Purity and Formulations

In chemistry, a pure substance is a single element or compound that is not mixed with any other substance. That is a much stricter meaning than the one used in everyday life.

In shops and adverts, "pure" usually means natural, unprocessed, or with nothing artificial added. A carton labelled "pure orange juice" or "pure milk" fits that everyday meaning, but to a chemist both are mixtures: orange juice is water plus sugars, acids, colourings and pulp, and milk is water plus fats, proteins, sugars and minerals.

Substance	Everyday label	Chemically pure?	Why
Distilled water	Pure water	Yes	Only H₂O, one compound
Copper wire	Pure copper	Yes	Only Cu, one element
"Pure" orange juice	Pure	No	Water, sugars, acids, pulp mixed together
"Pure" milk	Pure	No	Water, fats, proteins, sugars mixed together

A pure substance in chemistry is one single element or one single compound. If it contains more than one substance, it is a mixture, no matter what the packaging says.

A pure element or compound melts and boils at specific, fixed temperatures. These sharp values are a fingerprint you can look up and check against.

Pure water, for example, melts (freezes) at exactly 0 °C and boils at exactly 100 °C at normal atmospheric pressure. Pure sodium chloride melts at 801 °C. Because the value is fixed, comparing a measured melting or boiling point against the known value for that substance tells you whether your sample is pure.

Pure substance	Melting point (°C)	Boiling point (°C)
Water	0	100
Ethanol	−114	78
Sodium chloride	801	1465
Oxygen	−219	−183

The key idea is that a pure substance changes state over a very narrow temperature. It does not melt a little at one temperature and finish melting several degrees later. That sharpness is what you test for.

Adding an impurity to a substance changes how it melts in two clear ways, and both are testable in the exam.

An impurity lowers the melting point of a substance, so it starts to melt below the expected value. An impurity also causes the substance to melt over a range of temperatures rather than at one sharp point. This is why grit (salt) is spread on icy roads: the salt lowers the melting point of ice so it melts below 0 °C.

Sample	Melting behaviour	Conclusion
Pure substance	Melts sharply at one fixed temperature	Pure
Impure substance	Melts below the expected value, over a range	Contains impurities

Worked example. Pure aspirin melts at 135 °C. A student's prepared sample begins to melt at 128 °C and finishes melting at 132 °C.

• The sample starts melting below 135 °C, and it melts over a range (128–132 °C) rather than at one temperature.
• Both signs point to impurity, so the student's aspirin is not pure.

Two signs of impurity: the melting point is lower than expected, and melting happens over a range of temperatures instead of at a single sharp point.

Exam questions often give a small table of data and ask you to decide, with a reason, whether a sample is pure. Use the two rules together and always quote the numbers.

Worked example. Three samples labelled as the compound stearic acid are tested. Pure stearic acid melts at 69 °C.

Sample	Melting range measured (°C)
A	69 (sharp)
B	55–63
C	63–68

• Sample A melts sharply at exactly 69 °C, matching the known value, so it is pure.
• Sample B melts over a wide range (55–63 °C), well below 69 °C, so it is impure and quite heavily contaminated.
• Sample C melts over a range (63–68 °C) below 69 °C, so it is impure, though the narrower, higher range suggests it contains less impurity than sample B.

To gain full marks you must name the evidence: state that a pure substance melts at a single fixed temperature, then compare the measured range and value against that.

A formulation is a mixture that has been designed as a useful product. Each component is added in a carefully measured quantity, and each one has a specific purpose in the final product.

This is different from a random mixture. In a formulation the recipe is deliberate: change the proportions and you change how the product performs. A paint that is 40% pigment behaves differently from one that is 20% pigment, so manufacturers control every component precisely.

A formulation is a mixture in which each component is present in a measured amount and has a particular job, so the product works exactly as intended.

Take household paint as an example of the components each doing a job:

Component	Purpose in the paint
Pigment	Gives the colour
Binder	Sticks the pigment to the surface as it dries
Solvent	Keeps the paint liquid so it spreads
Additives	Improve properties such as drying time or texture

Because each ingredient is measured and purposeful, paint is a formulation rather than an ordinary mixture.

The specification lists several everyday product types that are formulations. You should be able to recognise any of them and give a reason.

Formulation	Why it is a formulation
Fuels (e.g. petrol)	Blended hydrocarbons plus additives, mixed to give the right performance
Cleaning agents	Detergents, fragrances and other agents in set proportions
Paints	Pigment, binder, solvent and additives, each measured
Medicines	Active drug plus fillers and coatings, dosed precisely
Alloys	Metals mixed in fixed proportions to tune the properties
Fertilisers	Nutrient compounds combined in measured amounts
Foods	Processed foods blend ingredients to a controlled recipe

The reason a product counts as a formulation stays the same across every example: it is a mixture, made to a recipe, with each component measured out for a specific purpose. You do not need to know the trade names of the ingredients in any branded product.

(Note) The specification does not require you to name the components inside any specific proprietary (branded) product. You only need to identify that something is a formulation and explain why.

1. Using the everyday meaning of "pure"

If a question calls something "pure orange juice" or "pure honey", do not accept it as chemically pure. In chemistry, pure means a single element or compound, so anything with more than one substance in it is a mixture.

2. Forgetting that an impurity lowers the melting point

An impurity does not raise the melting point. It lowers it and spreads melting over a range. State both effects when you judge purity.

3. Not quoting the data in a purity answer

Saying "it is impure" is not enough for the marks. Compare the measured value with the known melting point and state that pure substances melt at a single fixed temperature.

4. Confusing a formulation with any old mixture

A formulation is not just any mixture. The examiner wants two ideas: each component is present in a measured quantity and each has a specific purpose in the product.

5. Trying to name branded ingredients

You are never asked to list the exact contents of a named brand of paint or medicine. Identify that it is a formulation and explain why, rather than guessing ingredients.