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Chemistry: Atoms to AP Chemistry · Module 6 of 15

Solutions

Concentration, solubility, dilution, and what happens to a solvent when something is dissolved in it.

In this module
  1. Solutions & Concentration
  2. Solubility & Factors Affecting It
  3. Dilutions
  4. Colligative Properties
  5. Check yourself

Solutions & Concentration

  • A solution is a homogeneous mixture of a solute dissolved in a solvent.
  • Molarity (M) = moles of solute ÷ liters of solution — the most common way to express concentration in chemistry.
Worked example

What is the molarity of a solution made by dissolving 2.0 moles of NaCl in enough water to make 4.0 L of solution?

Solution. M = moles ÷ liters = 2.0 ÷ 4.0 = 0.50 M.

Solubility & Factors Affecting It

  • Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a given temperature; a solution at this limit is saturated.
  • For most solids, solubility increases with temperature; for gases, solubility decreases as temperature increases.
Worked example

Why does a carbonated drink go flat faster when it's warm?

Solution. Gas solubility (like CO₂) decreases as temperature increases, so warmer soda holds less dissolved CO₂ and it escapes — goes flat — faster.

Dilutions

  • Dilution adds solvent to lower a solution's concentration without changing the moles of solute present.
  • Dilution equation: M₁V₁ = M₂V₂, where 1 is the concentrated (stock) solution and 2 is the diluted solution.
Worked example

How much water must be added to 100 mL of 6.0 M HCl to make a 1.0 M solution?

Solution. M₁V₁ = M₂V₂ → (6.0)(100) = (1.0)(V₂) → V₂ = 600 mL total, so 600 − 100 = 500 mL of water must be added.

Colligative Properties

  • Colligative properties depend on the number of dissolved particles, not their identity: adding solute lowers a solution's freezing point and raises its boiling point compared to the pure solvent.
  • This is why salt is used to melt ice on roads — it lowers the freezing point of water.
Worked example

Why does adding salt to icy roads help melt the ice?

Solution. Dissolved salt ions lower the freezing point of water below 0°C, so ice melts (or doesn't refreeze) at temperatures that would otherwise keep it frozen.

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. What is the molarity of a solution with 3.0 mol of solute in 1.5 L of solution?

2. How does increasing temperature typically affect the solubility of a gas in a liquid?

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