LEARNING GOALS:  Physical and chemical properties of dry ice

1. What is dry ice?

Put a flat chunk of dry ice on the counter and see how easy it "floats" on top of the counter when you give it a small push.  Also see how finely ground dry ice moves around on the counter. 

Does a puddle of liquid form?  No, Why not?             
Solid → Gas         ( Sublimation)

Compare this to an ice cube made of H₂O (s)                       
Solid  →  Liquid        (Melting)

Put a quarter on the edge of the dry ice.  What do you observe or hear?
      What is dry ice?  It is solid, frozen CO₂
We breathe in O₂ breathe out CO₂.  Are we going to run out of O₂ on earth?  
No.  Trees and plants take in CO₂, produce O₂.

Why does it hurt to touch the dry ice?  Have students touch it but only very briefly.

At room temperature and pressure CO₂ (s) → CO₂ (g).  In solid CO₂, there is a force of attraction between the molecules.  It requires heat to pull molecules away from each other.  The solid takes heat from the air molecules surrounding it.  When you touch dry ice for more than a few seconds it hurts.  It takes the heat from your hand when you touch it.

2. Can we blow up a balloon with solid carbon dioxide?

Students will see how the dry ice goes from a solid to a gas by putting a chunk of dry ice into a balloon. Stretch a balloon wide-open using both hands – put your fingers deep into the balloon to stretch it open as wide as you can.  Using a pair of tongs, have a student place a small piece or a couple small pieces of dry ice in the balloon.  Tie the balloon shut and watch it inflate as the dry ice sublimes (solid → gas).  One of the students can keep the balloon to be used for the liquid nitrogen station. 

What is the white frost on the outside of the balloon?  Some students think dry ice goes through the balloon.  Discuss water vapor in the air and how it condenses on the balloon because it is cold.  Feel how cold it is.

3.  Making Fog

Fill a plastic tub half-full with hot water from sink.  Pour one to two cups of small chunks of dry ice into the hot water, all at once.  What do you observe?What is the fog?  Why does it flow over the edge of the container and down to the ground? 

FOG:  The fog is not just CO₂ (g).  When you breathe out CO₂, it is clear.  The water vapor above the hot water condenses.  As the CO₂ sublimes, CO₂ (s) → CO₂ (g), it absorbs heat, making it very cold so water condenses (like fog or clouds).

Why does the fog drop to the floor?  It is heavier then the air.  Talk about fire extinguishers which contain CO₂ (l) which forms CO₂ (g), and drops down onto the fire, keeps O₂ (g) away from fire, so it extinguishes the fire.  Light a birthday candle and have the students put it in the fog.  They will see that the CO₂ (g) blows out the flame immediately.

Fill a 250-mL beaker about half-way with water.  Put a small chunk of dry ice in the water.  Students can see bubbling as the CO₂ (s) → CO₂ (g).

4. What is the difference between ice and "dry ice"?

Have students place ~70 mL of water in three 100 mL beakers.  Place the beakers on a stir plate.  Students put bromothymol blue indicator in each beaker – stir the solution with a stirring rod.  To one beaker add some ice; to the other beaker add a chunk of dry ice.  What do you see?
The CO₂ dissolves in water to produce carbonic acid, H₂CO₃: 

 CO₂ + H₂O →  H₂CO₃ → H⁺ + HCO₃^-  making the solution acidic.

Students will see a color change for the dry ice / water reaction and not for the regular ice dissolving in water.  Let other students try this. Explain what universal indicator is.

5.  What are the bubbles in soft drinks?

Talk about CO₂ (g) in sodas.  High temperature or high pressure (shaking soda) releases CO₂ (g).  Gases like carbon dioxide and oxygen dissolve in water – this is good because aquatic plants, fish and all living species in water need carbon dioxide and oxygen to survive.

      TESTING 7UP – IS SODA ACIDIC?

Add 7-Up to the third beaker containing 40 mL of water and indicator.  What do you see?

7-Up contains carbonic acid which reacts with water just like dry ice reacts with water!

The CO₂ dissolves in water to produce carbonic acid, H₂CO₃.  Carbonated beverages all have CO₂ dissolved in solution making the solution acidic.   CO₂ + H₂O →   H₂CO₃ →  H⁺ + HCO₃^-

So if we add dry ice to a bowl of juice at Halloween, is it safe to drink?