8.1Chapter 8: Pure Substances and Solutions

Pure substances — Alhussan Chemistry - Stage 8 Revision Notes & Topical Questions

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8.1 Pure substances — Purity, Melting Points and Boiling Points — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define pure substance and purity, and use melting and boiling points to judge whether a substance is pure.Pure substance = a single element or a single…

Everyday meaning
Why it matters
Impure substance
Everyday uses
Pure result
Pure sample

Part 1 of 6What pure means

Pure substance

Pure substance a single element or a single compound with nothing else mixed in; every particle is the same.

Pure silicon contains silicon atoms only and is used to make integrated circuits in phones.
Pure water contains water molecules only.

Everyday meaning

Everyday meaning in everyday life pure often means natural, unprocessed or with nothing added,
- such as pure orange juice,
- which differs from the scientific meaning.

Mixtures

Mixtures tap water, sea water, mineral water, air, steel, milk and soil are mixtures.
Tap water contains other substances such as chlorine and carbonates, so a vaccine company uses pure water instead.

Part 2 of 6Purity

Purity

Purity how much of a substance is in a mixture, often given as a percentage.

In 100 g of seawater there is 3.7 g of salt and 96.3 g of water, so the purity of the water is 96.3%.
Ibuprofen powder with 98% purity contains 98 g or more of ibuprofen in 100 g of powder.

Why it matters

Why it matters impurities can change properties and can be harmful,
- so medicines, food additives, electronic materials and fuels must meet purity standards.

Part 3 of 6Melting and boiling points as tests

Pure substance

Pure substance it melts and boils at definite, sharp temperatures that can be looked up in data tables.
Pure water melts at 0 °C and boils at 100 °C at normal pressure.

Impure substance

Impure substance impurities change the melting and boiling behaviour.
The melting point is lower than the pure value and the substance melts over a range of temperatures.
The boiling point is higher than the pure value and it boils over a range.
The more impurity, the greater the change.

Sample	Melting behaviour	Conclusion
Melts sharply at 80 °C (data value 80 °C)	Single temperature	Pure
Melts from 72 to 78 °C	Range, lower	Impure

Everyday uses
- salt on icy roads works because salt water freezes below 0 °C,
- and antifreeze lowers the freezing point and raises the boiling point of car coolant.

Part 4 of 6Cooling curve of stearic acid

Method

Method hot liquid stearic acid is poured into a test tube and left to cool, with the temperature recorded every minute.

Pure result

Pure result the temperature stays constant (about 70 °C) while the stearic acid freezes, which shows it is pure.
A graph of a pure substance has a flat section at its melting point.

Impure result

Impure result the temperature of an impure substance changes as it freezes, so there is no flat section.

Formula

Formula stearic acid is C₁₈H₃₆O₂, containing carbon, hydrogen and oxygen;
- ibuprofen, C₁₃H₁₈O₂, contains the same three elements.

Part 5 of 6Measuring a melting point

Put a small amount of solid in a thin glass capillary tube and attach it to a thermometer.

Place it in a beaker of water or oil heated slowly, while stirring.

Record the temperature when the solid starts to melt and when it has finished melting.

Compare the range with the data-book value.

Pure sample

Pure sample a narrow range that matches the data value shows it is pure.
- (less than about 2 °C)

Slow heating

Slow heating it is crucial for accuracy because the thermometer lags behind the sample if the temperature rises too fast.

Part 6 of 6Other tests and applications

Chromatography

Chromatography a pure dye makes a single spot, whereas a mixture makes several.

Industry

Industry purity is measured with more sophisticated instruments.

Uses of pure substances

Uses of pure substances pharmaceuticals, electronics and chemical experiments.
- (very pure silicon)

Purifying

Purifying crystallisation or distillation can purify a sample, and the melting point is then re-tested.

Key terms

Term	Meaning
Pure substance	a single element or compound with nothing mixed in.
Purity	how much of a substance is in a mixture.
Impurity	an unwanted substance mixed in.
Melting point	the temperature at which a solid melts.
Boiling point	the temperature at which a liquid boils.
Capillary tube	a very thin glass tube.