chemical reaction

the question asks that what'd be observed when there's a chemical reaction.
as you may know, in chemical reactions new substances are formed.

sometimes the new substances could be invisible for us; however, most of the time we can observe something, like:
change of color (of solution/solid/gas),
heat is given out / absorbed (temperature changes),
flame is ignited,
production of gas (bubbling of solution) or solid (precipitation), etc.

though it's not necessary for a reaction to have these changes in appearance, they strongly suggest that there's a reaction.

whether chemical reaction occured is normally detected by two ways:
*observation
*chemical tests

Examples of observation:
*Evolving gas(like acid+Mg)
*Change of colour of solution(e.g. in titration, you add methyl orange to observe change in pH)
*Heat released(e.g. 暖袋)
*Heat absorbed(e.g. 冰袋)
*disppearance of reactants(e.g. put Na in water - Na eventually "melts")
*Solid formed (e.g. lead(II) nitrate reacts with sodium sulphate gives PbSO4, which is solid)

Examples of chemical tests:
*Silver nitrate tests (if you want to see if silver ions form in a chemical reaction)
*rust indicator (if you want to prove rusting has occured, you put the object in potassium hexacyanoferrate(III) and see if it changes colour.)