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Of course, now I realize that the one tube is becoming very basic with CaOH and the other is getting rather acidic with nitric acid. Wear rubber gloves or your fingers may sting.
Likely, you are making chlorine gas because you used table salt for your catalyst. It is also known as mustard gas due to its yellow color, and was used as a chemical warfare agent in WWI.; it's dissolving back into the water and turning it yellow.
I just taught Honors Chemistry again after not doing so for ten years. I used Calcium Nitrate as a catalyst. NaCl does frequently produce noticeable amounts of chlorine gas, and sodium bicarbonate (baking soda) does produce carbon dioxide with the oxygen, which negates the relighting or a wooden splint effect. I get a very good 2:1 ratio of Hydrogen to Oxygen, which seams to be rather pure.