Physical and Chemical Properties

Introduction

So far this unit has described physical properties: colour, density, melting point, conductivity and more. But a full description of a substance also includes what it can turn into . Paper burns, iron rusts, baking soda fizzes with vinegar. These behaviours are chemical properties . Knowing both kinds of property tells chemists how to store, use and handle substances safely.

Core explanation

Chemical properties. A chemical property describes how a substance behaves in a chemical reaction — a change that produces one or more new substances. You can only observe a chemical property by trying to change the substance into something else.

Examples: - Flammability: petrol, wood and methane burn in air; water and sand do not. - Reactivity with water: sodium reacts vigorously with water; gold does not react at all. - Reactivity with acids: magnesium and zinc react with dilute acids to give hydrogen gas; copper does not. - Tendency to corrode: iron rusts in damp air; aluminium forms a thin protective oxide layer; gold does not tarnish. - Stability to heat: calcium carbonate breaks down into calcium oxide and carbon dioxide when strongly heated; sodium chloride does not break down. - Acidity or alkalinity: vinegar is acidic; soap solution is alkaline.

Physical properties, by contrast, can be observed without making a new substance (page 50).

The key test. After observing the property, is the original substance still there, unchanged? If so, the property is physical. If new substances have formed — shown by signs such as a gas being given off, a colour change, a new solid forming or a large amount of heat or light being released — the property is chemical.

Both matter for choosing materials. A material for a saucepan must have suitable physical properties (conducts heat, high melting point) and suitable chemical properties (does not react with food or corrode quickly). Stainless steel scores well on both.

Step-by-step reasoning

To classify "magnesium burns with a bright white flame":

1. Identify what happens: magnesium reacts with oxygen. 2. Ask whether a new substance forms: yes, white magnesium oxide powder. 3. Therefore observing this property changed the substance. 4. It is a chemical property.

Visual explanation

A two-column table: Physical — iron is grey, iron is magnetic, iron melts at 1538 °C, iron conducts electricity. Chemical — iron rusts in damp air, iron reacts with dilute hydrochloric acid to give hydrogen, hot iron reacts with sulfur to form iron sulfide. Arrows show that the physical tests leave iron as iron, while the chemical ones produce new substances.

Real-world analogy

A person's physical properties are like height and eye colour — observable at a glance. Their "chemical properties" are like how they react to a surprise or a challenge — you only find out by putting them in that situation, and they may be changed by it.

Real-world example

Fire safety labels on products report chemical properties such as flammability. Sofas, children's nightwear and aircraft seats are tested to see how easily they catch fire, and flame-retardant chemicals may be added to reduce their flammability. The physical properties, such as softness and colour, are chosen separately for comfort and appearance.

Why?

Why can a chemical property not be tested without changing the substance? A chemical property is about the reaction itself. To know whether wood burns, some wood must burn and turn into ash, carbon dioxide and water. The act of observing consumes the sample, unlike measuring density or colour.

Common misconception

"Any change is a chemical change." Melting, boiling, dissolving and cutting change the appearance or state of a substance but produce no new substance, so they are physical changes, and the related properties (melting point, solubility) are physical properties.

Worked example

Question: Classify each property of copper as physical or chemical: (a) conducts electricity; (b) turns green when exposed to damp air for years; (c) is ductile; (d) does not react with dilute hydrochloric acid.

Reasoning: (a) and (c) can be observed without changing copper. (b) involves a new substance (green copper carbonate compounds). (d) describes how copper behaves (or fails to react) with a chemical, which is chemical behaviour.

Answer: (a) physical; (b) chemical; (c) physical; (d) chemical.

Quick check

1. Is "petrol is flammable" a physical or chemical property? Answer: Chemical, because burning changes petrol into new substances.

Exam focus

Classification questions are common. Use the test "does observing it make a new substance?". Remember that reactivity, flammability, corrosion and decomposition are chemical properties, and that non-reaction with a chemical is also chemical information.

Advanced insight

Chemical properties are explained by the arrangement of electrons in atoms. Sodium reacts vigorously with water because it loses its single outer electron very easily, while gold's electrons are held so tightly that it resists almost all reactions. You will explore these explanations in the atomic structure and periodic table units.

Summary

Physical properties can be observed without changing the substance; chemical properties describe how it reacts to form new substances. Flammability, reactivity with water or acids, corrosion and decomposition on heating are chemical properties. Both kinds of property are needed to identify substances and choose materials safely.

Practice questions

1. Give two chemical properties of iron. Answer: Any two: rusts in damp air, reacts with dilute acids to give hydrogen, reacts with sulfur when heated. 2. Why is "sugar dissolves in water" a physical property but "sugar chars when heated strongly" a chemical property? Answer: Dissolving produces no new substance (sugar is recovered by evaporation); charring produces new substances such as carbon and water. 3. State one sign that a chemical property is being observed. Answer: Any one: a gas is given off, a colour change, a new solid forms, heat or light is released. 4. Suggest one physical and one chemical property needed in a material for food cans. Answer: Physical: strong and malleable (or impermeable). Chemical: does not react with or corrode from the food.