Pure substances and elements

2.1 Pure in the shop and pure in science

Packs of milk, ghee and spices often carry the word pure. In everyday language, pure means unadulterated: nothing cheap or of poor quality has been added. Adulteration is an illegal practice of adding such substances to increase the quantity or reduce the cost, and it spoils the quality and can even make the product hazardous.

In science, a pure substance contains nothing else. One kind of particle makes up the whole material, and no physical process can split it into different kinds of matter. Products that look pure to a shopper may be mixtures to a scientist. Milk contains water, fat, protein and other substances, so science classifies it as a mixture even though the packing calls it pure.

2.2 From water to its elements

Water is a pure substance at the level of its molecules. Cooling it gives ice and boiling it gives vapour, but both are still water. Passing electricity through acidified water breaks it down differently. Two test tubes filled with water are placed over the terminals of a 9 V battery, and gas bubbles collect at both terminals.

The gas from one tube burns with a pop sound when a burning candle is brought near it, which identifies hydrogen. The gas from the other tube makes the candle flame glow brighter, which identifies oxygen. The volume of hydrogen collected is about twice the volume of oxygen, which matches the fixed ratio in water molecules. Water is therefore built from two elements, hydrogen and oxygen.

2.3 Elements

The two gases produced from water are elements: pure substances that chemical reactions cannot split into simpler substances. Each element is made of identical particles called atoms, and the atoms of one element differ from those of every other element. Elements are the building blocks of all matter. Gold, silver, sulfur and carbon are common examples.

The atoms of most elements cannot exist independently. Two or more atoms of the same element combine to form a stable particle called a molecule. Two hydrogen atoms make one hydrogen molecule, and two oxygen atoms make one oxygen molecule.

2.4 Metals, non-metals and metalloids

Elements are grouped as metals and non-metals. Gold, silver, iron, magnesium and aluminium are metals, while carbon, sulfur, oxygen and hydrogen are non-metals. A few elements such as silicon and boron sit in between and are called metalloids.

At present 118 elements are known. Most exist as solids, eleven exist as gases at room temperature, and only two are liquids at room temperature: mercury (a metal) and bromine (a non-metal). Gallium and caesium are solids that become liquid near 30 °C. A single mobile phone uses more than 45 elements.

2.5 Why electrolysis is a chemical change

Boiling water and freezing water are physical changes: the particles are still water particles, and the changes can be reversed by cooling or heating. Passing electricity through acidified water is different. The water breaks into hydrogen and oxygen, which are new substances with their own properties. No simple physical treatment turns the collected hydrogen and oxygen back into water; the gases have to be reacted chemically.

This is why the change is described as chemical rather than physical. It also explains why water is a compound rather than a mixture: the elements inside it are joined chemically and cannot be separated by filtering, evaporating or magnetising.

2.6 Atoms, molecules and the particle view

An atom is the smallest particle of an element that keeps the properties of that element. A molecule is a particle made of two or more atoms joined together. The molecules of an element contain only atoms of that element, such as two hydrogen atoms in a hydrogen molecule or two oxygen atoms in an oxygen molecule. The molecules of a compound contain atoms of different elements, as in water, where two hydrogen atoms and one oxygen atom are joined.

The particle view makes the classification concrete. A pure element contains only one kind of atom. A pure compound contains molecules with a fixed combination of atoms. A mixture contains particles of different pure substances that are not joined chemically.

2.7 Elements around us

Metals such as iron, aluminium and copper are used for construction, wiring and utensils because they are strong, conduct electricity or conduct heat. Non-metals such as oxygen, nitrogen, sulfur and carbon are equally important: oxygen supports life and combustion, nitrogen makes up most of the air, and carbon is the basis of fuels, plastics and living things. Silicon and boron behave in between, and are called metalloids.

The state of an element at room temperature is another property worth knowing. Most elements are solids, eleven are gases, and only mercury and bromine are liquids; gallium and caesium melt just above room temperature.