Science · Solutions & reactions
Solutions, Concentration, and Solubility
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Start here: key ideas
- A solute dissolves in a solvent to make a solution.
- Concentration is solute amount divided by solution volume.
- Adding solvent lowers concentration.
- Solubility is the maximum amount that can dissolve under given conditions.
- Polar substances tend to dissolve in polar solvents.
What you’ll be able to do
- Identify solute and solvent.
- Calculate or compare concentration.
- Tell concentration from solubility.
- Predict effects of temperature and polarity.
Identify solution parts, compare concentrations, and predict simple solubility changes.
Know that a homogeneous mixture looks uniform throughout.
Solute and solvent
A solution is a uniform mixture. The solute is dissolved. The solvent does the dissolving and is often present in the greater amount.
In salt water, salt is the solute and water is the solvent. Solutions may be solid, liquid, or gas; they are not limited to water.
Optional examples beyond water
Roles also apply to nonliquid mixtures: nitrogen is the main solvent in air; copper can be the solvent in brass.
Comparing amounts per volume
Concentration tells how much solute is present in a given volume of solution. A simple form is concentration = solute amount ÷ solution volume.
Example: 10 g in 2 L is 5 g/L. Adding solute raises concentration if volume is otherwise unchanged. Adding solvent dilutes the solution and lowers concentration.
Worked example. A solution has 12 g of solute in 3 L. Its concentration is 12 ÷ 3 = 4 g/L. If the volume is doubled to 6 L by adding solvent while solute stays 12 g, the new concentration is 2 g/L.
Supporting unit: moles per liter
Molarity is moles of solute per liter of final solution: M = n/V. The volume is solution volume, not just the solvent poured in. With solute amount unchanged, M1V1 = M2V2. Worked dilution: 0.8 mol/L × 25 mL ÷ 100 mL final volume = 0.20 mol/L. The volume ratio is unitless because both volumes use mL; 100 mL is the final volume, not 100 mL added.
How much can dissolve?
Solubility is the greatest amount of solute that can dissolve under certain conditions. A saturated solution holds that maximum. Extra solute remains undissolved.
Many solids become more soluble as temperature rises, but gases usually become less soluble in warmer liquids. Polar solutes tend to dissolve in polar solvents; nonpolar solutes tend to dissolve in nonpolar solvents.
Concentration tells what is present now. Solubility tells the maximum that could dissolve. A dilute solution can still be saturated if the substance has very low solubility.
Dissolving speed is not solubility. Stirring or crushing often makes a solid dissolve sooner without increasing its final solubility at that temperature. To read a solubility table, compare the added mass with the maximum for the stated solvent mass and temperature. If the solvent mass doubles, the maximum dissolved mass doubles at the same conditions.
At fixed temperature, increasing a gas’s partial pressure above a solution generally increases how much of that gas dissolves. Opening a carbonated drink lowers the carbon dioxide pressure above it, so dissolved gas escapes as bubbles. This rule concerns gas pressure and the same gas, liquid and temperature—not pressure effects on every solid solute.
OpenStax College, Chemistry (CC BY archive) · CC BY 3.0 · https://github.com/philschatz/chemistry-book/blob/a6ac3918e61cbedf0589c5b43e748dc8a7edf5e2/resources/CNX_Chem_11_03_solubility.jpg · https://creativecommons.org/licenses/by/3.0/ · Original source image reproduced unchanged; explanatory caption added.
Key terms
- solute: The substance that is dissolved.
- solvent: The substance that does the dissolving.
- solution: A uniform mixture of solute and solvent.
- concentration: The amount of solute in a given amount of solution.
- solubility: The maximum amount of solute that can dissolve under stated conditions.
- saturated: Holding the maximum dissolved solute under the current conditions.
Summary
- A solution contains a solute and solvent.
- Concentration compares solute amount with solution volume.
- Dilution adds solvent and lowers concentration.
- Solubility is a maximum under stated conditions.
- Like dissolves like is a useful polarity guide.
Solute gets dissolved; solvent does the dissolving. More solute or less volume means greater concentration.
Sources
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