C27: Pure Substances and Formulations
Understanding purity in chemistry, how melting and boiling points indicate purity, and the purpose and composition of formulations.
What is a Pure Substance?
In everyday language, "pure" means something that is free from contamination. In chemistry, a pure substance is a single element or compound that contains only one type of atom or one type of molecule throughout.
A pure substance has a sharp, specific melting point and a sharp, specific boiling point. These fixed points can be used to identify the substance and confirm its purity.
Examples of pure substances include:
- Distilled water (H₂O only)
- Carbon dioxide (CO₂ only)
- Iron (Fe only)
- Sodium chloride (NaCl only)
Melting and Boiling Point Analysis
The melting point and boiling point of a substance can be used to determine whether it is pure. A pure substance melts and boils at a single, fixed temperature. A mixture melts and boils over a range of temperatures.
Worked Example: Identifying Purity from Melting Point Data
A sample of aspirin is heated. It begins to melt at 134 °C and is fully melted at 138 °C. Pure aspirin has a melting point of 136 °C.
The sample melts over a range of 4 °C rather than at a single sharp temperature. This means the sample is not pure — it contains impurities.
Impurities lower the melting point of a substance and raise the boiling point. The greater the amount of impurity, the wider the melting or boiling range.
Worked Example: Boiling Point Analysis
Pure water boils at exactly 100 °C at standard pressure. A sample of tap water begins to boil at 99.8 °C and continues boiling up to 100.3 °C.
Since the boiling occurs over a range rather than at a single point, the tap water is not pure. It contains dissolved substances (impurities) such as mineral ions.
Pure Substances vs Mixtures
| Property | Pure Substance | Mixture |
|---|---|---|
| Composition | Single element or compound | Two or more substances combined |
| Melting point | Sharp, fixed temperature | Melts over a range of temperatures |
| Boiling point | Sharp, fixed temperature | Boils over a range of temperatures |
| Effect of impurities | No impurities present | Impurities lower melting point and raise boiling point |
| Separation | Cannot be separated by physical means | Can be separated by physical methods (filtration, distillation, chromatography) |
| Examples | Distilled water, oxygen, sodium chloride | Tap water, air, seawater, rock salt |
Formulations
A formulation is a mixture that has been designed as a useful product. It is made by mixing specific quantities of different substances (components) in carefully measured proportions so that the product meets its required purpose.
Formulations are essential in everyday life. The exact composition of a formulation is carefully controlled to ensure it works correctly and consistently.
Each component in a formulation has a specific function. Changing the proportions of the components changes the properties and performance of the product.
Common Formulations
| Formulation | Components | Purpose of Each Component |
|---|---|---|
| Medicines (e.g. tablets) | Active drug, binder, filler, coating | Drug treats condition; binder holds tablet together; filler adds bulk; coating makes it easy to swallow |
| Paints | Pigment, binder (resin), solvent | Pigment provides colour; binder holds pigment to surface; solvent dissolves components for easy application |
| Fertilisers | Nitrogen compounds, phosphorus compounds, potassium compounds | N promotes leaf growth; P promotes root growth; K promotes flower and fruit growth |
| Cleaning products | Active ingredient, surfactant, fragrance, water | Active ingredient removes dirt/stains; surfactant lowers surface tension; fragrance gives pleasant smell |
| Fuels | Hydrocarbon mixtures, additives | Hydrocarbons provide energy; additives improve performance and reduce emissions |
| Alloys | Metal with other elements | Other elements improve strength, hardness or resistance to corrosion |
Why Formulations Matter
Formulations are not pure substances. They are carefully designed mixtures where each component plays a specific role. The ratio of each component is critical to the performance and safety of the product.
Worked Example: Fertiliser Formulation
A farmer needs a fertiliser to promote root growth in young plants. The NPK ratio on the fertiliser bag shows the percentage of nitrogen (N), phosphorus (P₂O₅) and potassium (K₂O).
A fertiliser labelled 10:20:10 contains 10% nitrogen, 20% phosphorus pentoxide and 10% potassium oxide. The high phosphorus content makes it suitable for root growth.
Choosing the wrong NPK ratio could harm the crop or waste money, so the formulation must match the plant's needs.
Testing Purity in the Laboratory
To test whether a substance is pure, measure its melting point or boiling point and compare it to known data for the pure substance:
- If the measured value matches the reference value exactly, the substance is likely pure.
- If the substance melts or boils over a range of temperatures, it is a mixture.
- If the melting point is lower than expected, impurities are present.
Worked Example: Testing the Purity of Paracetamol
A student synthesises paracetamol and wants to check its purity. Pure paracetamol melts at 169 °C.
The student's sample starts melting at 165 °C and is fully liquid at 168 °C.
Since the sample melts over a range (165–168 °C) and starts melting below 169 °C, the sample is impure. The impurities have lowered the melting point.
Everyday Purity vs Chemical Purity
In everyday language, "pure" often means "nothing added" or "natural". For example, "pure orange juice" means no added sugar or water, but it is still a mixture of many chemical compounds. In chemistry, this orange juice is not a pure substance.
Common misconceptions:
- "Pure" on a food label does not mean the product is a pure substance in chemistry terms.
- Spring water and mineral water are mixtures — they contain dissolved minerals.
- Only distilled or deionised water is chemically pure water.
Separating Mixtures
Since formulations are mixtures, their components can be separated by physical methods. The choice of method depends on the type of mixture:
| Separation Method | Type of Mixture | How It Works |
|---|---|---|
| Filtration | Insoluble solid + liquid | Filter paper allows liquid through but traps solid |
| Evaporation | Soluble solid + liquid (solution) | Liquid evaporates, leaving solid behind |
| Simple distillation | Liquid + dissolved solid | Liquid is boiled off and condensed; solid remains |
| Fractional distillation | Miscible liquids | Liquids separated by their different boiling points |
| Chromatography | Dissolved substances | Substances separated by their different solubilities |
In the exam, you may be given melting point or boiling point data and asked to determine whether a substance is pure. Remember: a pure substance has a sharp melting/boiling point, while a mixture melts or boils over a range of temperatures.
Practice Questions
1. A sample of stearic acid begins to melt at 67 °C and is fully melted at 71 °C. Pure stearic acid melts at 69.3 °C. Is the sample pure? Explain your answer.
No, the sample is not pure. It melts over a range of temperatures (67–71 °C) rather than at a single sharp point. Impurities are present, which have lowered the melting point and widened the melting range.
2. Explain why tap water is not a pure substance in chemistry terms.
Tap water contains dissolved substances such as mineral ions (calcium, magnesium, chloride, fluoride) and treatment chemicals. It is a mixture, not a single compound, so it is not chemically pure.
3. Describe the role of each component in a paint formulation.
Pigment provides the colour. Binder (resin) attaches the pigment firmly to the surface being painted. Solvent dissolves the components so the paint can be spread easily and then evaporates as the paint dries.
4. A fertiliser has an NPK ratio of 15:5:20. State the percentage of each component and suggest which type of plant growth this fertiliser would best support.
15% nitrogen, 5% phosphorus pentoxide, 20% potassium oxide. The high potassium content would best support flower and fruit growth.
5. Explain the difference between the everyday meaning of "pure" and the chemical meaning of "pure substance".
In everyday language, "pure" means nothing has been added or the product is natural — for example, "pure orange juice" means no added sugar. In chemistry, a pure substance is a single element or compound with only one type of particle throughout, and it has a sharp melting and boiling point.
Maths Skills
Interpreting Melting Point Data
Melting point data can be used to determine whether a substance is pure or a mixture.
A pure substance has a single, sharp melting point. For example, pure water melts at exactly 0 °C.
A mixture melts over a range of temperatures and starts melting below the melting point of the pure substance. The greater the amount of impurity, the lower the melting point and the wider the range.
Example interpretation: A sample of aspirin melts between 128–135 °C. The data book value for pure aspirin is 136 °C. Since the sample melts over a range and below the expected value, it is impure.
Calculating purity: If a substance melts at a single temperature that matches the data book value, it is pure. If it melts over a range, the width of the range indicates the degree of impurity. A melting range of 1–2 °C suggests high purity; a range of 5 °C or more suggests significant impurity.
Common Misconceptions
Misconceptions About Purity and Melting Points
Pure substances always melt at a higher temperature than mixtures.
Pure substances melt at a single, specific temperature, whereas mixtures melt over a range of temperatures. The melting point of a mixture is actually lower than the pure substance — impurities disrupt the crystal lattice, making it easier to break apart. So a mixture starts melting at a lower temperature and continues over a range.
A formulation is an impure substance.
A formulation is a deliberate mixture designed to have useful properties — it is not "impure" in the sense of being accidentally contaminated. Formulations like medicines, paints and fuels are carefully designed mixtures where each component has a specific purpose. Purity in chemistry means a single substance; formulations are mixtures by design.
In everyday language and chemistry, "pure" means the same thing.
In everyday language, "pure" often means nothing has been added (e.g. "pure orange juice"). In chemistry, a pure substance is a single element or compound with only one type of particle throughout. Orange juice is a mixture, so it is not chemically pure.
6-Mark Question
Explain how melting point data can be used to determine the purity of a substance.
A pure substance has a sharp, specific melting point because all the particles are identical and require the same amount of energy to overcome the intermolecular forces holding them in the solid structure. When a substance is heated and melts at a single temperature that matches the data book value, this confirms it is pure. If a sample contains impurities, the melting point is lowered and the substance melts over a range of temperatures rather than at a single point. This is because the impurity particles disrupt the regular arrangement of the pure substance, weakening the structure and allowing it to break apart at a lower temperature. The more impurity present, the lower and wider the melting range. To test purity, the sample is slowly heated and the temperature at which it starts and finishes melting is recorded. A narrow melting range close to the data book value indicates high purity; a wide range significantly below the expected value indicates impurity. Melting point apparatus can be used for accurate measurement.
AO3: Analysis and Evaluation
Interpreting Melting Point Data
Three samples of benzoic acid (data book melting point: 122 °C) were tested:
- Sample A: melted at 121.8–122.2 °C
- Sample B: melted at 117–121 °C
- Sample C: melted at 122 °C (sharp)
Evaluate the purity of each sample and explain your reasoning.
Answer: Sample C is pure — it has a sharp melting point exactly matching the data book value. Sample A is very nearly pure — its melting range is only 0.4 °C wide and centred on 122 °C, suggesting only a tiny amount of impurity. Sample B is impure — it melts over a 4 °C range and the melting starts 5 °C below the pure value. The lower starting temperature and wider range both indicate significant impurity is present that disrupts the crystal structure.
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