Page 70 - SCERT Kerala State Syllabus 8th Standard Basic Science Textbooks English Medium Part 1_Neat

Doesn't the smell linger even after the camphor has disappeared
completely? Here also, the particles of camphor are not visible.
Now can't you infer that camphor has spread in air as extremely
small particles.
Isn't it the same that happens when water taken in a watch glass
evaporates?

It may therefore be learnt that all the substances like sugar candy,
potassium permanganate, water and camphor are made of minute
particles.
Similarly, pure substances like gold, silver etc., are also made of
extremely small particles.
Can pure substances be further decomposed?

Fill one quarter of a boiling tube with sugar and close it with
cotton. Then heat the boiling tube strongly (Figure 5.1). What can
be observed? What is seen on the sides of the boiling tube?
•

•
Fig. 5.1
Heating of sugar What are the substances obtained by heating sugar?

Now you know that carbon and water are the components of
sugar. Carbon obtained by heating sugar cannot be further
divided. But, in 1806 Sir Humphry Davy discovered that when
electricity is passed through water it splits into hydrogen and
oxygen. In fact, Davy's finding was made possible by the
discovery of Henry Cavendish that hydrogen burns in oxygen to
give water. Sugar, a pure substance can be further divided into
Sir Humphry Davy
(1778 - 1829) carbon, hydrogen and oxygen. Similarly, another pure substance,
water can be divided into hydrogen and oxygen. At the same
time, pure substances like carbon, hydrogen, oxygen, gold and
silver cannot be further divided into its components by chemical
reactions.
Elements and Compounds

Don't you now realise that pure substances are of two kinds? Of
these, the pure substances which cannot be further decomposed
through chemical processes are called elements. Find more
examples of elements and list them.
Henry Cavendish • Hydrogen •
(1731 - 1810)
Basic Science VIII
Basic Science VIII 69 69