Boiling point of sugar syrup

Plain water boils at 212°F at sea level, and the Exploratorium suggests checking a candy thermometer in boiling water to confirm it reads that. Sugar syrup does not stay at that mark. As a sugar syrup is cooked, water boils away, the sugar concentration increases, and the temperature rises.

That makes temperature a stand-in for concentration. At sea level the museum's table runs from the thread stage at 230 to 235°F and about 80 percent sugar, through soft-ball at 235 to 240°F (85 percent), firm-ball at 245 to 250°F, hard-ball at 250 to 265°F (92 percent), soft-crack at 270 to 290°F and hard-crack at 300 to 310°F (99 percent).

At 320°F, the clear-liquid stage, all the water has boiled away. Past that the sugar browns, with the brown-liquid stage at 338°F and burnt sugar at 350°F.