Substances and Mixture

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A pure substance:
• contains atoms of only one kind.
• It has fixed physical and chemical properties like boiling point, melting point, valency, density  • contains only one chemical identity, e.g. one element or one compound. • 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

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. A mixture:
• can be separated into 2 or more substances by physical or mechanical means • contain more than one chemical substance
• 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 • they do not have a fixed composition

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

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. | |[pic] | |liquid in liquid |Wine is a mixture of mostly ethanol and water. | |[pic] | |solid in solid |Alloys, such as brass, are made up of a mixture of metals. | |[pic] | |gas in liquid |Soft drinks, such as cola, are mixtures of mainly carbon dioxide gas and water. | |[pic] | |solid in liquid |Sea Water is a mixture of salts dissolved in water. | |[pic] | |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. Separating the Components of a Mixture

Most laboratory work in biology requires the use of techniques to separate the...
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