Mixing Acids and Bases

The experiment that we were doing was making invisible ink.  We mixed baking soda in one vial with water and in the other vial we mixed citric acid with water.  We had a special yellow paper that had a chemical in it to where once a base (which is baking soda) was mixed with it, it would turn red.  And once an acid was mixed with it (which was citric acid, obviously) well, it would turn back yellow, thus making it look like your writing vanished.  

I made a video to demonstrate.

How we made the stuff is we added four medium scoops (the scoops came with our science kit) of baking soda to water and the same amount of citric acid to water.  If you got a vial the size of ours and filled it 3/4 full of water, it would work.  The baking soda mixture will turn the paper red and the acid mixture will turn it back yellow.  

We used Q-tips dipped in each mixture to write.  You start with the base mixture and your message will appear red on the paper.  Then you go over the same message with your acid mixture and the message "disappears" because the acid turns the red back to yellow.

Usually if you mix an acid with a base (like vinegar and baking soda) it will fizz up and start bubbling.  When I went to pour the vials down the sink (after we were done with the experiment) the baking soda and the citric acid mixed and they started fizzing and bubbling right there in the sink.  That was very interesting because I had only done that experiment (mixing a base with an acid) a couple times before.  And then my mom asked me if there was any chance the RV would explode and I said, "No.  Definitely not."  Here's the video: