Lesson 1 of 10 · 8 min

Types of solutions and concentration

NCERT §1.1; §1.2

The mechanic tops up the radiator with a 'coolant' that looks like plain water. It is 620 g of ethylene glycol in 1.00 kg of water. How strong is it, and which way of saying 'strong' will still be true when the engine is hot?

The story this chapter follows: A road trip up to Leh

A family drives from the plains up to Leh, where the air pressure is only about 0.66 bar. The car's radiator holds a coolant of 620 g of ethylene glycol (10.0 mol) in 1.00 kg of water, a crate of soda sealed at 2.5 bar of CO₂ rides in the boot, and the first-aid kit carries a 5% glucose drip. Those numbers carry every lesson, from concentration to osmotic pressure.

The lesson in notes

In short

A solution is a homogeneous mixture: its composition and properties are the same in every part. The chapter deals with binary solutions, which have two components.

The component present in the largest amount is usually called the solvent, and it decides whether the solution is a solid, liquid or gas; every other component is a solute.

Solute and solvent can each be a gas, a liquid or a solid. Examples: camphor in nitrogen gas (solid in gas), oxygen in water (gas in liquid), glucose in water (solid in liquid), hydrogen in palladium (gas in solid), sodium amalgam (mercury in sodium, liquid in solid) and copper in gold (solid in solid).

Mass percentage (w/w) = mass of the component × 100 / total mass of the solution. A 10% glucose solution by mass is 10 g of glucose with 90 g of water; commercial bleach carries 3.62% sodium hypochlorite by mass.

Volume percentage (V/V) is used for liquids in liquids. A 35% (V/V) solution of ethylene glycol, used as antifreeze in car engines, lowers the freezing point of water to 255.4 K (−17.6 °C).

Mass by volume percentage (w/V), the grams of solute in 100 mL of solution, is the usual unit in medicine and pharmacy.

Parts per million = parts of the component × 10⁶ / total parts of all components, for trace amounts such as pollutants. Fluoride at 1 ppm in drinking water prevents tooth decay, while 1.5 ppm mottles the teeth.

Mole fraction: x_A = n_A / (n_A + n_B) in a binary mixture, and the mole fractions of all components add up to 1.

Molarity (M) is moles of solute per litre of solution; molality (m) is moles of solute per kilogram of solvent.

Mass percentage, ppm, mole fraction and molality do not change with temperature, because they are built from masses; molarity does, because the volume of a solution changes with temperature.