Testing Purity with Boiling Points

Introduction

Melting points test the purity of solids; boiling points do the same job for liquids. Pure water boils at 100 °C at sea level, and the temperature stays exactly there until all the water has gone. Salty water boils at a higher temperature, and that temperature keeps creeping upwards as the water boils away. Measuring the boiling point therefore tells you whether a liquid is pure.

Core explanation

Pure liquids have fixed boiling points. At a given pressure, every pure liquid boils at its own fixed temperature. At standard atmospheric pressure (about 101 kPa):

Pure liquid Boiling point --- --- Water 100 °C Ethanol 78 °C Propanone (acetone) 56 °C Cyclohexane 81 °C

While a pure liquid boils, the temperature stays constant , because all the energy supplied is used to separate particles into the gas rather than to raise the temperature.

Dissolved solids raise the boiling point. When a non-volatile substance such as salt or sugar is dissolved in water, the solution boils above 100 °C. This is called boiling point elevation . Dissolved particles take up some of the positions at the surface and hold on to water molecules, so fewer water molecules escape at any given temperature. The solution must be heated further before bubbles of vapour can form.

The boiling point keeps changing. As an impure liquid boils, the pure solvent leaves as vapour but the dissolved substance stays behind. The remaining solution becomes more concentrated, so its boiling point rises further. Instead of a steady temperature, you see a gradually rising one. This is a clear sign of a mixture.

Mixtures of liquids. If the impurity is another liquid, the mixture usually boils over a range between the two boiling points, and the composition of the vapour differs from the liquid. This difference is the basis of fractional distillation, covered later in the unit.

Pressure matters. Boiling points depend on the surrounding pressure. At the top of a high mountain, water boils below 100 °C; in a pressure cooker, above it. A boiling point test for purity must therefore compare values measured at the same pressure, usually by correcting to standard pressure.

Summary of evidence.

Observation Conclusion --- --- Boils at the data-book value, temperature steady Probably pure Boils above the expected value Contains dissolved impurity Temperature rises steadily during boiling Mixture

Step-by-step reasoning

To judge purity from boiling data:

1. Check the pressure is close to standard, or correct for it. 2. Compare the boiling temperature with the data-book value. 3. Watch whether the temperature stays constant as the liquid boils. 4. A steady temperature at the expected value indicates purity; a higher or rising one indicates a mixture.

Visual explanation

Picture the surface of boiling water as a crowded exit door. In pure water every particle at the door is a water molecule ready to leave. In salt water, some places at the door are occupied by dissolved ions that cannot escape, and they tug at nearby water molecules. Fewer water molecules get out, so extra heating is needed to reach boiling.

Real-world analogy

Imagine trying to leave a busy stadium when some of the exits are blocked by people who are not leaving. Fewer fans get out each minute, and more push is needed to empty the stands. The blocking people are like dissolved particles that raise the boiling point.

Real-world example

Car engine coolant is a mixture of water and ethylene glycol (antifreeze). The dissolved glycol raises the boiling point of the coolant well above 100 °C, especially under the pressure inside the cooling system, and also lowers its freezing point, so the engine is protected in both hot and cold weather.

Why?

Why does a pure liquid boil at a steady temperature? At the boiling point, the energy supplied goes into overcoming the forces between particles so that they escape as vapour, not into making the particles move faster. Since every particle is the same, conditions do not change as the liquid boils, and the temperature stays level.

Common misconception

"Adding salt to water makes it boil faster." Salt actually raises the boiling point slightly, so the water must get hotter before it boils. The rise from a pinch of salt in cooking is very small, well under 1 °C, so it makes no noticeable difference to cooking time.

Worked example

Question: A colourless liquid starts boiling at 101.5 °C at standard pressure and the temperature slowly rises to 103 °C as it boils. Evaporating a sample leaves white crystals. What is the liquid?

Reasoning: The boiling point is just above 100 °C and rises during boiling, which suggests an aqueous solution of a dissolved solid. The crystals left on evaporation confirm a dissolved substance.

Answer: It is an impure sample of water containing a dissolved solid, such as a salt solution, not pure water.

Quick check

1. What happens to the boiling point of water when a solid is dissolved in it? Answer: The boiling point is raised above 100 °C.

Exam focus

Remember the pattern: impurities lower melting points but raise boiling points. Also mention that a pure liquid boils at a constant temperature, while a mixture boils over a range. State that data-book values apply at standard pressure.

Advanced insight

The size of the boiling point rise depends on the number of dissolved particles, not their identity. One mole of sodium chloride splits into two moles of ions, so it raises the boiling point roughly twice as much as one mole of sugar. Properties that depend only on the number of solute particles are called colligative properties.

Summary

A pure liquid boils at a fixed temperature at a given pressure and stays at that temperature while boiling. Dissolved impurities raise the boiling point and cause it to rise during boiling as the solution becomes more concentrated. Comparing measured boiling behaviour with data-book values, at the same pressure, tests the purity of a liquid.

Practice questions

1. Pure ethanol boils at 78 °C. A sample boils between 79 °C and 84 °C. Is the sample pure? Explain. Answer: No. It boils above the expected value and over a range rather than at one steady temperature. 2. Explain why the temperature of boiling salt water gradually rises as it boils. Answer: Water leaves as vapour but the salt stays behind, so the solution becomes more concentrated and its boiling point increases. 3. Why must boiling points be compared at the same pressure? Answer: Boiling point depends on pressure, so a different pressure would give a different value even for a pure liquid. 4. Compare the effect of an impurity on the melting point and on the boiling point of a substance. Answer: An impurity lowers the melting point and raises the boiling point, and it spreads both changes of state over a range.