Page 82 - SCERT Kerala State Syllabus 8th Standard Basic Science Textbooks English Medium Part 1_Neat

Think about the differences between the two types of changes
given in the table. In physical changes, only the arrangement of
molecules within the substance changes. This is a temporary
change. But in a chemical change, one substance gets completely
transformed into a new substance. That is, new molecules are
formed. This is a permanent change.

During a physical change, only a change in the arrangement of
molecules occurs. Hence it can be easily brought back to its original
state. In the case of chemical changes, new molecules are formed.

Let us familiarise ourselves with a few more chemical changes.
Take two beakers and fill three-fourth of them with water. Drop
a small piece of sodium in one beaker. Notice the formation of
a gas by the vigorous reaction between sodium and water. After
the reaction, add two drops of phenolphthalein in both the
beakers. What do you observe?
The presence of which substance is indicated by the colour
change in the beaker to which sodium was added?

Fig. 6.2
The alkali formed by the reaction of sodium with water is sodium
hydroxide. Metals like sodium and potassium react vigorously
with water to form hydrogen gas and the corresponding alkali.
Write the reactants and products involved in the above reaction.
Reactants

Fig. 6.3
Products

Write down the balanced chemical equation of this reaction.

You know that heat and light are produced when magnesium
burns in air. Let us consider a few other chemical changes.

Basic Science VIII 81