Why doesn't the Alka-Seltzer start effervescing on its own (without water) ?!
Wanna bet that I go to the same school, have the same teacher, and have the same homework as you do? Anyway, the reason why Alka-Seltzer doesn't start effervescing on its own is because the citric acid is solid. In it's solid form, it doesn't react with baking soda. You need to dilute the citric acid and that's why you need the water. Once the citric acid is diluted, THEN it can react with the baking soda. Get it?
Because the Bicarb of soda needs water to activate it !