CBSE Class 7 Answered

But, during a chemical change, substances are changed into different substances. By one clue, for example, colour change wont give the hypothesis for the theory which may be nullified or rejected. So, we see other two- three clues along with colour change such as temperature change, liberation of gas etc.

So, the main concept is that different products formed during chemical change gives different clues to know that its a chemical change. But energy production and evolution of gas is the mndatory clues required to know a chemical reaction in 90% cases.

So, during a chemical reaction, we have to go for two clues at maximum.

For example, the explosion of fireworks is an example of chemical change. Energy can also be released during a chemical change.  The fireworks release energy in a form of light alsp. So, we see there are two clues-light emission and energy production.