What Are Physical Properties?

Introduction

How do you describe a piece of copper to someone who has never seen it? You might say it is reddish-brown, shiny, bends without breaking, conducts electricity and melts at a very high temperature. Each of these descriptions is a physical property . Physical properties let chemists identify substances, compare materials and choose the right material for a job — all without changing what the substance is.

Core explanation

Definition. A physical property is a characteristic that can be observed or measured without changing the substance into a new substance . Measuring the density of copper, looking at its colour or melting it (which can be reversed by cooling) all leave it as copper.

Common physical properties.

- Appearance: colour, lustre (shininess), transparency. - State at room temperature: solid, liquid or gas. - Density : mass per unit volume. - Melting point and boiling point . - Hardness : resistance to scratching. - Malleability and ductility : ability to be hammered into sheets or drawn into wires. - Strength , brittleness and elasticity . - Solubility : how much dissolves in a solvent. - Thermal conductivity and electrical conductivity . - Magnetism .

The next pages explore each of these in turn.

Qualitative and quantitative. Some properties are qualitative — described in words ("yellow", "shiny", "brittle"). Others are quantitative — measured with numbers and units ("density 2.70 g/cm³", "melts at 660 °C"). Quantitative properties are more useful for identification because they can be compared precisely with data tables.

Physical changes. Processes that change a substance's state or shape without making a new substance, such as melting, boiling, dissolving or cutting, are physical changes . Measuring a physical property sometimes involves a physical change (such as melting), but the substance is still the same substance afterwards.

Why they matter. Physical properties decide how materials are used: copper for electrical wiring because it conducts well and is ductile, glass for windows because it is transparent and hard, and rubber for tyres because it is elastic.

Step-by-step reasoning

To decide whether a property is physical:

1. Imagine observing or measuring it. 2. Ask: after the observation, is the sample still the same substance? 3. If yes, it is a physical property. 4. If the observation required turning the substance into something new (for example, burning it), the property is chemical (see page 63).

Visual explanation

Property Qualitative or quantitative? Example for copper --- --- --- Colour Qualitative Reddish-brown Lustre Qualitative Shiny when polished Density Quantitative 8.96 g/cm³ Melting point Quantitative 1085 °C Electrical conductivity Both Excellent conductor Malleability Qualitative Can be hammered into sheets

Real-world analogy

Describing a substance's physical properties is like describing a person's appearance for an identity card: height, eye colour and hair colour describe the person without changing who they are. A chemical property is more like describing how the person behaves in a particular situation.

Real-world example

Forensic scientists identify tiny fragments of glass, paint or fibre found at crime scenes by measuring physical properties such as density, refractive index and colour, then comparing them with samples from a suspect's clothing or car. Because physical tests do not destroy the evidence, the samples can be kept for court.

Why?

Why are quantitative properties preferred for identification? Words such as "silvery" or "fairly hard" describe many different substances and are open to interpretation. A number such as "melts at 327 °C" narrows the possibilities down sharply and can be checked by anyone with the right equipment.

Common misconception

Students sometimes think melting is a chemical property because the substance "changes". Melting only changes the state; the particles are the same, and cooling reverses the change. The melting point is therefore a physical property.

Worked example

Question: Classify each as a physical property or not: (a) sulfur is yellow; (b) sulfur melts at 115 °C; (c) sulfur burns with a blue flame.

Reasoning: Colour and melting point can be observed without making a new substance. Burning turns sulfur into sulfur dioxide, a new substance.

Answer: (a) physical; (b) physical; (c) not physical — it is a chemical property.

Quick check

1. Is "density 7.87 g/cm³" a qualitative or quantitative property? Answer: Quantitative, because it is a measured number with a unit.

Exam focus

Learn the definition: "observed or measured without changing the substance into a new substance". You may be asked to pick physical properties from a list that includes chemical ones, or to give two physical properties that make a material suitable for a given use.

Advanced insight

Some physical properties depend strongly on conditions. Boiling points are quoted at standard pressure, densities at a stated temperature, and conductivity changes with temperature. Scientific data tables always state the conditions, so that values can be compared fairly.

Summary

A physical property can be observed or measured without turning the substance into a new one. Examples include colour, lustre, state, density, melting and boiling points, hardness, malleability, solubility, conductivity and magnetism. Properties can be qualitative or quantitative; quantitative ones are best for identification. Physical properties guide how materials are chosen and used.

Practice questions

1. Give three physical properties of water. Answer: Any three: colourless, liquid at room temperature, density 1.00 g/cm³, melts at 0 °C, boils at 100 °C. 2. Why is "reacts with acid to form a gas" not a physical property? Answer: Observing it requires the substance to react and form new substances, so it is a chemical property. 3. Name one qualitative and one quantitative physical property of gold. Answer: Qualitative: yellow and shiny (or malleable). Quantitative: density 19.3 g/cm³ (or melting point 1064 °C). 4. Why is copper used for electrical wires? Refer to two physical properties. Answer: It is an excellent electrical conductor and it is ductile, so it can be drawn into wires.