# Characteristics of chemical reactions [closed]

Characteristics of chemical reactions are certain characteristics that tell us whether a chemical reaction has occurred or not.

Some important characteristics of chemical reactions are given below :

• Evolution of a gas
• Formation of a precipitate
• Change in color
• Change in temperature
• Change in state

I was wondering whether all chemical reactions can be identified by one or more of these characteristics. Since there is an extremely wide range of chemical reactions, I think that these are just the major characteristics of chemical reactions and there may exist some reactions that exhibit none of these characteristics.
Am I right about this? If yes, what are some examples of chemical reactions that show none of these characteristics?

Thanks!

• No they can't. The majority of organic chemistry reactions show none of these characteristics and you don't know any reaction has occurred until you run some form of diagnostic on the reaction mixture. – Waylander Jun 23 '20 at 12:06
• @Waylander And how does it hold up with inorganic chemistry? – Rajdeep Sindhu Jun 23 '20 at 12:20
• I'm biased as I'm an organic chemist. You are IMHO more likely to see one or more of the characteristics with an inorganic reaction. – Waylander Jun 23 '20 at 13:12
• Thanks ${}{}{}$ – Rajdeep Sindhu Jun 23 '20 at 13:13
• 'Change in Temperature' is but one example of an energy manifestation, as in the formation of light. Also, transfers of electrons, as occurring in a battery cell, for example, may display no evident signs of a reaction occurring. – AJKOER Jun 24 '20 at 14:21

I think you are right about this judgment. There are certainly some outliers which show none of these characteristics.

One example I can think about is the catalytic reduction of nitrate to nitrite, by either transition metal catalysts or biocatalysts. And I think some reactions in liquid phase show none of the characteristics you mentioned above.

Still, I think the key here is that chemical reactions always involve the breaking-forming of chemical bonds and the change of electron cloud distribution. I think that's the essential characteristic of chemical reactions.

Thanks! Let me know if you have more questions.

• Thanks! That clears it all, for now. – Rajdeep Sindhu Jun 23 '20 at 23:43

While most chemical reactions can be classified by their characteristics, the overwhelming majority of reactions can be grouped into the following:

• Synthesis/Combination
• Decomposition
• Displacement
• Double displacement
• Reduction/Oxidation (Redox)

These reactions will exhibit their own telling signs of occurrence, however, you are right in your estimation that reactions exist which will not exhibit signs to the naked eye and will require indicators.

• I edited your answer a bit, hope that's fine. You'll see it once it's accepted. – Rajdeep Sindhu Jun 23 '20 at 13:46

One characteristic is always present in chemical reactions. That is the change in energy. This is because in a chemical reaction, bonds are always broken and formed.

In endothermic reactions, temperature of the reacting mixture decreases. Or in cases where heat is not involved, it is to be noted that photochemical reactions actually absorb light and show quite noticeable characteristics as mentioned above.

In exothermic reactions, energy is produced, which can increase temperature, produce electricity and even produce light energy. In situations where chemical reactions show no noticeable change is because the change in temperature is either too small to measure or is affected by external factors like temperature, pressure and others.

Of course a chemical reaction always does not show noticeable characteristics; characteristics visible to the naked eye. Those characteristics may be too small to observe.