Temperature Definition - Kinetic Energy & Evaporation

In summary, temperature measures the average kinetic energy of particles and the reason water evaporates is due to some particles having high kinetic energy and escaping. Freezing is a collective event and at the triple point, ice, liquid water, and water vapor can coexist in equilibrium.
  • #1
ffleming7
25
0
I am in high school chemistry, and we are learning about gases. We were told that temperature is a measurement of the average kinetic energy of the particles. We were told that the reason water evaporates is because some of the particles in the water have high kinetic energy (higher than the boiling point) and escape and become gases. I was wondering if there is ever particles in the water that have very low kinetic energy and solidify, just like some particles evaporate.
 
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  • #2
That's how ice is made :smile:
 
  • #3
1) Consider liquid water in contact with a gaseous mixture of air and water
vapour. Water evaporates when the vapour pressure of the liquid water
is greater than the partial pressure of water vapour in the gas; i.e.
when the rate at which molecules leaving the liquid is greater than the
rate at which they are entering the liquid from the gas. Water will
evaporate only until there is a state of (dynamic) equilibrium between
the liquid and the vapour.
2) In general, molecules of water near the liquid-gas boundary may escape
from the liquid provided they have a small energy necessary to overcome
any surface forces. Molecules with a whole range of kinetic energy can
leave the liquid, not just those with extremely high energies.
(Incidentally, what do you mean by kinetic energy higher than the boiling
point?)
3) Freezing is a collective event. A single molecule cannot "freeze". At any
given instant, there may be many molecules with small or even zero
kinetic energy but these will be jostled by their more energetic
neighbours within a very short time, so there is no local freezing
possible at temperatures above the freezing point.
 
  • #4
Just a little more - pkleinod's explanation tells us why water can exist as a gas and a liquid together in stable equilibrium at, say, 10C, but not water and ice. But at 0C, it is possible for all three to exist in a stable equilibrium with each other and ice will form and melt in a stable equilibrium much the same way water will condense and evaporate in a stable equilibrium.
 
  • #5
Thank you russ_watters. Yes, at the triple point, ice, liquid water and water vapour can coexist at equilibrium. Just a small correction: the triple point of water does not occur at 0 o C (=273.15 K). The temperatures and pressures are the following:
For ice I, water(l), water(g): T = 0.0099o C (273.16 K);
P = 0.00603 atm (611 Pa; 4.58 mm Hg)
1 atm = 101 325 Pa.
This is a small point in the context of the original post.
 

1. What is temperature and how is it defined?

Temperature is a measure of the average kinetic energy of the particles in a substance. It is defined as the degree of hotness or coldness of an object.

2. How is temperature related to kinetic energy?

Temperature and kinetic energy are directly proportional to each other. As the temperature increases, so does the average kinetic energy of the particles within a substance.

3. What is the role of kinetic energy in evaporation?

Kinetic energy plays a crucial role in the process of evaporation. As the temperature of a substance increases, the average kinetic energy of the particles also increases, causing them to move faster and escape the surface of the substance, resulting in evaporation.

4. How does temperature affect the rate of evaporation?

Temperature has a significant impact on the rate of evaporation. As the temperature increases, the rate of evaporation also increases due to the increased kinetic energy of the particles, resulting in faster movement and escape from the surface of the substance.

5. Can temperature affect the boiling point of a substance?

Yes, temperature can affect the boiling point of a substance. The boiling point is the temperature at which a substance changes from a liquid to a gas. As the temperature of a substance increases, so does its kinetic energy, eventually reaching a point where it can overcome the intermolecular forces holding the substance together, resulting in boiling.

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