Solutions & Concentration

A solution is a homogeneous mixture in which one substance, called the solute, is evenly dispersed in another substance, called the solvent. Solutions are central to chemistry because reactions often happen in water or other liquids where particles can move and collide easily. Understanding concentration helps scientists describe how much solute is present in a given amount of solution. This matters in medicine, environmental testing, cooking, and laboratory work.

Molarity is one of the most common ways to measure concentration, and it tells how many moles of solute are dissolved per liter of solution. Dilution changes concentration by adding more solvent while keeping the amount of solute constant. Solubility describes the maximum amount of solute that can dissolve under specific conditions such as temperature. Together, these ideas explain how to prepare solutions, predict whether solids will dissolve, and calculate concentrations accurately.

Key Facts

Solution = solute + solvent
Molarity: M = n/V, where n is moles of solute and V is liters of solution
Moles from mass: n = m/MM, where MM is molar mass
Dilution equation: M1V1 = M2V2
Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a given temperature
Unsaturated solutions can dissolve more solute, saturated solutions cannot, and supersaturated solutions contain more dissolved solute than normally stable

Vocabulary

Solution: A homogeneous mixture in which solute particles are evenly distributed throughout a solvent.
Solute: The substance that is dissolved in a solution, usually present in the smaller amount.
Solvent: The substance that does the dissolving, usually present in the larger amount.
Molarity: The concentration of a solution expressed as moles of solute per liter of solution.
Solubility: The maximum amount of a substance that can dissolve in a solvent under specific conditions.

Common Mistakes to Avoid

Using liters of solvent instead of liters of solution, which is wrong because molarity is defined with total solution volume after mixing.
Forgetting to convert milliliters to liters, which gives molarity values that are off by a factor of 1000.
Assuming dilution changes the number of moles of solute, which is wrong because adding solvent changes concentration but not the amount of solute present.
Confusing solubility with dissolving speed, which is wrong because solubility is about how much dissolves at equilibrium, not how fast it dissolves.

Practice Questions

1 What is the molarity of a solution made by dissolving 0.50 mol of NaCl in enough water to make 2.0 L of solution?
2 How much 6.0 M HCl is needed to prepare 250 mL of 1.5 M HCl?
3 A student stirs sugar into water and notices that after a while extra sugar remains at the bottom. Explain whether the solution is unsaturated, saturated, or supersaturated and why.