Question

How could we know that a change has taken place?

Answer 1

There are four signs that a chemical reaction has occurred: a change in color, a change in temperature, formation of a gas or formation of a precipitate. A precipitate is a solid that forms when two liquid solutions are mixed. As matter changes in a chemical reaction, it can either absorb or release energy.
OR ...
We can tell if a chemical reaction has taken place when one or more of the following things happen: There hasbeen a colour change inside thereaction flask. A gas has formed. Usually we know a gas has formed when we can see bubbles...

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Answer 2

Answer:

when properties and change in colour ,size,shape etc then we will know that change has taken place but in Physical change properties can't be changed