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🍭 Sugar stages: the candy thermometer explained

The glass in the pot is not timing dinner. It is watching water leave, and naming what the syrup will be when it cools.

5
lessons
~20 min
to learn
🔬 Science
subject
Adults
level
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What you’ll learn

  1. The pot that will not stay at a hundredExplain that a candy thermometer reports leftover water — concentration — not elapsed time, and that the stage names describe the cooled syrup.A sugar syrup boils hotter as water leaves. The Exploratorium's stage names are predictions about the cooled drop. Time, pot shape and altitude all change how fast that climb happens.
  2. Water leaves. The number follows.Connect boiling-point elevation and saturation to the rising number on a candy thermometer, and say honestly where the dilute Kb formula fails.Sucrose lowers water's vapour pressure. As water boils away, concentration rises and so does the boil. OpenStax and Purdue describe the dilute case; candy stages at 80–99 percent sugar are empirical maps, not napkin calculations.
  3. What the cooled drop is trying to beGroup the candy stages into thread, ball and crack families and match each to leftover water, a cold-water test, and typical candies.Thread is still a syrup. Ball is a stiffness test (fudge through marshmallow). Crack is almost no water (taffy through brittle). Past 320 °F the water is gone and sugar itself breaks into caramel.
  4. The glass and the cup of cold waterDescribe how to read a candy thermometer, why to calibrate it in boiling water, and how altitude shifts every printed number.Use the glass and the cup together. Keep the bulb in the syrup, not on the steel. Check 212 °F in plain water at sea level. CSU: subtract 2 °F per 1,000 feet of elevation.
  5. The night the fudge went grainyDistinguish crystalline from amorphous candy, explain invert sugar and other interfering agents, and leave one thing to notice plus one open question.Fudge wants tiny crystals; a lollipop wants none. Corn syrup, invert sugar and fat jam the sucrose lock. A stray seed or a humid night can undo a perfect number on the glass.

Questions this course answers

According to the Exploratorium, what does the highest temperature of a sugar syrup tell you?

The Exploratorium's candy-stages page says the highest temperature the syrup reaches tells you what it will be like when it cools. The names — thread, ball, crack — are names for that cooled state.

At sea level, about what Fahrenheit temperature does the Exploratorium give for the start of hard-crack?

Hard-crack is 300–310 °F at sea level, with about 99 percent sugar. Soft-ball is much lower, around 235. Caramel brown liquid is higher, 338.

In your own words, why does a pot of sugar syrup boil hotter and hotter as it cooks?

OpenStax / LibreTexts and Purdue treat boiling-point elevation as a colligative effect: more solute particles, lower vapour pressure, higher boil. In the candy pot the concentration keeps rising because water is leaving.

Name the kind of property that depends on how many solute particles are present, not on which chemical they are.

LibreTexts / OpenStax: in the dilute ideal-solution model, colligative properties depend on the total concentration of solute species. Sucrose and ethylene glycol at the same molality are predicted to produce the same boiling-point elevation; concentrated candy syrup is outside that simple model.

Match each Exploratorium stage-family to a candy that lives there

The Exploratorium assigns fudge, pralines and fondant to soft-ball; caramels to firm-ball; taffy and butterscotch to soft-crack; toffee, nut brittles and lollipops to hard-crack.

Put these on the climb in the order water leaves the pot

Water leaves, concentration rises, the cooled drop gets stiffer, then the water is gone and caramelisation begins. Thread, ball, crack, then caramel.

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