Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: endothermic dissolving

Learn more physics!

Most recent answer: 02/18/2013
Q:
For some reason I wondered about the temperature of water (honestly, this is not a homework question). I added Salina, a powder that makes water bubble, probably CO2, used for digestion. I used a thermometer to test water temp before and after, and the temp of the water decreased. I repeated the experiment several times. Why does the water temp decrease? For an example, 200mL of water at 27 deg C dropped to 24.5 C when 15 G of Salina was added. Why? many thanks John Bradford
- John (age 45)
DUBBO, NSW, Australia
A:
Some substances release heat when they dissolve in water (that's called exothermic) and others soak up heat (called endothermic). Evidently you have something endothermic.
You might wonder why this would dissolve. After all, the second law of thermodynamics says that things proceed toward a condition of maximum entropy, i.e. maximum number of quantum states. Cooling things down reduces the entropy. The dissolving can still happen, though, since the extra entropy the molecules get by running around in solution can more than compensate for the entropy lost by a small amount of cooling.

Mike W.

(published on 02/18/2013)

Follow-Up #1: endothermic reactions

Q:
Following your answer I repeated the experiment with an additional observation of temp reading for a longer period of time. I noticed the temp indeed increased quickly once the bubbles had subsided. I also raised the water temp by five degrees before adding the powder, measuring the time involved when using both the lower and higher temp of water at the start of the experiment. As expected, the reaction time and intensity of the reaction was increased, while the water temp still dropped then rose to a final temp slightly higher than initial temp. Thankyou for your help in this matter
- John Bradford (age 45)
Dubbo NSW Australia
A:
Thanks for the follow-up. I wonder if there was some sort of other, slower reaction going on that brought the final temperature slightly higher than the initial one. Or, small effects can often be caused by subtle experimental complications, e.g. hanging out near the glass might transfer a little heat to it.

Mike W.

(published on 02/18/2013)

Follow-up on this answer.