Introduction

Everything around us — water, iron, wood, plastic, gold, salt — is made of matter. Every type of matter has its own unique set of characteristics that help us identify it and decide how to use it. These characteristics are called properties. Scientists divide the properties of matter into two main categories: physical properties and chemical properties. Understanding the difference between these two types of properties helps us explain why certain materials are used for specific purposes in everyday life.

1. What Are Physical Properties?

A physical property is a characteristic of a substance that can be observed or measured without changing the chemical composition of the substance. In other words, when you observe a physical property, the substance remains the same substance — it does not turn into anything new.

Common physical properties include:

Colour The colour of a substance is one of the most easily observed physical properties. Gold is yellow, copper is reddish-orange, and sulphur is bright yellow. Observing colour does not change the substance at all.

Shape and Size The shape of an object can be changed — for example, by cutting or bending — without changing what the substance actually is. A copper wire bent into a curve is still copper.

Density Density is the amount of mass packed into a given volume. It is calculated as mass divided by volume. Lead is very dense, which is why it is heavy for its size, while wood is less dense, which is why it floats on water.

Melting Point The melting point is the temperature at which a solid substance turns into a liquid. For example, ice melts at 0°C and iron melts at about 1538°C. The melting point is a fixed physical property that helps identify a substance.

Boiling Point The boiling point is the temperature at which a liquid turns into a gas (vapour). Water boils at 100°C at sea level. The boiling point is used to separate and identify substances.

Solubility Solubility is the ability of a substance to dissolve in a solvent such as water. Salt and sugar dissolve easily in water and are said to be soluble. Sand and chalk do not dissolve in water and are said to be insoluble. Solubility is a physical property because dissolving does not create a new substance — the salt or sugar can be recovered by evaporating the water.

Thermal Conductivity Thermal conductivity is the ability of a material to conduct (transfer) heat. Metals such as copper and aluminium are excellent thermal conductors, which is why they are used to make pots and pans. Wood and plastic are poor conductors of heat (insulators), which is why they are used for handles.

Electrical Conductivity This is the ability of a substance to allow electric current to pass through it. Copper and silver are excellent electrical conductors. Rubber and plastic are insulators — they do not allow electricity to pass through.

Hardness Hardness describes how resistant a substance is to being scratched. Diamond is the hardest natural substance known, while talc is very soft. Hardness is measured using the Mohs scale.

Lustre Lustre refers to how shiny a surface appears. Metals typically have a high lustre, which is why they are used in jewellery and decorative items.

State of Matter Whether a substance is a solid, liquid, or gas at room temperature is a physical property. Mercury is a metal that is liquid at room temperature, while most metals are solid.

2. What Are Chemical Properties?

A chemical property is a characteristic that describes how a substance reacts with other substances or changes into a different substance under certain conditions. To observe a chemical property, the substance must undergo a chemical change — a new substance is formed.

Common chemical properties include:

Flammability Flammability is the ability of a substance to catch fire and burn in the presence of oxygen. Petrol, wood, and natural gas are highly flammable. Water and sand are not flammable. Flammability is a chemical property because when a substance burns, it is chemically changed into new substances such as carbon dioxide and water vapour.

Reactivity Reactivity describes how readily a substance reacts with other substances such as acids, water, or oxygen. Sodium is highly reactive — it reacts vigorously with water, producing hydrogen gas and heat. Gold, on the other hand, is almost completely unreactive, which is why it does not corrode or tarnish and remains shiny for thousands of years.

Tendency to Rust (Corrosion) The tendency of iron to rust when exposed to oxygen and water is a chemical property. Iron has a high tendency to corrode, while stainless steel has a much lower tendency because of the chromium it contains.

Acidity and Alkalinity The ability of a substance to act as an acid or a base is a chemical property. Vinegar is acidic and reacts with metals and carbonates. Sodium hydroxide is alkaline and reacts with acids to form salts and water.

Toxicity Toxicity describes how harmful a substance is to living organisms when they are exposed to it. This is a chemical property because it relates to how a substance reacts with the chemistry of living cells.

Combustibility Combustibility refers to how easily a substance can burn completely. Coal and charcoal are highly combustible and are used as fuels.

3. Key Difference: Observing vs Changing

The most important distinction between physical and chemical properties is this:

• Physical properties can be observed or measured without changing what the substance is. The substance keeps its chemical identity.
• Chemical properties can only be observed when the substance undergoes a chemical change and becomes something new.

For example:

4. How Physical Properties Determine the Uses of Materials

The physical properties of a material directly determine how it is used in everyday life.

• High melting point of tungsten — Tungsten has the highest melting point of any metal (about 3422°C). This makes it ideal for use in the filaments of light bulbs and in high-temperature industrial equipment.
• High thermal conductivity of copper and aluminium — These metals transfer heat efficiently, making them the best choice for cooking pots, heat sinks in computers, and radiators.
• Low density of aluminium — Aluminium is strong but lightweight, making it ideal for aircraft construction, car bodies, and food packaging (aluminium foil).
• High electrical conductivity of copper — Copper is used in electrical wires, cables, and circuit boards because it allows electric current to flow with very little resistance.
• Solubility of salt and sugar — These substances dissolve readily in water, which is why they are used in food preparation and preservation.
• Hardness of diamond — Diamond is the hardest natural substance, making it useful for cutting tools, drill bits, and grinding equipment.
• Flexibility of rubber — Rubber can be stretched and compressed without breaking, making it ideal for tyres, seals, and elastic bands.

5. How Chemical Properties Determine the Uses of Materials

Chemical properties are equally important in determining how materials are used.

• Flammability of petrol and natural gas — These fuels are used in engines and heating systems precisely because they are highly flammable and release large amounts of energy when burned.
• Non-flammability of carbon dioxide — CO₂ does not burn and prevents burning, which is why it is used in fire extinguishers.
• Low reactivity of gold — Gold does not react with most chemicals, does not rust, and does not tarnish. This makes it ideal for jewellery, electrical contacts in computers, and dental work.
• Reactivity of zinc — Zinc is more reactive than iron, so it is used to coat iron (galvanisation) to protect it from rusting.
• Tendency of iron to rust — Because engineers know iron rusts, they use stainless steel or coated iron in environments where moisture is present, such as in kitchen sinks and outdoor structures.
• Non-reactivity of nitrogen — Nitrogen does not react easily with food, so it is pumped into food packaging to keep food fresh and prevent it from oxidising.
• Reactivity of acids — Hydrochloric acid reacts with certain metals and minerals, making it useful in cleaning metal surfaces and manufacturing processes.

6. Summary

Physical and chemical properties are both essential tools for understanding and using materials in daily life. Physical properties — such as colour, density, melting point, boiling point, solubility, and thermal conductivity — describe what a substance is like without changing it. Chemical properties — such as flammability, reactivity, and tendency to corrode — describe how a substance behaves when it changes into something new. Together, these properties explain why scientists and engineers choose specific materials for specific uses, from the wires in your home to the aircraft in the sky.

Key Terms to Remember

• Physical property — a characteristic observed without changing the substance’s chemical composition
• Chemical property — a characteristic observed only when a substance undergoes a chemical change
• Density — mass per unit volume of a substance
• Melting point — temperature at which a solid becomes a liquid
• Boiling point — temperature at which a liquid becomes a gas
• Solubility — ability of a substance to dissolve in a solvent
• Thermal conductivity — ability to transfer heat
• Flammability — ability to catch fire and burn
• Reactivity — how readily a substance reacts with other substances
• Corrosion — gradual destruction of a metal through chemical reaction with its environment