Question

What are “hallmarks” of chemical reactions, that is,how can you observe that a chemical reaction has taken place?

Answer 1

A Chemical reaction is accomponied by some physical or chemical changes to distinguish it from the reactants. These changes may include the change of state (solid reactants may get converted to liquid or gasous state), change in color (different from the reactants), sometimes change in odour (smell) which can be perceived.

Change in physical properties can be verified by estimating the melting point (if the product is a solid) or boiling point (if it is a liquid), change in density (which can be measured), change in viscosity (if it is a liquid), change in refractive index etc. You can estimate the percentage composition of the product by elemental anlayser (percentage of C, H, N, S ) and distinguish it from the reactants.

If you dont see any of these changes we have to perceive the chemical changes by using appropriate instruments like the Infra red spectrometer, Nuclear Magnetic Resonance Spectrometer (NMR), Mass spectrometer. Although these instruments are costly, they are confirmatory test to see weather a reaction has happend or not.

Some reactions are slow and some are fast. So we have to analyse the products at regular intervals to see the progress of the reaction