Hard Candy Chemistry

Because of hard candy's glass-like properties it is often used in movies when a wine glass or beer bottle is going to be smashed into one's head in a film. Usually, if you know ahead of time that a glass filled with liquid will be smashed, you can see the candy glass degenerating a bit. It will get milky, cloudy looking or start to melt a bit. I read they used to use sheets of candy for window panes that people would get thrown out of during a cowboy saloon fight. Now that they have glass that won't splinter they don't do that anymore.
Ah, trivia. What would I be without you?

"Then we're going to add a little bit of acid as well."
No explanation? I'm sure there must be a good reason for this, as opposed to, say, adding a little bit of rat poison.

My guess would be the acid is lemon juice, for flavoring. Just a guess.

Or maybe he'll be listening to Stairway to Heaven later, and needs some strong candies to go with it.

from what I've read the acid simply adds a tartness and is a flavoring.

Probably citric acid like in lemons and oranges.

Citric Acid affects the crystallization process. It helps the sugar recrystallize in the correct formation. Various other things, like corn syrup, can do the same thing. Corn syrup will help you if you have a problem with your sugar candy being grainy.