Water is a very versatile substance; it can be a solid, liquid, or gas. It has the ability to change into all these states of matter that make water ideal for making different types of weather. Children do not understand that water is a common component of air and can be "pulled" out.

The amount of moisture, wind speed, topography, and temperature are important for the immediate weather. Pressure, wind direction, and movement of air masses are important to predict the weather. Water in the atmosphere moves around and causes all kinds of weather. The physical properties of water allow this compound to move easily from one state of matter to another. Rain is the liquid form. Snow, sleet, and hail are the solid form. Water vapor is the gaseous form.

When a liquid is converted to gas, it is termed evaporation. Condensation is when vapor turns back to a liquid. Melting occurs when a solid is changed to a liquid, and freezing is the revere process. When a solid goes directly to a gas and vice versa, this is called sublimation.

Sublimation  is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. An example of sublimation is dry ice turning from a solid directly to a gas.

The reverse of sublimation is deposition or desublimation. In desublimation a gas becomes a solid without going through the liquid phase.  One example of deposition is frost where in sub-freezing air, water vapor changes directly to the solid form, ice, without first becoming a liquid.  

Condensation which is what this sweat on a glass is called, produces dew. Dew does not fall like rain. It is just water vapor that becomes liquid on a solid surface that has been cooled below a certain temperature. This cooling usually happens during a clear, cool night, but disappears once the morning Sun evaporates the dew. Frost is formed like dew, but at temperatures below freezing. The water vapor changes directly to small, fine frost crystals without condensing into water drop first.

The dew point is the temperature at which the atmosphere is saturated with water vapor. A given volume of air containing higher amount of water vapor has a higher dew point than the same volume of drier air. The dew point gives an indication of the humidity. In meteorology the dew point is applied in the prediction of where and when clouds will form.

In this lab the students will take a close look at what evaporation, condensation, and precipitation mean, by actually creating the different phases. These words are best defined by watching the phenomena. Otherwise, the terms mean very little. Although students live through weather system after system, they rarely observe what is actually occurring.

1.  Go over the Water Cycle and make sure students understand the vocabulary words. Two that they may not know are transpiration, infiltration and sublimation/deposition (desublimation). 

2.  Read  Snow Crystals and Flakes and discuss that snow is frozen vapor.  Snow crystals can be the same, especially when they are small.  However, snowflakes which are crystals that adhere are probably not ever the same.  At the end of the poem you can see the pattern how snowflakes come from the clouds.  The first time snowflakes were photographed were in the early 1920's by Wilson A. Bentley, known worldwide as the "Snowflake Man."

3.  The lab helps the students understand condensation and precipitation.  If you do these examples slowly, students will be able to see these phenomena occurring all around them, just by observing!

Experiment 1. 
Place some ice in a small clear dish or watchglass. Pour very hot water (almost boiling) into a beaker. Place the crucible on top of the beaker. In a little while the water will condense on the upper side of the crucible and a few drops will fall as "rain." This is an example of condensation.

Experiment 2.
Fill a beaker about 1/3 full of water. Add 1 piece of ice and place the thermometer in the beaker. Wait a few minutes. If there is no condensation on the outside of the beaker, add another piece of ice. Continue until "dew" forms on the outside of the glass. Record the temperature at this point, this is called the "dew point."

Experiment 3. 
Put two scoops of ice into a beaker and one scoop of salt. When you start to see ice form on the outside of the beaker, measure the temperature of the ice and salt mixture. This is the temperature it would have to be on this day for frost to form. The salt is added to quickly melt the ice into water vapor, frost is nothing more than frozen water vapor. When you make ice cream, the salt is used in a similar manner.

Frost is an example of deposition/desublimation.  A gas turning directly to a solid.

The next activity has the students making ice cream in a bag.  It can help them understand how even things we eat use scientific principle.

the chemistry of ice, salt and exothermic reactions

1/2 cup milk (heavy whipping cream is preferred)
1/2 teaspoon vanilla
1 tablespoon sugar
4 cups crushed ice
4 tablespoons salt
2 quart size Zip-loc bags
1 gallon size Zip-loc freezer bag
a hand towel or gloves to keep fingers from freezing as well!

Mix the milk, vanilla and sugar together in one of the quart size bags. Seal tightly, allowing as little air to remain in the bag as possible. Too much air left inside may force the bag open during shaking. Place this bag inside the other quart size bag, again leaving as little air inside as possible and sealing well. By double-bagging, the risk of salt and ice leaking into the ice cream is minimized. Put the two bags inside the gallon size bag and fill the bag with ice, then sprinkle salt on top. Again let all the air escape and seal the bag. Wrap the bag in the towel or put your gloves on, and shake and massage the bag, making sure the ice surrounds the cream mixture. Five to eight minutes is adequate time for the mixture to freeze into ice cream.

When salt comes into contact with ice, the freezing point of the ice is lowered. Water will normally freeze at 32 degrees F. A 10% salt solution freezes at 20 degrees F, and a 20% solution freezes at 2 degrees F. By lowering the temperature at which ice is frozen, we are able to create an environment in which the milk mixture can freeze at a temperature below 32 degrees F into ice cream.