Mid-Summative Exam 6 Review
Evaporation – Process that happens when heat energy is added to liquid water, turning it into water
vapor.
In order for this to happen the molecules need to (absorb/ release) energy which causes the
water molecules to move (more quickly / more slowly). This causes the water molecules to change
into a (gas / liquid / solid).
Condensation – Process that occurs when heat energy is released from water vapor, turning it into
liquid water.
In order for this to happen, the molecules of water vapor in the air need to (absorb/ release)
energy which causes the molecules of water vapor to move (more quickly / more slowly). This
causes the molecules of water vapor to change into a (gas / liquid / solid).
Humidity – the amount of water vapor in the air.
Relative Humidity – the amount of water vapor in the air compared to how much water vapor the
air can hold.
Relative humidity is measured as a percentage. When the relative humidity is 100% the air is
saturated. When temperature increases and the amount of water vapor in the air stays the same, the
relative humidity will (increase / decrease ).
Dew Point – the temperature at which water vapor in the atmosphere condenses from a gas to a
liquid. This is also the temperature at which air is saturated. Dew point temperature is reached when
air is cooled to the point at which it is saturated.
How Clouds Form
Air near the ground absorbs energy through (conduction/convection). The temperature of the air
(increases/decreases) causing the density to (increase/decrease), therefore the air (rises/sinks).
As the air (rises/sinks), the temperature of the air (increases/decreases), causing the water vapor in
the air to (evaporate/condense), forming liquid droplets of water. The liquid water needs
(condensation nuclei / gas) on which to form. As more and more water droplets
(condense/evaporate), they form a visible cloud. As the water droplets continue to grow larger,
they can become too heavy and (precipitation/evaporation) occurs.
Evaporation and Condensation
How does the kinetic energy change during evaporation?
The K.E. of the liquid water molecules increases, causing the molecules to move quickly and change
from a liquid to a gas.
How does the kinetic energy change during condensation?
The K.E. of the water vapor decreases, causing the gas molecules to move more slowly and change
from a gas to a liquid.
Why does sweating help keep a person cool? Explain in detail, and be sure to include:
a. changes in the state of the water
b. transfer of kinetic energy (direction of energy flow)
-
In response to an increase in body temp, sweat moves from the inside of your body through
your sweat pores to the surface of your skin.
Heat energy from your body flows into the sweat droplets, causing the water molecules to
move more quickly.
In response, the molecules of liquid water turn into a gas carrying the energy into the
atmosphere.
Why does condensation form on the outside of your glass of iced tea? Explain in detail and include:
a. the changes in the state of the water
b. transfer of kinetic energy (direction of energy flow)
-
Warmer (faster moving) molecules of air, including water vapor, strike the outside of the
cold glass of iced tea.
Energy from the air is transferred to the cold glass, causing the water vapor molecules in
the air to slow down.
The slower moving water vapor molecules stick to the cold surface of the glass forming a
layer of condensation on the outside of the glass.
If the amount of water vapor in the air stays the same, but the air temperature increases, what
happens to the relative humidity?
_____The Relative Humidity would decrease________________________________________
What do you think would happen to water vapor that condenses on a surface that has a temperature
below 0°C? What is this type of dew called?
____ After the water vapor condenses it would freeze, forming frost.________________________
Explain why the weather balloon pops as it reaches a high altitude.
As the weather balloon continues to rise the outside air pressure decreases. The gas inside to
balloon pushes outward, expanding the balloon until it bursts.
Relative Humidity Practice
Part 1: Using the Relative Humidity Calculation Chart (p. 79)
1. Dry Bulb: 35 °C
Wet Bulb: 26 °C
Difference:___9°C__
R.H.___49%______
2. Dry Bulb: 17°C
Wet Bulb: 10°C
3. Dry Bulb: 30 °C
Wet Bulb: 14 °C
Difference:___7°C__
R.H.__40%_____
Difference:_16°C__
R.H.___13%____
Part 2: R.H. Calculations – Using the table on page 31 of the lab book.
Show all work! Example: Current vapor / saturation-max x 100 = R.H. %
4. The temperature of a sample of air is 10 ºC. If there is 7g of water vapor in a kg of the air what
is the relative humidity? What would the relative Humidity be if the air is heated to 20ºC?
Air at 10°C can hold 7g of water vapor, so 7g/7g x 100 = 100% R.H.
If the air is heated to 20°C then it can hold 14g of water vapor. If there is still 7g of water vapor,
then 7g/14g x 100 = 50% R.H.
5. There is 10 g of water vapor in a kg of air at 25°C air. What is the current relative humidity? To
what temperature would this air need to cool to for it to become saturated (reach the dew point)?
Air at 25°C can hold 20g of water vapor. If it is holding 10g, then 10g/20g x 100 = 50% R.H.
If the air is holding 10g of water vapor then it would have to cool to 15°C to become saturated.
Part 3: Additional R.H. calculations using the chart below. Show all work.
Title: Grams of water vapor needed to saturate 1 kg of air at various temperatures.
Temperature
-10°C
0°C
5°C
10°C
15°C
20°C
25°C
30°C
Water Vapor
2.0 g
3.5 g
5.0 g
7.0 g
10.0 g 14.0 g 20.0 g 26.5 g
(g)
35 °C
35.0 g
6. What is the relative humidity of a kilogram of air at 20°C that contains:
Air at 20°C can hold as much as 14g of water vapor.
a. 10 g of water vapor
10g/14g x 100 = 71.4 % R.H.
b. 5 g of water vapor
5g/14g x 100 = 35.7% R.H.
c. 14 g of water vapor
14g/14g x 100 = 100% R.H.
d. 8 g of water vapor
8g/14g x 100 = 57.1% R.H.
7. A kilogram of air contains 3.5 grams of water vapor. Its relative humidity is 50%. What is the
temperature of the air? At what temperature would the air reach a relative humidity of 100%?
If 3.5g of water vapor = 50 % R.H., then 7g of water vapor = 100% R.H. Air with 7g of water vapor
saturates air at a temperature of 10°C. To saturate the air with only 3.5g of water vapor, the air
would need to be cooled to 0°C.