Elements, Compounds and Mixtures

Elements, Compounds and Mixtures

Name: _______________________( ) Class: ______ Date: _________

1) Elements

Sodium chloride (salt) is a white solid. If you melt it at a high temperature and then pass an electric current through the molten liquid, it breaks down (decomposes) into a silvery liquid and a yellow-green gas. The silvery liquid is molten sodium and the gas is chlorine. This decomposition reaction is summarised below:

sodium chloride sodium + chlorine

(silvery liquid) (yellow-green gas)

No matter how hard you try, it is impossible to break down sodium or chlorine into simpler substances. Sodium and chlorine are examples of elements.

An element is a substance that cannot be broken down into simpler substances by ordinary chemical methods.

There are about 110 known elements. All substances are made up of these 110 elements. For example, water is made up of two elements called hydrogen and oxygen. Sugar is made up from the elements hydrogen, oxygen and carbon. Common salt is made up from the elements sodium and chlorine. The human body is made up of about 10 main elements and small amounts of many others. The element calcium is found in our bones and the element iron in our blood.

Each element has a name and symbol. The symbol is based on either the element's English name or its name in another language. For example, the symbol for the element iron is Fe - which comes from the Latin name for iron, ferrum. Some examples of common elements and their symbols are given in the table below.

Elements are classified as metals or non-metals, depending on how well they conduct electricity. Metals are good conductors of electricity; non-metals are usually very poor electrical conductors. Most elements are metals; only 22 elements are non-metals.

Metals can easily be recognised by their physical properties. A comparison of the physical properties of metals and non-metals is shown in the table.

2) Atoms

We know that matter is made up of tiny particles. These particles are called atoms. An atom is the smallest particle of any element

Atoms are very small. Each atom has a diameter of about 0.1 nanometre or 0.1 X 10 -9 metre. It would take about 1 000 000 000 000 000 or 1015 atoms to completely cover the head of a pin! We cannot see an atom with our naked eye. But if we look through an electron microscope, an instrument that can magnify objects millions of times, we might be able to see a crude picture of atoms as shown.

The bumps are individual atoms on the surface of a crystal of silicon. This is the best picture of atoms taken so far. A special electron microscope was use.

Each element consists of a particular type of atom. The atoms of each element are different from the atoms of any other element. For example, all hydrogen atoms are the same (although there can be slight differences between them – e.g. isotopes) and are different from atoms of carbon or iron.

3) Physical and Chemical Changes

In a physical change, no new substance is formed. These changes are easily reversed. An example is the dissolving of salt in water. The solution still consists of salt and water but does not contain any new substance. The change can "easily be reversed by evaporating the water to crystallise the salt. Other examples of physical changes include melting, boiling, mixing two solids or two liquids, and dissolving solids in solvents such as petrol and alcohol.

In a chemical change, a new substance is formed. The new substance has different properties such as different melting points and different chemical reactions. You can often recognise a chemical change from two observations:

1. The new substance has a different appearance, such as a different colour or a different physical state. For example, the rusting of iron is a chemical change. The iron is a shiny grey colour but the rust produced is a brown colour.

2. A lot of heat is often given out in a chemical change. For example, the burning of a candle or cylinder gas in air is a chemical change. These chemical changes produce heat. The heat from such changes is used to provide us with energy.

Chemists call chemical changes chemical reactions.

4) Compounds and Mixtures

Compounds

A compound is a substance containing two or more elements joined together by a chemical reaction. A compound has very different properties from its elements.

Magnesium oxide is a compound made of two elements: magnesium and oxygen. It is made by burning magnesium in air.

The heat and light show that a chemical reaction took place. The elements are a silvery solid and a colourless gas. The compound has different properties: it is a white solid.

Water is a compound made up of two elements: hydrogen and oxygen. It can be made by applying a lighted splint to a mixture of hydrogen and oxygen.

The heat and light produced by the lighted splint show that a chemical reaction took place. The element hydrogen is a flammable gas. The compound water has very different properties: it is a liquid and does not burn.

The figure below shows the changes in the particles when water is formed from its elements. Each `circle' represents one atom. So the particles of hydrogen, for example, consist of two atoms joined together. In the mixture of hydrogen and oxygen, before reaction, the hydrogen and oxygen atoms are not joined together. When the reaction takes place, the hydrogen and oxygen atoms join together to form the particles of water. Each particle of water consists of two hydrogen atoms and one oxygen atom joined.

The atoms of oxygen and hydrogen are tightly joined together in water

A particular compound always contains the same elements. These elements are always present in a fixed proportion in the compound. For example, the percentage composition of hydrogen and oxygen in water is always: 11% hydrogen, 89% oxygen. Water can be obtained from rain, rivers, the sea or a tap. It can be in the solid, liquid or gaseous state. In every case, it has the same percentage composition of hydrogen and oxygen.

When the elements combine together to form a compound, this compound cannot be separated again into its elements by physical means - such as filtering, distillation or dissolving in water. The atoms are firmly joined together. The atoms of different elements in a compound can only be broken apart in chemical reactions.

Mixtures

In a mixture of elements, the atoms of the elements are not joined together (see the mixture of hydrogen and oxygen shown ealier). The percentage of each element in a mixture is not always the same. A mixture does not have a fixed composition. A mixture of oxygen and hydrogen, for example, can have any composition of each element.

Brass is a mixture of two metals, copper and zinc. The percentage of zinc in most brass samples varies between 10% and 60%. Each element in a mixture retains its properties. A mixture can easily be separated by physical means such as filtration, crystallisation or distillation.

A mixture can also be made up of different compounds, like the black and white rock crystals in granite. A `picture' of a mixture of two compounds is shown in the left figure.

A good way of distinguishing a single compound from a mixture of compounds, is from its melting point and its boiling point. A single compound has a fixed melting point and (at sea level) a fixed boiling point. For example, pure water (a single compound) melts at 0°C and boils at 100°C. Butter (a mixture of compounds) melts over the range 30°C to 50°C. This means butter starts melting at 30°C and is completely melted at 50°C. Kerosene fuel used for jet aircraft (also a mixture of compounds) boils over the range of 170°C to 250°C.

Many common substances are mixtures - including air, sea water, concrete and cooking oil. Most metals are used as mixtures because they are stronger than pure metals. That is why brass is used for the pins of electric plugs instead of pure copper. Such mixtures of metals are called alloys.

A comparison of the characteristics of compounds and mixtures is summarised as in the table below.

Adapted from Chemistry for “O” level 3rd Edition by JGR Briggs Pg 38-44

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