To understand the difference between a pure substance and a mixture, let us understand the difference between a physical and a chemical change.
The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point etc. are the physical properites.
The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance.
Although ice, water and water vapour all look different and display different physical properties, they are chemically the same.
Both water and cooking oil are liquid but their chemical characteristics are different. They differ in odour and inflammability. We know that oil burns in air whereas water extinguishes fire. It is this chemical property of oil that makes it different from water.
Burning is a chemical change. During this process one substance reacts with another to undergo a change in chemical composition. Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a chemical reaction.
During burning of a candle, both physical and chemical changes take place.
On the basis of their chemical composition, substances can be classified either as elements or compounds.
Robert Boyle was the first scientist to use the term element in 1661. Antoine Laurent Lavoisier (1743-94), a French chemist, was the first to establish an experimentally useful definition of an element. He defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
Elements can be normally divided into metals, non-metals and metalloids.
Metals usually show some or all of the following properties:
They have a lustre (shine).
They have silvery-grey or golden-yellow colour.
They conduct heat and electricity.
They are ductile (can be drawn into wires).
They are malleable (can be hammered into thin sheets).
They are sonorous (make a ringing sound when hit).
Examples of metals are gold, silver, copper, iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature.
Non-metals usually show some or all of the following properties:
They display a variety of colours.
They are poor conductors of heat and electricity.
They are not lustrous, sonorous or malleable.
Examples of non-metals are hydrogen,oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.
Some elements have intermediate properties between those of metals and non-metals, they are called metalloids; examples are boron, silicon, germanium etc.
A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
Divide the class into two groups. Give5 g of iron filings and 3 g of sulphur powder in a china dish to both the groups.
• Mix and crush iron filings and sulphur powder.
• Mix and crush iron filings and sulphur powder. Heat this mixture strongly till red hot. Remove from flame and let the mixture cool.
Groups I and II
• Check for magnetism in the material obtained. Bring a magnet near the material and check if the material is attracted towards the magnet.
• Compare the texture and colour of the material obtained by each group.
• Add carbon disulphide to one part of the material obtained. Stir well and filter.
• Add dilute sulphuric acid or dilute hydrochloric acid to the other part of the material obtained.(Note: teacher supervision is necessary for this activity).
• Perform all the above steps with both the elements (iron and sulphur) separately.
The gas obtained by Group I is hydrogen, it is colourless, odourless and combustible– it is not advised to do the combustion test for hydrogen in the class.
The gas obtained by Group II is hydrogen sulphide. It is a colourless gas with the smell of rotten eggs.
You must have observed that the products obtained by both the groups show different properties, though the starting materials were the same.
Group I has carried out the activity involving a physical change whereas in case of Group II, a chemical change (a chemical reaction) has taken place.
The material obtained by group I is a mixture of the two substances. The substances given are the elements– iron and sulphur.
The properties of the mixture are the same as that of its constituents.
The material obtained by group II is a compound.
On heating the two elements strongly we get a compound, which has totally different properties compared to the combining elements.The composition of a compound is the same throughout. We can also observe that the texture and the colour of the compound are the same throughout.
So we can generalise the differences between mixtures and compounds as given below (Fig.1)
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