grade 7 Solubility & Solubility Curve

Solubility & Solubility Curve

Dissolving mixes a solute evenly through a solvent. When no more will dissolve the solution is saturated, and solubility is the maximum grams in 100 g of water. Most solids dissolve better as temperature rises, and cooling can recrystallize excess solute. Change the salt amount and the temperature and saturation and the solubility curve appear.

Dissolving and Saturation

Salt amount20 g

Temperature40°C

💡 What is Dissolving?

①Dissolving: solute spreads evenly in the solvent

②Solute: substance that dissolves (salt, sugar)

③Solvent: substance that dissolves it (water)

④Saturated solution: maximum solute dissolved, no more accepted

Solubility Curve

Temperature40°C

🔍 Reading a Solubility Curve

①Solubility: max grams of solute per 100 g of water

②Most solids: temperature↑ → solubility↑

③KNO₃ shows a steep increase with temperature

④NaCl is nearly constant with temperature

Solubility and Recrystallization

Solubility Definition

S = solute (g)100 g water

Depends on temperature and solute type

Recrystallization

Hot saturated → cool → crystals precipitate

Cooling lowers solubility so excess crystallizes out

Precipitation Amount

Precipitate = hot solubility − cold solubility

e.g. KNO₃ cooled 60°C→20°C precipitates 110 − 32 = 78 g

🧪 Recrystallization Principle

①Dissolve a lot at high temperature → cool

②Lower temperature → less can dissolve

③Excess precipitates as crystals

④Most effective for substances with large solubility differences (KNO₃)

Exam Key Points

Core Summary

Solids: temp↑ → mostly ↑ | Gases: temp↑ → ↓

Solids generally dissolve more when hotter; gases the opposite

🎯 Exam Key Points

①Solubility: max solute per 100 g water

②Saturated solution: no more dissolves

③Recrystallization: pure crystals via temperature change

④NaCl is hard to recrystallize (small variation)

⑤Gas solubility: decreases with temp, increases with pressure (soda)

Examples and Unit Test

Example 1

At a certain temperature, salt dissolved in 100 g of water up to 36 g and then no more dissolved. What is the solubility of salt at this temperature, and the state of this solution?

Solubility is the maximum amount of solute (in g) that can dissolve in 100 g of water, and a state where no more dissolves is a saturated solution.

Since up to 36 g dissolves in 100 g of water and no more, the solubility is 36 and this is a saturated solution.

▸ Solubility = 36 (g per 100 g water) / saturated solution

Solubility is the maximum solute that dissolves in 100 g of water, and that state is a saturated solution.

Example 2

The solubility of potassium nitrate is 110 at 60°C and 32 at 20°C. If a saturated solution at 60°C (100 g water) is cooled to 20°C, how much potassium nitrate crystallizes out?

On cooling, the solubility drops, and the decrease crystallizes out (amount = high-temperature solubility - low-temperature solubility).

110 - 32 = 78, so 78 g crystallizes out.

▸ 78 g

Amount crystallized = high-temp solubility - low-temp solubility; a bigger difference gives more crystals.

Unit-test style

Which statement about solubility is correct?

①Solubility is the maximum solute that dissolves in 100 g of solvent

②For most solids, higher temperature gives lower solubility

③A saturated solution can still dissolve more solute

④For a gas, higher temperature gives higher solubility

⑤All substances have the same solubility regardless of temperature

▸ ① Solubility is the maximum solute that dissolves in 100 g of solvent

Solubility is defined as the maximum amount of solute (in g) that dissolves in 100 g of solvent (water) at a given temperature.

For most solids, higher temperature gives higher solubility (②), a saturated solution dissolves no more (③), for a gas higher temperature gives lower solubility (④), and solubility depends on the substance and temperature (⑤).

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Pure Substances & Mixtures

Properties of Gases

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