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Compound

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notes

Compounds:

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

Mixtures Compounds
Elements or compounds just mix together to form a mixture and no new compound is formed. Elements react to form new compounds.

A mixture has a variable composition.

The composition of each new substance is always fixed.
A mixture shows the properties of the constituent substances. The new substance has totally different properties.
The constituents can be separated fairly easily by physical methods. The constituents can be separated only by chemical or electrochemical reactions.

experiment

Group 1:

  • Activity: Mix and crush iron filings and sulphur powder.
  • Observation: is a mixture of the two substances i.e. iron and sulphur. The properties of the mixture are the same as that of its constituents.

Group 2:

  • Activity: Mix and crush iron filings and sulphur powder. Heat this mixture strongly till red hot. Remove from flame and let the mixture cool.
  • Observation: The material obtained by group II is a compound (iron sulphide) which has totally different properties compared to the combining elements.
  • Inference: Group 1 mixture will show some magnetic properties as in a mixture, individual properties are retained and we all know that iron is magnetic! Iron sulphide is non-magnetic as it has reacted with sulphur and formed a covalent bond and hence no free electrons are present in the outermost shell.
    Iron sulphide does not react with carbon sulphide but reacts with hydrochloric acid. Metal salts react with acids and displaces hydrogen atoms and hence hydrogen sulphide is obtained which has the smell of rotten eggs.
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