Dry ice is added to a glass cylinder containing a dilute solution of sodium hydroxide and universal indicator. The carbonic acid that forms as the carbon dioxide bubbles through the solution neutralizes the base, changing the indicator from purple to blue to green to amber.
This demo can be used to illustrate the use of indicators or to introduce the concept of non-metal oxides acting as acid anhydrides. I sometimes like to do this demo in the cylinder that I have just done the alkali metal reactivity demo in because that demo leaves you with a dilute basic solution in the cylinder. It doesn't necessarily fit any instructional sequence well, but it's just fun. Allow 10 minutes to perform this demo.
Carbon dioxide is an acid anhydride, meaning that it reacts with water to produce an acid - in this case carbonic acid: CO2(aq) + H2O(l) ==> H2CO3(aq) This acid can then be neutralized by a strong base: H2CO3(aq) + 2 OH- ==> CO32-(aq) + 2 H2O(l) Universal indicator is actually a mixture of several common indicators. This is apparent upon close observation because the color changes are not really a smooth continuum, but rather a sucession of fairly sudden changes corresponding to the endpoints of each individual indicator.
- large glass cylinder filled about 1/3 of the way with .01M NaOH solution
- dropper bottle of universal indicator
- insulated container with several chunks of dry ice in it
- add several dropperfuls of universal indicator solution to the cylinder
- add several chunks of dry ice to the cylinder
- the color of the solution will slowly change
Dry ice is extremely cold. It can cause frostbite. Avoid touching it. If you must touch it briefly, make sure that your hands are dry so it doesn't frezze to your skin.
Be sure to bring a waste container when you break down this demo, otherwise the cylinder will tump over on the way back to the lab. But don't stopper the waste container if there's any dry ice left in it. Pressure could build up. You can get dry ice at science stores