Investigation of the Factors Involved in the Formation of the Fog When Dry Ice Is Placed

Investigation of the Factors Involved in the Formation of the Fog when Dry Ice is Placed in Water

Introduction Clouds and fogs are made of extremely tiny droplets of liquid water suspended in air. “Fogs” are distinguished from clouds” in that the water droplets in a cloud originate from water vapor high in the atmosphere. On the other hand, the water droplets in a fog do not originate from water high in the atmosphere, but rather from sources of low-lying water such as ponds and lakes. Clouds and fogs both depend upon two physical changes for their formation: evaporation of water into the atmosphere followed by condensation into tiny droplets.

When dry ice is placed in water, a fog is made. Have you ever placed dry ice in water to make a fog? Have you ever wondered how this fog is produced? In this laboratory experiment, you will investigate several factors involved in the formation of the dry-ice-in-water fog. Your goal in this experiment is to try to explain how the fog is produced when dry ice is placed in water.

Materials 100 mL graduated cylinder, several large test tubes, dry ice pellets, ice water, room temperature water, hot water, glycerol, MicroLab system, thermistor, light sensor, 91% isopropyl alcohol (ethanol, acetone or other similar solvents may be substituted). CAUTION: Do not handle dry ice with bare hands. Glycerol and isopropyl alcohol are flammable. No open flames.

Procedure Part A: Investigation of the source of the fog

1. Place about 50 mL of water into a graduated cylinder. Precisely measure the mass of water in the cylinder.

2. Find and record the temperature of the water. Look up and record the vapor pressure of water at this temperature.

3. Add a piece of dry ice that is 3 – 6 grams in weight to the water. YOU WILL NEED TO USE APPROXIMATELY THE SAME MASS OF DRY ICE FOR ALL TRIALS: HOT, COLD, AND ROOM TEMPERATURE WATER!!

4. Allow the dry ice to sublime.

5. Once the dry ice has completely sublimed, find and record the mass of water that remains.

6. Repeat steps 1 – 5 using cold water.

7. Repeat steps 1 – 5 using hot water.

Data

trial / Temp of water / Vapor pressure of water at this temp / Mass of dry ice used / Starting water mass / Final water mass / Change in mass of water / Relative thickness of fog produced
Cold
Room temp
Hot

Questions

1. Did the mass of the water increase, decrease, or remain the same as a result of the addition of dry ice?

2. The fog produced is made of water. Where did the water come from to make this fog?

3. What is the relationship between vapor pressure of the water and the thickness of the fog produced?

Part B: Observation of the fog produced in various liquids

1. Place about 10 mL of room temperature water in a test tube.

2. In a separate test tube, place about 10 mL of alcohol.

3. Place about 10 mL of glycerine in a third test tube.

4. Look up and record the vapor pressure of each liquid used.

5. Add similar sized pieces of dry ice (about 1 – 2 grams) to each test tube.

6. Note the relative thickness of each fog produced. If desired, light from a source can be directed through the top of the test tube. The amount of light passing through the test tube can be measured with MicroLab or a similar data logger. This allows for a quantitative measure of cloud thickness: thicker clouds will block more light than thinner ones.

Data

Liquid / Vapor pressure / Relative thickness of cloud produced
Water
Alcohol
Glycerin

Questions

1. What is the relationship between vapor pressure of the water and the thickness of the fog produced? Is this similar to what you found in part A, above?

2. Only liquids with a high enough ______seem to form clouds.

(If you can’t answer this question, see your instructor before moving on).

At this point, you should recognize that only liquids with a high enough vapor pressure form fogs when dry ice is placed in them. However, high vapor pressure can’t be the only contributing factor. Alcohol has a much higher vapor pressure than water, yet it only forms a thin, wispy fog when dry ice is placed in it. What else is going on here? These next two sections are designed to help you think through some other factors that might be involved in producing a thick, cloudy fog.

Part C: Observation of the bubble size produced in various liquids

1. Place about 10 mL of room temperature water in a test tube.

2. In a separate test tube, place about 10 mL of alcohol.

3. Place about 10 mL of glycerin in a third test tube.

4. Look up and record the surface tension of each liquid used.

5. Add similar sized pieces of dry ice (about 1 – 2 grams) to each test tube.

7. Note the size (or sizes) of bubble(s) that surrounds the piece of dry ice when it is in the liquid. Feel free to use a slow motion video app on a smart phone to aid these observations.

Data

Liquid / Surface tension / Relative thickness of cloud produced / Size of bubble(s) produced
Water
Alcohol
Glycerin

Questions

1. What is the relationship between surface tension of the liquid and the size of bubbles produced when dry ice is placed in the liquid?

2. To get a thick, rich fog a liquid needs to have both a high ______and a high

______. The liquid needs to form bubbles that are ______.

(If you can’t answer this question, see your instructor before moving on).

3. Why might the formation of large bubbles be important in forming a thick fog?

Part D: Ability of the dry ice bubble(s) formed to insulate the solid piece of dry ice from the bulk liquid into which it is placed.

1. Place about 10 mL of room temperature water in a test tube.

2. Use MicroLab or a similar data logger to record the time-dependent temperature of the water.

3. Add a piece of dry ice (about 1 – 2 grams) to the water. YOU WILL NEED TO USE ABOUT THE SAME MASS FOR ALL TRIALS: WATER, ALCOHOL, AND GLYCERIN!!

4. Record the temperature until the dry ice has completely sublimed away.

5. Repeat steps 1 – 4 for alcohol and glycerin.

Data

Liquid / Mass of dry ice used / Change in temperature observed / Time required for dry ice to sublime / Relative thickness of fog produced
Water
Alcohol
Glycerin

Questions

1. Which liquids did the best job insulating the solid dry ice from the bulk liquid?

2. What happens to the vapor pressure of liquids as temperature drops?

3. To get a very thick fog, it is important for a bubble-film of gas to surround and insulate the solid piece of dry ice from the bulk liquid. Why might you think this is important? Of the liquids tested, which have this property?

4. Liquid requirements for a thick, cloudy fog:

Vapor pressure:______Surface tension: ______

Size of bubble formation: ______Insulating ability:______

Formation of gaseous film around solid dry ice: ______

Which liquid has all of these characteristics? ______

Part E: Getting glycerin to form a fog.

1. Place about 10 mL of glycerin in a test tube.

2. Heat the glycerin for no more than 20 seconds in the microwave. CAUTION!! USE HOT GLOVES!!

3. Upon heating the glycerin, its vapor pressure rises.

4. The hot glycerin should have all of the characteristics necessary to produce a thick, cloudy fog.

5. CAREFULLY READ THROUGH THIS STEP BEFORE CONDUCTING IT. While wearing hot gloves, tilt the graduated cylinder to the side. Be sure the mouth of the graduated cylinder is pointed away from you and others. Using tongs, gently slide a piece of dry ice into the hot glycerin. Be prepared for some rapid bubbling of hot glycerin which might eject out of the graduated cylinder!

6. Once the dry ice has safely been added to the graduated cylinder, record your observations.

______

Concluding Exercise

Reflect on the observations made during these experiments. Based on your observations, propose an explanation – on the molecular level – for how the fog is produced when dry ice is placed in water or any other liquid. Back up your proposed explanation with data and observations. If desired, extend your explanation by proposing new experiments to conduct.

Keep in mind that – just like clouds in our atmosphere – the fog formed in this experiment requires some water to evaporate and then condense.