Is Gold a Pure Substance? Elements and Mixtures
Pure gold is a pure substance, because it is an element made of only one kind of atom (Au, atomic number 79). Gold jewellery is usually not pure: 18-carat gold is an alloy of 75% gold with copper and silver, which is a mixture. A pure substance has a fixed composition and fixed properties.
What is a pure substance?
A pure substance is a material made of only one kind of particle, with a fixed composition and fixed properties, such as a fixed melting point and boiling point. There are two kinds:
- Elements: one type of atom only, such as gold (Au), oxygen (O₂) or iron (Fe).
- Compounds: two or more elements chemically bonded in a fixed ratio, such as water (H₂O) or sodium chloride (NaCl).
Both can be shown by a single symbol or formula. All the elements are listed on the periodic table.
Is gold an element or compound?
Is gold an element or compound? It is an element. Gold has the symbol Au (from the Latin aurum) and atomic number 79, so every gold atom has 79 protons. It cannot be broken down into simpler substances by chemical reactions, which is the test for an element. Gold also has fixed properties: it melts at 1,064 °C and has a density of 19.3 g/cm³.
When is gold not a pure substance?
Pure gold is very soft, so jewellers mix it with other metals to make it harder. The mixture is an alloy, which is a type of mixture of metals. The carat scale tells you how much gold it contains:
| Carat | Gold content | Type |
|---|---|---|
| 24 carat | About 99.9% | Pure substance (element) |
| 22 carat | 91.7% | Alloy (mixture) |
| 18 carat | 75% | Alloy (mixture) |
| 14 carat | 58.3% | Alloy (mixture) |
| 9 carat | 37.5% | Alloy (mixture) |
So the answer is: 24-carat gold is a pure substance, and 18-carat gold is a mixture. Alloys do not have a fixed composition (two 18-carat rings may have different amounts of copper and silver), and their melting point is a range, not a single value.
Pure substance vs mixture
A mixture is two or more substances that are physically combined, not chemically bonded, so each keeps its own properties.
| Feature | Pure substance | Mixture |
|---|---|---|
| Composition | Fixed | Varies |
| Particles | One kind (element or compound) | More than one kind |
| Melting and boiling points | Sharp, fixed values | A range |
| Separating | Needs a chemical reaction | Physical methods (filtering, distillation) |
| Examples | Gold, water, salt | Air, sea water, brass |
A useful test: a pure substance melts at one exact temperature, while a mixture melts over a range. That is how scientists check the purity of a chemical in a lab.
Is salt a pure substance?
Is salt a pure substance? Table salt (sodium chloride, NaCl) is a compound, and so a pure substance, since it always contains sodium and chlorine ions in a 1 : 1 ratio. But the salt in your kitchen is often not completely pure, as it can contain anti-caking agents or iodine. And salt water is a mixture, since the salt and water are only mixed and can be separated by evaporation or distillation.
How to classify any material
Use this order of questions:
- Can it be separated by physical methods such as filtering, evaporating or using a magnet? If yes, it is a mixture.
- If no, can it be broken down into simpler substances by a chemical reaction? If yes, it is a compound.
- If no, it is an element.
Applied to examples: air (mixture), water (compound), oxygen (element), brass (mixture of copper and zinc), diamond (element, carbon), steel (mixture).
Why does it matter?
Chemists need pure substances to get reliable results, and engineers choose between pure metals and alloys for the right properties: copper is a good conductor but soft, while brass is harder. Pure silicon is needed in computer chips, with impurities added in carefully controlled amounts. To see how the particles in different states of matter are arranged, try states of matter.
Common mistakes
- Thinking that "pure" means "natural". Pure water made in a lab is as pure as water from a spring can be, if it contains only H₂O.
- Believing that a compound is a mixture because it contains different elements. In a compound they are chemically bonded in a fixed ratio.
- Calling all jewellery gold a pure substance. Only 24-carat gold is.
- Mixing up pure in everyday speech (clean) and in chemistry (one substance only).
Frequently asked questions
Is gold a pure substance or a mixture?
Pure gold (24 carat) is a pure substance, because it is an element made of only gold atoms. Gold jewellery below 24 carat, such as 18-carat gold, is an alloy of gold with other metals, so it is a mixture.
What is a pure substance in chemistry?
A pure substance is made of one type of particle only, so it has a fixed composition and fixed properties. Elements and compounds are both pure substances, while mixtures are not.
Is water a pure substance?
Pure water (H₂O) is a pure substance, a compound of hydrogen and oxygen in a fixed 2 : 1 ratio. Tap water and sea water are mixtures, since they contain dissolved minerals and gases.
What is the difference between a pure substance and a mixture?
A pure substance has a fixed composition and a sharp melting point, and it can only be separated by chemical reactions. A mixture has a variable composition, melts over a range and can be separated by physical methods.
Try it yourself
Open States of Matter and watch how the particles of a pure substance behave as you heat it. Then list five materials around you and classify each one as an element, a compound or a mixture, using the three questions above.
See it for yourself, in 3D and free.
Try it in the States of Matter