terms of particles, why a gas is less soluble in warm water.
Dissolved gas molecules gain kinetic energy as the water warms, so more of them have enough energy to escape from the liquid into the air; fewer stay dissolved.
Challenge
1At 60 °C, 80.0 g of KNO₃ is dissolved in 100 g of water. Using the data (110 g at 60 °C, 63.9 g at 40 °C, 31.6 g at 20 °C), describe what happens as it cools to 20 °C, including about when crystals first appear and how much solid forms by 20 °C.
At 60 °C it is unsaturated (80.0 < 110). Crystals first appear when the solubility drops to 80.0 g, between 40 °C and 60 °C (about 47 °C from the curve). By 20 °C: 80.0 − 31.6 = 48.4 g of crystals.
2A litre of water at 5 °C is saturated with oxygen from the air. It is warmed to 25 °C in a sealed full bottle and then opened. Predict and explain what you would see in the bottle before it is opened.
Small bubbles form on the inside walls, because O₂ (and N₂) are less soluble at 25 °C than at 5 °C; the extra dissolved gas comes out of solution as the water warms. (You see the same tiny bubbles in a glass of cold tap water left to warm up.)
3A hot spring's water is supersaturated with calcium carbonate as it reaches the surface and cools and releases CO₂. Suggest why rock deposits (travertine) form around the spring, using ideas about solubility.
Underground, high pressure keeps more CO₂ dissolved, which helps keep more calcium carbonate dissolved. At the surface the pressure drops, CO₂ escapes, and the water can no longer hold as much calcium carbonate, so it precipitates as solid rock around the spring.
Quiz
1. For most solid solutes, raising the temperature
2. For gases dissolved in water, raising the temperature
3. A point below a solubility curve represents
4. A saturated solution of KNO₃ in 100 g water at 40 °C (63.9 g) is cooled to 20 °C (31.6 g). Mass of crystals:
5. Which change increases the amount of CO₂ that dissolves in water?
6. Thermal pollution harms fish mainly because warm water
Big idea (E3.3): temperature changes solubility (solids usually up, gases down); pressure changes the solubility of gases only.
How to help: Ask your teen to read a value off the solubility curve and explain what 'below the curve' means. Ask why a warm soda goes flat faster, or why trout like shaded streams.
Common difficulties:
- Applying the 'hotter dissolves more' rule to gases. Gases are the opposite.
- In cooling problems, subtracting in the wrong order, or forgetting to scale when the water is not 100 g.
- Reading graph values carelessly: encourage them to draw light guide lines with a ruler.
Safety: open carbonated drinks over a sink, pointed away from faces; a grown-up should handle hot tap water.