There are several observations that we can identify some chemical reaction have taken place with in a system.
- Change in temperature
-usually when a reaction takes place heat will be absorbed or released which result in a temperature change.
-eg: put a piece of sodium in water. you will feel or observe that the temperature is increased
- Change in colour
-Some times we see a colour change in the media when a reaction takes place.
-eg:put NaOH and `KMnO_4` solution to a river water sample.you will see the color will change to brown
- Emission of gas
-sometime when a reaction took place a gas can be removed from the system
-eg:put a piece of sodium in water. you will observe that a gas is emitting by the forming of bubbles
- smell or odor
-When some reactions take place the formation of some substances can give us a smell or odor.
-eg: Add HCl to a FeS solution. You will taste the strong odor of `H_2S`
Other than this there are several clues we can identify that a chemical reaction had taken place.
- Change in PH
- Dissolving of reactants
- Forming of precipitates