When a solid dissolves in a liquid, its particles spread evenly throughout the liquid, forming a solution. At particle level, solvent molecules are attracted to solute particles, pull them away from the surface, and surround them — continuing until the solid has fully dissolved or the solution becomes saturated.
What are solute, solvent, and solution?
Three terms form the basic vocabulary of solutions:
- Solute: the substance that dissolves (e.g. salt, sugar).
- Solvent: the liquid that does the dissolving (e.g. water, ethanol).
- Solution: the mixture formed when the solute dissolves in the solvent.
A solution is a homogeneous mixture — it has the same composition throughout. If you take a sample from the top and a sample from the bottom of a well-stirred salt solution, both contain the same concentration of salt. This distinguishes a solution from a suspension (e.g. muddy water), where particles settle over time.
Water is the most common solvent in everyday life and is called the universal solvent because it dissolves a wider variety of substances than any other liquid.
What is solubility and how is it measured?
Solubility is defined as the maximum mass (in grams) of a solute that will dissolve in 100 g of water at a specified temperature to form a saturated solution.
Units: g per 100 g water (also written g/100 g or g per 100 cm³ in practice).
| Substance | Solubility at 20 °C | Solubility at 60 °C |
|---|---|---|
| Sodium chloride (salt, NaCl) | 36 g / 100 g water | 37 g / 100 g water |
| Potassium nitrate (KNO₃) | 32 g / 100 g water | 110 g / 100 g water |
| Copper(II) sulfate (CuSO₄) | 20 g / 100 g water | 40 g / 100 g water |
| Carbon dioxide (CO₂, gas) | 0.17 g / 100 g water | 0.058 g / 100 g water |
Key observations from this table:
- Most solid solutes: solubility increases with temperature.
- Gases: solubility decreases with temperature (important for understanding fizzy drinks and dissolved oxygen in rivers).
What is a saturated solution?
A saturated solution contains the maximum possible amount of dissolved solute at a given temperature. If you add more solute to a saturated solution, it will not dissolve — it simply sinks to the bottom.
Worked example: At 20 °C, the solubility of potassium nitrate (KNO₃) is 32 g per 100 g of water.
Question: What is the maximum mass of KNO₃ that will dissolve in 250 g of water at 20 °C?
Solution:
- 32 g dissolves in 100 g water.
- In 250 g water: 32 × (250 ÷ 100) = 32 × 2.5 = 80 g of KNO₃
If you try to dissolve more than 80 g in 250 g of water at 20 °C, the excess stays undissolved.
How does temperature affect solubility?
For most solid solutes, dissolving is an endothermic process — it absorbs energy from the surroundings. By Le Chatelier's principle (introduced more fully at GCSE level), raising temperature shifts the equilibrium towards dissolution, so more solid dissolves as temperature rises.
For gases, the reverse is true: dissolving is exothermic. Raising temperature shifts the equilibrium towards the gas leaving solution, so gases are less soluble at higher temperatures. This is why:
- Warm fizzy drinks go flat faster — CO₂ is less soluble in warm water.
- Warm river water holds less dissolved oxygen than cold water — a problem for fish in hot weather.
A solubility curve is a graph of solubility (y-axis, g/100 g water) against temperature (x-axis, °C). Points on the line represent saturated solutions; points below the line represent unsaturated solutions; points above the line (briefly achievable by careful cooling) represent supersaturated solutions.
How can you separate a dissolved solid from a solution?
Several separation techniques apply:
- Evaporation: heat the solution to evaporate the solvent. Good for heat-stable solutes (e.g. salt from seawater), but destroys crystals.
- Crystallisation: slowly evaporate some solvent, then allow the solution to cool slowly. As temperature falls, solubility decreases and the excess solute deposits as well-formed crystals. This technique is used industrially to purify substances such as sugar and salt.
- Filtration: if the solid is insoluble (not a true solution), filtration separates the solid from the liquid. Filtration does not work for dissolved solutes — they pass through the filter paper with the solvent.
What affects the rate of dissolving?
Solubility tells you the maximum amount that will dissolve; the rate of dissolving depends on conditions:
| Factor | Effect on rate | Particle explanation |
|---|---|---|
| Increasing temperature | Faster | Particles have more kinetic energy; more frequent, energetic collisions with solute surface |
| Stirring or shaking | Faster | Removes saturated solution from around the solute, replacing it with fresh solvent |
| Smaller particle size (crushing) | Faster | More surface area exposed to solvent; more contact between solute and solvent particles |
Frequently asked questions
Why does table salt (NaCl) dissolve in water?
Water molecules are polar — one end is slightly positive (hydrogen side) and the other is slightly negative (oxygen side). Sodium chloride is made of positively charged Na⁺ and negatively charged Cl⁻ ions in a giant lattice. Water molecules are attracted to these ions: the negative oxygen end surrounds Na⁺ ions, and the positive hydrogen end surrounds Cl⁻ ions. This attraction pulls individual ions away from the lattice surface one by one, until all the salt has dissolved (or the solution becomes saturated).
What is the difference between soluble, insoluble, and slightly soluble?
A soluble substance dissolves readily in a given solvent — typically more than 1 g per 100 g of water at room temperature. An insoluble substance does not dissolve meaningfully, remaining as a solid even after stirring. A slightly soluble substance dissolves only in small amounts — calcium carbonate (limestone) dissolves about 0.0013 g per 100 g of water at 20 °C, which is enough to make water "hard" but not enough to observe visually. The distinction matters in chemistry because many reactions form insoluble precipitates that drop out of solution.
Why do fish die in very warm water?
Warm water holds less dissolved oxygen than cold water, because the solubility of gases decreases with increasing temperature. If a river or lake warms significantly — due to hot weather, discharge of warm industrial cooling water, or sewage decomposition — dissolved oxygen levels fall. Fish and other aquatic organisms that rely on dissolved oxygen for respiration may suffocate. This is one of the ecological impacts of thermal and organic pollution in waterways.
Can a solution become too concentrated to stay as a solution?
Yes — if the concentration exceeds the solubility at that temperature, excess solute comes out of solution. On cooling a saturated solution, solubility falls, so some of the dissolved solute crystallises out. In a carefully prepared supersaturated solution (e.g. honey), the concentration actually exceeds solubility, but no crystals form until a "seed" crystal or disturbance triggers rapid crystallisation — a process you might have seen when honey suddenly goes solid.
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