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Pure Substances and Mixtures

Key Concepts

A pure substance:
  • cannot be separated into 2 or more substances by physical or mechanical means

  • is homogeneous, ie, has uniform composition throughout the whole sample

  • its properties are constant throughout the whole sample

  • its properties do not depend on how it is prepared or purified

  • has constant chemical composition

A mixture:

  • can be separated into 2 or more substances by physical or mechanical means

  • displays the properties of the pure substances making it up

  • its composition can be varied by changing the proportion of pure substances making it up

  • heterogeneous substances, ones with non-uniform composition throughout the sample, are always mixtures

Pure Substances

Elements and compounds are both examples of pure substances.

Pure substances cannot be separated into simpler substances by physical or mechanical means such as sifting, filtering, crystallization, distillation, etc.
eg, distilling pure water (H2O) does not separate water into hydrogen and oxygen, it only produces water vapour.

Pure substances display a sharp melting and boiling point.
On a graph of temperature vs time, this is shown as flat line where the temperature does not change over time until all the pure substance has melted or boiled.

Pure substances will have a constant appearance, colour and density throughout the sample.

Pure substances have constant chemical composition, eg,

  • pure water (H2O) is always composed of 2 hydrogen atoms and 1 oxygen atom chemically bonded.
  • pure gold (Au) is only made up of gold atoms

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Some examples of mixtures are given below:

Type of Mixture Example
gas in gas The atmosphere is a mixture of gases, mostly nitrogen and oxygen.

liquid in liquid Wine is a mixture of mostly ethanol and water.

solid in solid Alloys, such as brass, are made up of a mixture of metals.

gas in liquid Soft drinks, such as cola, are mixtures of mainly carbon dioxide gas and water.

solid in liquid Sea Water is a mixture of salts dissolved in water.

solid in gas Smoke is mixture of tiny solid particles in atmospheric gases.

Homogeneous mixtures do not display a sharp melting point, they melt over a range of temperatures.
Sharpness of the melting point is often used to determine whether a substance is pure or impure (mixture).

On a temperature vs time graph there is no flat line during which the temperature remains constant over time. Instead, there will be a slope indicating that the components of the mixture are melting.

Mixtures can be separated into the pure substances making them up by physical or mechanical means because each pure substance retains its own properties.

Some methods for separating the components of a mixture include:

separation technique property used for separation example
Sifting (sieving) particle size alluvial gold is separating from smaller soil particles using a sieve

Visual Sorting colour, shape or size gold nuggets can be separated from crushed rock on the basis of colour

Magnetic Attraction magnestism magnetic iron can be separated from non-magnetic sulfur using a magnet

Decanting density or solubility liquid water can be poured off (decanted) insoluble sand sediment
less dense oil can be poured off (decanted) more dense water

Separating Funnel density of liquids in a separating funnel, less dense oil floats on top of more dense water, when the valve is open the water can be poured out from under the oil

Filtration solubility insoluble calcium carbonate can be separated from soluble sodium chloride in water by filtration

Evaporation solubility and boiling point soluble sodium chloride can be separated from water by evaporation

Crystallization solubility slightly soluble copper sulfate can be separated from water by crystallization

Distillation boiling point ethanol (ethyl alcohol) can be separated from water by distillation because ethanol has a lower boiling point than water

Quick Question

Question 1

Which of these is a mixture?

(a) Steel

(b) Iron

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