Solutions
Class 12 · Chemistry
4 ideas in Class 12 Chemistry, explained in plain words, with games and quizzes to practise.
Ways of Expressing Concentration
Part of: Concentration and Solubility
Concentration counts solute per solution: mass percent and ppm for parts, mole fraction for share, molarity as moles per litre of solution, and molality as moles per kilogram of solvent. Molarity shifts with temperature while molality does not.
Why it matters. Labs, medicines and colligative laws all need the right unit. Mixing litres of solution with kilograms of solvent ruins every calculation.
Formulas
- Mole fraction: x = n / n(total)
- Molarity: M = moles / litres of solution
- Molality: m = moles / kg of solvent
Key words
- Mole fraction
- : Moles of one part over total moles in the mixture.
- Molarity
- : Moles of solute per litre of solution.
- Molality
- : Moles of solute per kilogram of solvent.
- ppm
- : Parts per million, for very dilute quantities.
Try it in two minutes
- Convert each mass to moles first.
- Divide by litres of solution for molarity.
- Divide by kilograms of solvent for molality.
Try at home. Label a salt solution with all four units.
Solubility and Henry's Law
Part of: Concentration and Solubility
Gas solubility grows with its partial pressure: mole fraction equals pressure over Henry's constant. Larger constants mean sparingly soluble gases. Cold water holds more gas, soda stays fizzy under pressure, and divers breathe helium mixes to dodge nitrogen bubbles.
Why it matters. Fizzy drinks, aquariums and scuba safety all follow this law. Inverting the constant's meaning predicts solubility backwards.
Formulas
- Henry: p = KH x x
- Solubility rises with pressure, falls with temperature
- Larger KH means lower solubility
Key words
- Henry's law
- : Gas mole fraction equals its partial pressure over Henry's constant.
- Henry's constant
- : Pressure per unit mole fraction; larger means less soluble.
- Bends
- : Pain from nitrogen bubbles on fast ascent after dives.
Try it in two minutes
- Read the gas partial pressure.
- Divide by Henry's constant for mole fraction.
- Compare constants to rank solubilities.
Try at home. Explain why a warm soda goes flat faster.
Vapour Pressure and Raoult's Law
Part of: Raoult's Law and Colligative Properties
Each volatile part pushes vapour in proportion to its mole fraction times its pure pressure, summing to the total. Ideal pairs obey this at all mixes; positive-deviation pairs push harder with weaker bonds and boil low, while negative-deviation pairs cling and boil high, both freezing into constant-boiling azeotropes.
Why it matters. Distillation, perfumes and fuels all depend on vapour behaviour. Expecting every mixture to separate cleanly ignores azeotropes.
Formulas
- Raoult: p1 = x1 x p1(0), total = p1 + p2
- Ideal pairs have matching A-A, B-B and A-B bonds
- Azeotropes distil without changing composition
Key words
- Raoult's law
- : Partial pressure equals mole fraction times pure vapour pressure.
- Ideal solution
- : Mixture obeying Raoult's law at every composition.
- Azeotrope
- : Mixture boiling unchanged at a fixed composition.
- Positive deviation
- : Higher vapour from weaker unlike bonds, boiling low.
Try it in two minutes
- Multiply each mole fraction by its pure pressure.
- Add the two partial pressures.
- Check bonds: weaker means higher vapour, stronger means lower.
Try at home. Explain why ethanol-water resists full separation.
Colligative Properties and van't Hoff
Part of: Raoult's Law and Colligative Properties
Dissolved particles lower vapour pressure, lifting boiling point by Kb times molality and dropping freezing point by Kf times molality, while osmosis pushes solvent across membranes with pressure CRT. Salts that split multiply effects by van't Hoff i, and clumping ones shrink it.
Why it matters. Antifreeze, desalination, food preservation and molar-mass finding all use these counts. Forgetting i for salts halves every answer.
Formulas
- Elevation: dTb = i x Kb x m
- Depression: dTf = i x Kf x m
- Osmotic: pi = i x C x R x T
Key words
- Colligative property
- : Change depending on particle count, not identity.
- Osmotic pressure
- : Pressure stopping solvent flow across a membrane.
- van't Hoff factor
- : Observed over ideal particle count ratio.
- Reverse osmosis
- : Forcing water back through a membrane to desalinate.
Try it in two minutes
- Count particles per formula with i.
- Multiply molality by Kf or Kb.
- Multiply molarity by RT for osmotic pressure.
Try at home. Explain why salt melts ice on roads.
What your child will practise
- Colligative Properties and van't Hoff: Candy Quick Fire (Quick Fire)
- Solubility and Henry's Law: Desert Puzzle (Clue Puzzle)
- Solubility and Henry's Law: Desert Quick Fire (Quick Fire)
- Vapour Pressure and Raoult's Law: Space Quick Fire (Quick Fire)
- Ways of Expressing Concentration: Ocean Puzzle (Clue Puzzle)
- Quiz: Colligative Properties and van't Hoff (8 questions)
- Quiz: Solubility and Henry's Law (8 questions)
- Quiz: Vapour Pressure and Raoult's Law (8 questions)
- Quiz: Ways of Expressing Concentration (8 questions)