12. Which of these process is always endothermic?

Question

1. burning
2. condensation
3. freezing
4. Boiling

Answer 1

Hi Ghoson!

 

These two facts (one of which has already been given) will cover most of the answers to all the problems you posted:

 

a)  Endothermic reactions are ones were heat is taken in by the reaction; exothermic is when heat is given off by a reaction

 

b) a chemical change occurs when two or more substances combine to form new substances by a chemical reaction.  If no new substances are created, the reaction is not a chemical change (since none of the substances has changed in that way).

 

For this question, you need to determine which of these processes requires heat energy to be taken in, rather than given off.  A rule of thumb is that, for a given substance, solids tend to have less internal energy than liquids, which in turn have less internal energy than gases.  So if a material goes from gas-->liquid or liquid-->solid, it is going from higher to lower internal energy, which means it has to give energy away.  If it transitions the other way, it is going to a higher internal energy form, which means it has to take energy in.

 

And you can feel the heat energy coming off typically when something burns, so you can tell that process gives off heat energy.

 

Hope this helps!  If you would like to talk about it more specifically, just let me know.