Substance types and determining purity
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Explainer Video
Summary
Substance types and determining purity
In a nutshell
Substances have different compositions and can be pure or impure. This summary will cover the differences between elements, compounds, pure and impure compounds and give examples.
Types of substances
Understanding different types of substances in chemistry is important. These are explained below.
SUBSTANCE | Definition | EXAMPLES |
| Element | Elements contain atoms with the same number of protons in their nucleus. They exist as atoms or molecules. Elements are found in the periodic table, so any element listed on there could be used as an example. | Hydrogen (), all atoms have one proton. Oxygen (), all atoms have eight protons. |
| Compound | Compounds contain two or more elements. They can be formed by chemically joining elements from the periodic table. They exist as molecules or ionic structures. | Water () consists of chemically joined and atoms. Copper sulfate () consists of chemically joined , and atoms. |
| Pure substance | Pure substances are the opposite of mixtures (impure). Pure substances contain only one element/compound. | Pure water contains only molecules. |
| Mixture | Mixtures contain multiple elements/compounds that are not chemically joined. These can be separated out. | Air is a mixture of elements, such as and , and compounds such as . Crude oil is a mixture of hydrocarbons. |
| Formulation | Formulations are mixtures with a precise purpose. Each component is present in a specific quantity and contributes to the properties required for function. They are important in everyday life and the pharmaceutical industry. | Paint consists of pigment, solvent and binder. Metal alloys consist of multiple different metals. |
Determining purity
The melting or boiling point indicates how pure a substance is.
- Pure substances melt/boil at a specific temperature.
- Mixtures melt/boil at a range of temperatures, that are usually lower or higher than pure substances due to impurities.
This can therefore be used to determine how pure your substance is.
You test purity by measuring the melting/boiling point of a substance and comparing it to the expected value if it were pure.
The closer the measured value to the actual value, the purer the sample.
Impurities lower the melting point, increase the boiling point and increase the range at which melting/boiling occurs.
Example
A student made aspirin in the lab and measured its melting point. It melted between and , however the textbook says it should melt at . Is the sample pure? Explain your answer.
The sample is not pure because it melted over a range of temperatures, not a specific one. The melting point is also lower than pure aspirin.
Using graphs to determine purity
This graph shows the substance is pure because the plateau in the graph shows it melts at the fixed temperature of . A sharp melting point always indicates purity. If it were a mixture, the range would be much greater.
Separating mixtures
Mixtures can be separated in various different ways.
Examples
- Filtration
- Crystallisation
- Distillation
- Chromatography
FAQs - Frequently Asked Questions
What are compounds?
Compounds contain two or more elements. They can be formed by chemically joining elements from the periodic table.
How does the melting and boiling point differ between pure substances and mixtures?
Pure substances melt/boil at a specific temperature. Mixtures melt/boil at a range of temperatures.
What is the difference between a mixture and a pure substance?
Pure substances only contain one element/compound. Mixtures contain multiple elements/compounds that are not chemically joined.