Matter and its classification

Why chemistry matters

Look around you. The medicine in a strip of tablets, the fertiliser on a field, the screen of a phone and the clean water in a tap all come from chemistry. Chemistry is the study of matter: what it is made of, what it is like, and how it changes.

A chemist works in two ways. First, a chemist sorts substances and describes them. Then a chemist measures them, with numbers and units. This lesson does both.

Three states of matter

Anything that has mass and takes up space is matter. Matter is made of tiny particles, and it exists in three states.

Heating or cooling changes the state. Ice melts to water, and water boils to steam. The particles stay the same; only their arrangement and their motion change.

Mixtures and pure substances

A mixture has two or more substances, mixed in any ratio. Each keeps its own properties.

• A homogeneous mixture is the same all through. Air, sugar dissolved in water and the alloy brass are examples.
• A heterogeneous mixture is not the same all through. You can see its parts, as in sand mixed with iron filings.

A pure substance has a fixed composition. There are two kinds.

• An element has only one kind of atom. Copper, silver and gold are elements. In some elements the atoms join in small groups: oxygen gas is made of $\text{O}_2$ molecules.
• A compound has atoms of two or more elements joined in a fixed ratio by mass. Water always holds hydrogen and oxygen in the mass ratio $1 : 8$.

A compound is not a mixture of its elements. Its properties are new. Hydrogen burns and oxygen helps burning, but water puts out a fire. Physical methods, such as filtering or distilling, can separate a mixture. Only a chemical reaction can break a compound into its elements.

Physical and chemical properties

A physical property can be measured or seen without changing the identity of the substance. Colour, smell, melting point, boiling point and density are physical properties.

A chemical property shows only in a chemical change, when the substance becomes something new. Whether a substance burns, whether it is acidic, and how it reacts with water are chemical properties.

The SI units

Scientists all over the world use the International System of Units (SI). It has seven base units.

Every other unit is built from these. A prefix makes a unit larger or smaller by a power of ten.

So $1\ \text{nm} = 10^{-9}\ \text{m}$, and $1\ \text{mg} = 10^{-3}\ \text{g}$.

Mass, volume and density

Mass is the amount of matter in a body. It is the same everywhere. Weight is the pull of gravity on that mass, so it changes from place to place. A stone weighs less on the Moon, but its mass does not change. Chemists find mass with a balance, in grams or kilograms.

Volume is the space a body takes up. The SI unit is the cubic metre, $\text{m}^3$. In the laboratory we use smaller units: $1\ \text{L} = 1\ \text{dm}^3 = 1000\ \text{cm}^3$, and $1\ \text{mL} = 1\ \text{cm}^3$.

Density is mass per unit volume:

Its SI unit is $\text{kg m}^{-3}$, and chemists often use $\text{g cm}^{-3}$. A solid denser than water sinks in it, and a less dense one floats.

Temperature

Three scales are in use. On the Celsius scale, water freezes at $0\ ^\circ\text{C}$ and boils at $100\ ^\circ\text{C}$. On the Fahrenheit scale, the same points are $32\ ^\circ\text{F}$ and $212\ ^\circ\text{F}$. The SI scale is the kelvin scale. It starts at absolute zero, the lowest temperature possible.

A kelvin temperature is never negative, and it is written without a degree sign: $300\ \text{K}$, not $300\ ^\circ\text{K}$.

Practise with new numbers

Try it in Python

The program works out the density of four samples, and says whether each floats in water.

Common mistakes

• Calling brass or air a compound. They are mixtures. Their composition can change, and their parts keep their own properties.
• Mixing up mass and weight. Mass does not depend on gravity. Weight does.
• Writing a degree sign with kelvin. Write $300\ \text{K}$, never $300\ ^\circ\text{K}$.
• Dividing the wrong way. Density is mass divided by volume, so its unit has the volume underneath.

Remember these

• Matter is a pure substance (an element or a compound) or a mixture (homogeneous or heterogeneous).
• A compound has its elements in a fixed ratio by mass, and it has new properties.
• A physical property keeps the substance the same. A chemical property shows in a chemical change.
• The seven SI base units are the metre, kilogram, second, ampere, kelvin, mole and candela.
• $d = \dfrac{m}{V}$, $T = t_C + 273.15$ and $t_F = 1.8\, t_C + 32$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Sort these into elements, compounds and mixtures: filtered sea water, carbon dioxide, an iron nail, a bowl of mixed nuts.

2. Butter melts in a warm pan, and paper burns in a flame. Which shows a chemical property?

3. An astronaut carries a $2\ \text{kg}$ bag to the Moon. What happens to its mass and to its weight?

4. Normal body temperature is $37\ ^\circ\text{C}$. What is it in kelvin?

5. Write $2.5\ \text{mg}$ in kilograms.