Candy recipes tell you to boil sugar syrup until it reaches various temperatures on a candy thermometer: the soft ball stage at about 237 degrees Fahrenheit (114 degrees Celsius), the hard crack stage at 305 degrees Fahrenheit (152 degrees Celsius), and so on.
Different cookbooks will give you slightly different temperatures for the various stages.) The longer you boil it, the thicker the syrup gets and the higher the temperature becomes. But you can boil pure water as long and as hard as you like, and it will never get any hotter.
Obviously there’s something going on in that bubbling sugar-water syrup that’s very different from what goes on in plain boiling water.
Whenever something, almost anything, is dissolved in water, the boiling temperature goes up. And sugar is no exception. So any sugar-and-water solution will boil higher temperature than plain water will. The more concentrated the solution or, put another way, the more dissolved material the water contains, the higher its boiling temperature will be.
For example, a solution of two cups of sugar in one cup of water (yes, it’s possible) won’t start boiling until 217 instead of 212 degrees Fahrenheit (103 instead of 100 degrees Celsius). But then as you continue to heat it, many of the water molecules will boil off as vapor, and the sugar solution will become more and more concentrated.
It will develop a higher and higher proportion of sugar to water. The more concentrated it becomes, the higher its boiling temperature becomes, so the longer you boil it, the hotter it gets. Because of this, candy recipes can use temperature as an indication of how concentrated the sugar syrup is and therefore how hard or sticky it’ll be when cooled.
If you boil sugar syrup long enough, all the water will eventually be gone and you’ll be left with nothing but melted sugar in the pot, at about 365 degrees Fahrenheit (185 degrees Celsius). At about the same time, it will begin to caramelize, a polite word for the actual destruction of sugar molecules into a complex assortment of other chemicals that have intriguing flavors in spite of their generally frightful chemical compositions.
The yellow-to-brown color transition corresponds to the buildup of more and bigger particles of carbon, which is the ultimate decomposition product of sugar. Heat sugar just a little too long, though, and you’ll be left with a charred, black mess of still-sweet, but thoroughly inedible, charcoal.