Boiling Point

Introduction

When water is heated in a kettle, small bubbles appear, then large bubbles rise and burst, and steam pours from the spout. The water is boiling . The boiling point is another quantitative, characteristic property that helps chemists identify liquids and separate mixtures by distillation. Unlike melting point, it depends noticeably on pressure — which is why cooking is harder on a high mountain.

Core explanation

Definition. The boiling point is the temperature at which a liquid boils: bubbles of vapour form throughout the liquid, not just at the surface. At sea level, pure water boils at 100 °C. The reverse change, gas to liquid, is condensation , and for a pure substance it occurs at the same temperature.

Some boiling points (°C, at standard atmospheric pressure):

Substance Boiling point --- --- Nitrogen −196 Oxygen −183 Ethanol 78 Water 100 Mercury 357 Sodium chloride 1465 Iron 2862

Boiling versus evaporation. Evaporation happens only at the surface of a liquid and at any temperature: puddles dry up on a cool day. Boiling happens throughout the liquid, at one particular temperature for a given pressure, and is much faster. Both change liquid into gas.

Temperature stays constant while boiling. A pure liquid that is boiling stays at its boiling point, however strongly it is heated, until it has all turned to gas. The energy supplied is used to separate particles completely from one another. Turning up the heat under a boiling pan of water makes it boil faster, not hotter.

Pressure affects boiling point. A liquid boils when the pressure of its vapour becomes equal to the pressure of the atmosphere above it. At lower atmospheric pressure, such as on a high mountain, less vapour pressure is needed, so the liquid boils at a lower temperature. At the top of Mount Everest, water boils at about 70 °C. In a pressure cooker, the pressure is raised, so water boils at about 120 °C, cooking food faster.

Step-by-step reasoning

To explain why food takes longer to cook on a mountain:

1. Atmospheric pressure is lower at high altitude. 2. Water boils when its vapour pressure equals the atmospheric pressure. 3. Lower atmospheric pressure is reached at a lower temperature. 4. So the water boils below 100 °C. 5. Food cooked in water that cannot get as hot cooks more slowly.

Visual explanation

A heating curve from ice at −20 °C to steam at 120 °C shows two plateaus: a shorter one at 0 °C (melting) and a longer one at 100 °C (boiling), with sloping sections between them. The longer boiling plateau shows that much more energy is needed to turn water into steam than to melt ice. A side panel shows bubbles forming throughout a beaker during boiling, but only surface particles escaping during evaporation.

Real-world analogy

Evaporation is like a few students slipping out of a party early through the front door, one at a time. Boiling is like the moment the whole party breaks up at once, with people leaving from every room simultaneously.

Real-world example

Oil refineries separate crude oil into petrol, kerosene, diesel and other fractions by heating it and letting the vapours condense at different heights of a tall column. Each fraction has a different boiling range, so boiling points are the key property that makes the separation possible.

Why?

Why does boiling need so much more energy than melting? During melting, particles only need to loosen enough to slide past one another; they stay close together. During boiling, particles must be separated completely and pushed far apart against the attraction of their neighbours, which requires much more energy.

Common misconception

"Bubbles in boiling water are air." Small bubbles appearing early in heating are dissolved air coming out of solution, but the large bubbles in rapidly boiling water are water vapour — gaseous water formed inside the liquid.

Worked example

Question: Using the table, state the state of ethanol at (a) 25 °C, (b) 90 °C. Ethanol melts at −114 °C.

Reasoning: (a) 25 °C is between −114 °C and 78 °C: liquid. (b) 90 °C is above the boiling point of 78 °C: gas.

Answer: (a) liquid; (b) gas.

Quick check

1. Give one difference between boiling and evaporation. Answer: Boiling happens throughout the liquid at a fixed temperature; evaporation happens only at the surface and at any temperature below the boiling point.

Exam focus

State boiling points with conditions ("at atmospheric pressure"). Explain the effect of altitude or pressure cookers using vapour pressure and atmospheric pressure. On heating curves, label both plateaus and explain that energy is used to overcome forces between particles.

Advanced insight

Liquid nitrogen boils at −196 °C, so at room temperature it boils vigorously. Because a boiling liquid stays at its boiling point, liquid nitrogen provides a convenient constant-temperature bath for freezing biological samples and cooling scientific detectors, used only with proper insulation and ventilation.

Summary

The boiling point is the temperature at which a liquid forms vapour bubbles throughout; condensation occurs at the same temperature. Boiling differs from evaporation, which happens at the surface at any temperature. Boiling temperature stays constant until all the liquid has vaporised, and it depends on external pressure — lower at high altitude, higher in a pressure cooker.

Practice questions

1. Why does turning up the heat under boiling water not make it hotter? Answer: The extra energy is used to turn more water into steam; the temperature stays at the boiling point. 2. Why do pressure cookers cook food faster? Answer: Higher pressure raises the boiling point of water (to about 120 °C), so food cooks at a higher temperature. 3. Is nitrogen a solid, liquid or gas at −150 °C? (mp −210 °C, bp −196 °C) Answer: Gas, because −150 °C is above its boiling point. 4. Why is the boiling plateau on a heating curve for water longer than the melting plateau? Answer: Much more energy is needed to separate particles completely (boiling) than to let them slide past each other (melting).