Predicting State from Melting and Boiling Points

Introduction

Is bromine a solid, a liquid or a gas on a summer's day? What about oxygen on the Moon's dark side, or iron in a furnace? You do not need to see a substance to answer. If you know two numbers — its melting point and its boiling point — you can predict its state at any temperature. This simple skill appears in almost every chemistry examination and helps chemists decide how substances must be stored and handled.

Core explanation

The number line rule. Think of a temperature scale as a line, with the substance's melting point and boiling point marked on it. The two points divide the line into three regions:

- Below the melting point: the substance is a solid . - Between the melting point and the boiling point: the substance is a liquid . - Above the boiling point: the substance is a gas .

At exactly the melting point, solid and liquid can both be present; at exactly the boiling point, liquid and gas can both be present.

Worked data (at standard pressure):

Substance Melting point (°C) Boiling point (°C) State at 25 °C --- --- --- --- Oxygen −218 −183 Gas Bromine −7 59 Liquid Ethanol −114 78 Liquid Water 0 100 Liquid Sulfur 115 445 Solid Iron 1538 about 2860 Solid

Take care with negative numbers. The trickiest cases involve temperatures below 0 °C. Remember that −200 is lower than −183, so oxygen at −200 °C is below its boiling point and is a liquid (it is still above its melting point of −218 °C). Sketching a number line avoids mistakes.

Why it works. Below the melting point, the particles do not have enough energy to escape from their fixed positions. Between the two points, they can move past one another but cannot fully separate. Above the boiling point, they have enough energy to overcome the attractions entirely.

Pressure. These predictions assume standard pressure. At different pressures, especially for boiling points, the answer can change.

Step-by-step reasoning

To predict the state of a substance at a given temperature:

1. Write down its melting point and boiling point. 2. Draw a short number line and mark both values. 3. Mark the given temperature on the same line. 4. Read which region it falls in: solid, liquid or gas.

Visual explanation

Draw a vertical thermometer. Mark the melting point low down and the boiling point higher up. Shade the part below the melting point blue and label it "solid", the part between the two points green for "liquid", and the part above the boiling point red for "gas". Any temperature can then be placed on the thermometer and its colour read off.

Real-world analogy

Think of a building with three floors: the ground floor is "solid", the first floor is "liquid" and the top floor is "gas". The melting point is the ceiling of the ground floor and the boiling point is the ceiling of the first floor. Knowing a temperature is like knowing someone's height above the ground: you can tell which floor they are on.

Real-world example

Liquid nitrogen boils at −196 °C, so at room temperature it is far above its boiling point and boils away rapidly. It must be stored in insulated, vented flasks rather than sealed containers, because the gas it produces could otherwise build up dangerous pressure. Its state data guides how it is safely handled.

Why?

Why do we need both the melting and the boiling point to predict state? The melting point alone only tells us whether the substance is a solid or not; above it, the substance could be a liquid or a gas. The boiling point separates those two possibilities, so both numbers are needed.

Common misconception

"A substance with a negative boiling point must be a solid or liquid, because negative temperatures are cold." In fact, a negative boiling point means the substance is already a gas at room temperature, which is well above its boiling point.

Worked example

Question: Chlorine melts at −101 °C and boils at −34 °C. What is its state at (a) 20 °C, (b) −50 °C, (c) −120 °C?

Reasoning: (a) 20 °C is above −34 °C, the boiling point. (b) −50 °C is between −101 °C and −34 °C. (c) −120 °C is below −101 °C, the melting point.

Answer: (a) gas, (b) liquid, (c) solid.

Quick check

1. Bromine melts at −7 °C and boils at 59 °C. What is its state at 70 °C? Answer: Gas, because 70 °C is above its boiling point.

Exam focus

Examiners often give a table and ask which substance is a liquid at room temperature, or which is a gas at a stated temperature. Work each one through carefully, paying special attention to negative values. Show your reasoning by comparing the temperature with both the melting and boiling points.

Advanced insight

Only two elements are liquid at 25 °C and standard pressure: bromine and mercury. A few metals, such as caesium (melting point about 28 °C) and gallium (about 30 °C), melt just above room temperature — gallium will melt in a warm hand. This shows how the melting and boiling points of elements vary enormously depending on the attractions between their particles.

Summary

Melting and boiling points let you predict a substance's state at any temperature. Below the melting point it is a solid; between the melting and boiling points it is a liquid; above the boiling point it is a gas. Take care with negative temperatures by using a number line, and remember that these predictions assume standard pressure.

Practice questions

1. A substance melts at 44 °C and boils at 280 °C. What is its state at 25 °C? Answer: Solid, because 25 °C is below its melting point. 2. Ethanol melts at −114 °C and boils at 78 °C. What is its state at −120 °C? Answer: Solid, because −120 °C is below −114 °C. 3. Which substance in the table on this page is a gas at 25 °C? Explain. Answer: Oxygen, because its boiling point (−183 °C) is far below 25 °C. 4. What state is water in at 100 °C and standard pressure, and why is the answer not a single state? Answer: Liquid and gas can both be present, because 100 °C is exactly its boiling point, where the change of state is taking place.