Boiling Point

Introduction

Ask anyone at what temperature water boils and they will probably say 100 °C. That number is so reliable that it was once used to define the Celsius scale itself. Every pure liquid has its own boiling temperature, called its boiling point . Knowing boiling points lets chemists identify substances, predict which liquids will turn to gas in a warm room, and separate mixtures of liquids from one another.

Core explanation

Definition. The boiling point of a substance is the temperature at which it boils — changing from liquid to gas throughout the liquid — at a stated pressure. Unless told otherwise, boiling points are quoted at normal atmospheric pressure at sea level, about 101 kPa.

A fixed temperature. For a pure substance, boiling happens at one sharp, fixed temperature. If you heat pure water at sea level, its temperature rises until it reaches 100 °C. It then boils, and while it boils the temperature stays at 100 °C , however strongly it is heated. Extra energy produces more vapour rather than a higher temperature.

Every substance has its own value. Boiling point is a characteristic property, like density or melting point:

Substance Boiling point (°C) --- --- Oxygen −183 Nitrogen −196 Ethanol 78 Water 100 Mercury 357 Sodium chloride 1413 Iron about 2860

What boiling point tells us. At its boiling point, particles have just enough energy to escape from the liquid throughout its volume. A substance with weak attractions between particles needs little energy, so it has a low boiling point; oxygen and nitrogen are gases even on a freezing winter day. A substance with strong attractions, such as sodium chloride or iron, has a very high boiling point.

Pressure matters. The boiling point changes if the pressure above the liquid changes. That is why data books state the pressure, and why water boils at a lower temperature high on a mountain. For now, remember that "the boiling point" normally means the value at standard pressure.

Step-by-step reasoning

To find a boiling point from measurements:

1. Heat the liquid gently and record its temperature at regular intervals. 2. Watch the temperature rise steadily. 3. Note the temperature at which it stops rising while the liquid boils. 4. That constant temperature is the boiling point. 5. Compare it with a data table to help identify the liquid.

Visual explanation

Picture a graph of temperature against time for water being heated. The line climbs steadily, then flattens into a horizontal line at 100 °C while bubbles churn in the beaker. The height of that flat section on the temperature axis is the boiling point.

Real-world analogy

A boiling point is like the height of a fence around a playground. Children (particles) can only get out once they can jump high enough. Each substance has its own fence height; a low fence means particles escape easily at a low temperature.

Real-world example

Car radiators contain a mixture of water and antifreeze rather than pure water. The coolant system is also kept under pressure. Both measures raise the boiling temperature above 100 °C, so the coolant does not boil away in a hot engine.

Why?

Why does the temperature stop rising at the boiling point? The energy being supplied is used to separate particles from one another, overcoming their attractions, rather than making them move faster. Because the average speed of the particles does not increase, the temperature stays constant until all the liquid has boiled.

Common misconception

"Turning up the heat makes boiling water hotter." It does not. At a fixed pressure, pure water boils at 100 °C. More heat simply makes it boil faster, producing steam more quickly.

Worked example

Question: An unknown colourless liquid is heated. Its temperature rises to 78 °C, stays there while the liquid boils, and the whole sample boils away. Using the table, what could it be?

Reasoning: The liquid has a sharp, fixed boiling point, so it is likely to be pure. The value 78 °C matches ethanol in the table.

Answer: It is probably pure ethanol.

Quick check

1. What is the boiling point of pure water at standard pressure? Answer: 100 °C.

Exam focus

Learn a precise definition: the temperature at which a liquid boils at a given pressure. Know that a pure substance has a sharp, fixed boiling point and that the temperature stays constant during boiling. Be ready to read boiling points from tables and graphs.

Advanced insight

The Kelvin temperature scale is also used for boiling points: add 273 to a Celsius value. Water boils at 373 K and liquid nitrogen at 77 K. Scientists define a liquid's boiling point more exactly as the temperature at which the pressure of its vapour equals the pressure of the surroundings, which explains precisely why the boiling point depends on air pressure.

Summary

The boiling point is the temperature at which a liquid boils at a stated pressure, normally standard atmospheric pressure. A pure liquid boils at a sharp, fixed temperature, and its temperature stays constant while it boils. Each substance has its own boiling point, which is low when the attractions between particles are weak and high when they are strong.

Practice questions

1. Define boiling point. Answer: The temperature at which a liquid boils, changing to gas throughout the liquid, at a given pressure. 2. Water in a pan is boiling strongly at sea level. The cook turns the heat to maximum. What happens to the temperature of the water? Answer: It stays at 100 °C; the water just boils more rapidly. 3. Nitrogen boils at −196 °C and mercury at 357 °C. Which has the stronger attractions between its particles? Explain. Answer: Mercury, because more energy (a higher temperature) is needed to separate its particles. 4. Why do data tables state a pressure alongside boiling points? Answer: Because the boiling point changes with the pressure above the liquid.