Elements and compounds
Every Irish kitchen has a tub of table salt. It is made from sodium, a soft metal that fizzes violently in water, and chlorine, a poisonous gas. Yet you sprinkle it on your chips. How can two dangerous substances make something safe to eat? By the end of this page you will be able to explain it.
Elements
An element is a pure substance made of only one type of atom. It cannot be broken down into simpler substances by chemical methods.
Oxygen, iron, gold, copper and carbon are all elements. Each has its own symbol, such as O, Fe, Au, Cu and C. There are about 118 elements, and every substance around you is built from them.
Water is $\text{H}_2\text{O}$: two hydrogen atoms for every one oxygen atom. Carbon dioxide is $\text{CO}_2$: one carbon atom for every two oxygen atoms. Sodium chloride is NaCl: one sodium atom for every one chlorine atom. A compound can only be split into its elements by a chemical change, such as heating strongly or passing electricity through it.
Define an element and a compound, and give one example of each.
Define an element and a compound, and give one example of each.
Classify each substance as an element or a compound: oxygen, water, iron, carbon dioxide, sodium chloride, gold.
Classify each substance as an element or a compound: oxygen, water, iron, carbon dioxide, sodium chloride, gold.
Name the elements present in each compound: a) water ($\text{H}2\text{O}$) b) carbon dioxide ($\text{CO}2$) c) sodium chloride (NaCl)
Name the elements present in each compound:
- a) water ($\text{H}_2\text{O}$)
- b) carbon dioxide ($\text{CO}_2$)
- c) sodium chloride (NaCl)
Metals and non-metals
Most elements are either metals or non-metals. You can tell them apart by their physical properties.
| Property | Metals | Non-metals |
|---|---|---|
| Appearance | Shiny (lustrous) | Dull |
| Electrical conductivity | Good conductors | Poor conductors (carbon as graphite is an exception) |
| Heat conductivity | Good conductors | Poor conductors |
| When solid | Malleable (can be hammered) and ductile (can be drawn into wires) | Brittle (break or crumble) |
| State at room temperature | Mostly solid | Solids or gases |
Copper is a metal. It is a good conductor and ductile, so it is used in the wiring of Irish homes. A metal such as iron conducts heat well and is strong, so it is used for cooking pots. Sulfur is a yellow, brittle, dull non-metal: it would shatter if you hammered it, and it conducts heat poorly. Non-metals such as plastic handles are used on pots to stop your hand getting burned.
Copper is used in electrical wiring in Irish homes, and sulfur is a yellow, brittle solid. a) Classify each as a metal or a non-metal. b) Give three physical properties…
Copper is used in electrical wiring in Irish homes, and sulfur is a yellow, brittle solid.
- a) Classify each as a metal or a non-metal.
- b) Give three physical properties that distinguish metals from non-metals.
- c) Explain why copper is suitable for wiring.
The table gives properties of four elements. | Element | Appearance | Conducts electricity | Brittle when solid | |---|---|---|---| | W | shiny | yes | no | | X | dull…
The table gives properties of four elements.
| Element | Appearance | Conducts electricity | Brittle when solid |
|---|---|---|---|
| W | shiny | yes | no |
| X | dull yellow | no | yes |
| Y | shiny | yes | no |
| Z | dull black | no | yes |
- a) Which elements are metals?
- b) Which elements are non-metals?
- c) Explain how you decided.
Compounds and mixtures
Mixing is not the same as combining chemically. Elements and compounds are pure substances; a mixture is not, because it contains more than one substance. A solution is a homogeneous mixture (it looks the same throughout), while most other mixtures are heterogeneous.
In a mixture the substances are only mixed together. They keep their own properties and can be separated by physical methods such as a magnet, filtering or evaporation. The amounts can vary.
In a compound the elements are chemically joined. The compound has new properties, it has a fixed ratio of atoms, and it can only be split by a chemical change.
| Mixture | Compound | |
|---|---|---|
| Joined chemically? | No | Yes |
| Properties | Those of the parts | New, different from the elements |
| Composition | Can vary | Fixed ratio |
| Separation | Physical methods | Chemical change only |
This answers our opening question. Sodium and chlorine are chemically joined in NaCl, so the compound has new properties. Salt is neither a soft reactive metal nor a green gas. Sea salt dissolved in water is different again: it is a mixture of many substances, such as sodium chloride and other salts, in water, and its composition can vary.
Air is a mixture of nitrogen, oxygen, argon and carbon dioxide. Its gases keep their own properties, the proportions can vary, and they can be separated by physical methods. Water, by contrast, is a compound: hydrogen and oxygen are chemically joined in a fixed ratio.
Iron filings and sulfur powder are mixed in a dish. A magnet removes the iron. The mixture is then heated strongly, and a dark grey solid, iron sulfide, forms. The…
Iron filings and sulfur powder are mixed in a dish. A magnet removes the iron. The mixture is then heated strongly, and a dark grey solid, iron sulfide, forms. The magnet no longer attracts it.
- a) Was the first stage forming a mixture or a compound?
- b) Explain the evidence that iron sulfide is a compound.
- c) Why can the iron not be separated using a magnet after heating?
Sodium is a soft, reactive metal and chlorine is a poisonous green gas. Together they form sodium chloride, which is table salt. a) Is sodium chloride an element, a…
Sodium is a soft, reactive metal and chlorine is a poisonous green gas. Together they form sodium chloride, which is table salt.
- a) Is sodium chloride an element, a compound or a mixture? Justify.
- b) Compare the properties of sodium chloride with those of its elements and explain what this shows about compounds.
- c) Describe how the composition of sea salt dissolved in water differs from that of sodium chloride.
Air is a mixture of nitrogen, oxygen, argon and carbon dioxide. a) Classify each gas as an element or a compound. b) Explain why air is a mixture and not a compound.…
Air is a mixture of nitrogen, oxygen, argon and carbon dioxide.
- a) Classify each gas as an element or a compound.
- b) Explain why air is a mixture and not a compound. Give two reasons.
- c) A student says: "Water is a mixture of hydrogen and oxygen." Evaluate this statement.
A scientist is given a grey shiny solid. It conducts electricity, can be hammered into sheets and does not break down when heated. a) What kind of substance is it most…
A scientist is given a grey shiny solid. It conducts electricity, can be hammered into sheets and does not break down when heated.
- a) What kind of substance is it most likely to be? Justify with evidence.
- b) Which further test or observation could help show that it is a single element rather than a compound?
- c) Explain why a non-metal is generally unsuitable for making cooking pots, whereas a metal is suitable.