One can tell that a reaction is taking place by the emission of smoke.
How does one know that a reaction has taken place?
A reaction takes place when a new substance or substances are formed from one or a combination of more than one substance.
The initial substances are called reactants while the new substances formed are called products.
In the image, the burning of the magnesium results in the production of smoke. Thus, a new substance is being formed in the form of smoke.
More on chemical reactions can be found here: https://brainly.com/question/22817140
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