This drink smokes elegantly, changes from a calm blue to fuschia as it cools from room temperature to drinkably cold, and tastes like a dirty martini.
(For the non-alcoholic, dry ice-free version, see the color-changing sports drink)
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Step 1: Ingredients
Gin (or vodka, for a vodka martini) ~ Vermouth ~ Dry Ice ~ Baking Soda ~ Red Cabbage
Step 2: Red #$@%! Cabbage?
Yes. Red cabbage contains a water-soluble anthocyanin that is a pH indicator. At low pH (acid), it's red. It's purple at neutral pH, and goes blue then green as the solution becomes alkaline. You're not going to see the full range here, because we want the drink to be, well, drinkable.
To get the indicator, chop up a cabbage leaf, put it in a bowl with some water, and microwave until it's boiling (or just add boiling water and allow to steep). A purple pigment will stain the water.
Step 3: Build
Add a teaspoon of cabbage juice... I mean indicator solution - to the martini glass. Then add very small quantities of baking soda, just enough to turn the solution blue. Add gin (or vodka) and vermouth (~6:1) to the glass. You should have a pale blue clear liquid.
Step 4: Present
Give the recipient the drink. Tell them to watch closely, then add a chunk of dry ice. It will sink to the bottom of the drink, and bubble away happily, slowly cooling the drink. It will also neutralise the baking soda and change the colour of the martini. See it in action in the video above.
(that's not me saying "wow, that's wicked" by the way. This drink was mostly made as a way of keeping three small children entertained while their mother was out Christmas shopping. They liked the look of the cocktail, but thought the mocktail version was "really disgusting". I did warn them that it was just salty, cabbage-flavoured water...)
It should go without saying, but wait for the dry ice to disappear before trying to drink the cocktail. You do NOT want to swallow any dry ice!
Step 5: Taste
In the interests of science, I tried some of the baking soda/indicator mix straight. Baking soda tastes salty on its own, and rather overpowers the cabbage juice. The latter by itself doesn't taste of much; a slightly sweet, vegetably taste, nothing like what its violent colour suggests. Overall, it flavours the martini in a way reminiscent of a dirty martini (martini with olive brine added). So you'll only like it if you like dirty martinis, probably a pretty small constituency.
I didn't initially regard this drink as spooky in any way, so was surprised to see it appear on various "Creepy Halloween Foods" lists (dabbled, neatorama, mentalfloss, etc). The lists were great and well worth checking out (they included some really effective eyeball recipes - see here for my version).