A chemical reaction is formed of reactants and products. Reactants react with each other to give one or more product. Sometimes we can visualize observations that are characteristic of the reaction. Like for example a release of a gas (bubbles are formed) or a change of color or also the formation of a precipitate.
Rack of test tubes
We prepare 7 beakers of 7 different solutions (that we mark from 1→ 7). We put separately around 50 ml of: HCl
Using plastic droppers we mix 10 drops of one solution with 10 drops of another solution in a test tube, to get all in all 20 different test tubes. We then note the observation for each test tube.
Name of the unknown substance:
Thanks to the procedure we can determine the name of the unknown substance (knowing that it is the same than one of the other 6 solutions) by comparing the different observations. After comparing we realized that solution 5 (CaCl2) and solution 7 (unknown) gave the same observation no matter with what other solution they were mixed with. We can then conclude that solution 7 is CaCl2.
Chemical reactions with observable changes:
2HCl (aq) + Na2CO3 (aq) → H2O + CO2 (g) + 2 NaCl (aq)
HCl + K2CrO4 → 2KCl (aq) + H2CrO4 (aq) Orange color
NaOH(aq) + CaCl2 (aq) → NaCl2(aq) + Ca(OH)2
NaOH(aq) + CuSO4 (aq) → Na2SO4 (aq) + Cu(OH)2 (s)
Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2NaCl (aq)