Properties of Substances: Melting and Boiling Points, Gases, Conductors

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1. What is a property?

A property is something that describes what a substance is like or how it behaves — for example its colour, hardness, whether it is magnetic, whether it floats. A property is the same every time for that substance: pure copper always conducts electricity; pure water always boils at the same temperature at normal air pressure. Scientists use properties to identify substances and to choose the right material for a job.

2. Melting point and boiling point

The melting point is the temperature at which a solid turns into a liquid. The same temperature is the freezing point (liquid → solid). The boiling point is the temperature at which a liquid boils and turns into a gas quickly. Each substance has its own melting point and boiling point — that is why they are properties.

How to work out the state at any temperature — compare the temperature with the melting point (mp) and the boiling point (bp):

1. Temperature lower than the mp → solid.
2. Temperature between the mp and the bp → liquid.
3. Temperature higher than the bp → gas.

Using melting point to test purity: pure water freezes at exactly 0 °C and boils at 100 °C. If salt is dissolved in it, it freezes below 0 °C and boils above 100 °C. So if a 'water' sample boils at 103 °C, it is not pure water. (This is why salt is spread on icy roads in cold countries.)

3. Gases have properties — including mass

Gases are hard to notice because most are invisible, but they are real substances made of particles, just like solids and liquids. In a gas the particles are far apart and move quickly in all directions.

Each gas particle has a tiny mass, so a gas has mass. One litre of air has a mass of about 1.2 g — small, but it can be measured on a sensitive balance. Another example: an unopened bottle of fizzy drink has a greater mass than the same bottle after it has been opened and left to go flat, because carbon dioxide gas has escaped.

4. Electrical and thermal conductivity

Electrical conductivity describes whether a substance lets electricity pass through it. An electrical conductor lets electricity flow; an electrical insulator does not. Thermal conductivity describes whether a substance lets heat (thermal energy) pass through it easily. A thermal conductor lets heat pass quickly; a thermal insulator slows heat down. Both are properties: they depend on what the substance is.

Testing electrical conductivity: put the material in the gap of a simple circuit. If the lamp lights, the material is an electrical conductor.

5. Investigation: which metal is the best thermal conductor?

Rods of different materials were stood in a beaker of hot water. A small bead was stuck to the top of each rod with wax. When heat travels up the rod, the wax melts and the bead falls. The time for each bead to fall was measured.

• Independent variable: the material of the rod. Dependent variable: the time for the bead to fall.
• Control variables: same length and thickness of rod, same temperature of water, same amount of wax, same size of bead, same depth in the water.
• Conclusion: copper is the best thermal conductor (shortest time); glass is a thermal insulator.
• Risk: hot water can scald — stand the beaker on a flat surface, do not lean over it, and let rods cool before touching.

Worked examples

Worked example 1 — Substance X melts at −7 °C and boils at 59 °C. What is its state at (a) −20 °C (b) 25 °C (c) 80 °C? [3 marks]

1. (a) −20 is lower than −7 (the mp) → solid.
2. (b) 25 is between −7 and 59 → liquid.
3. (c) 80 is higher than 59 (the bp) → gas.

Common mistake: thinking −20 is 'higher' than −7 because 20 is bigger than 7.

Worked example 2 — Use the table of substances. Which substance has a boiling point 22 °C lower than water's? [1 mark]

Answer: 100 − 22 = 78 °C, which is the boiling point of ethanol.

Worked example 3 — Mona weighs a bottle of fizzy water: 520 g. She opens it, leaves it overnight, puts the lid back on and weighs it again: 516 g. (a) What mass of gas escaped? (b) What does this show about gases? [2 marks]

1. (a) 520 − 516 = 4 g.
2. (b) The gas (carbon dioxide) that escaped had mass — gases have mass.

Worked example 4 — Explain why the handle of a saucepan is made of plastic but the base is made of metal. [2 marks]

Answer: the metal base is a good thermal conductor, so heat passes quickly to the food (1); plastic is a thermal insulator, so the handle does not get too hot to hold (1). Common mistake: writing 'plastic does not conduct electricity' — the question is about heat.

Worked example 5 — Omar tests a pencil lead (graphite) in the conductivity tester and the lamp lights. He says, "Graphite must be a metal." Is he correct? [1 mark]

Answer: No — graphite is a non-metal that conducts electricity. Most non-metals are insulators, but graphite is an exception.

Practice

All pages in this course

Start here

• Checkpoint 6 Science: The Test and How to Prepare
• Thinking and Working Scientifically: Investigations, Variables, Tables and Graphs

Biology

• The Heart and Circulatory System
• The Lungs and Respiratory System
• Human Reproductive System and Puberty
• Diseases, Hygiene and the Body's Defences
• Food Chains, Food Webs, Energy and Toxins

Chemistry

• Properties of Substances: Melting and Boiling Points, Gases, Conductors
• Reversible Changes and Dissolving
• Boiling and Evaporation
• Chemical Reactions: Reactants, Products and Evidence

Physics

• Mass, Weight and Gravity
• Force Diagrams and Forces on Moving Objects
• Floating and Sinking
• Light: Reflection and Refraction
• Electric Circuits, Symbols, Series and Parallel

Earth and space

• Rocks and Fossils
• Types of Soil
• The Rock Cycle
• The Solar System and the Phases of the Moon

Practice tests

• Checkpoint 6 Science — Practice Test 1
• Checkpoint 6 Science — Practice Test 2
• Checkpoint 6 Science — Practice Test 3