What are 4 possible clues that a chemical reaction has taken place?

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jeew-m eNotes educator| Certified Educator

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
sk8terboi8160 | Student

to tell if a chemical reaction has occurred look for these four things:
1) color change
2) the formation of gas bubbles
3) formation of a precipitate
4) temperature change

plantcell | Student

A chemical reaction has products and reactants. A change in either could tell that a reaction has taken place. 

Formations of products --> precipitates, colour changes, gas bubbles observed, changes in pH

Loss of reactants --> Dissolving, colour change, change in pH

Reactions are endothermic or exothermic and may either take in energy or give out energy which can be in terms of heat and/or light

user9919475 | Student

The three changes that show chemical reactions takes place are First 

*cange in colour. 

*change in temperature. 

*change in state. 

   .   .   .   Extraas hints

*Evalution of gas. 

*Formation of ppt(precipitate)

rithvikvutukuri | Student

color change

temperature change



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