Chapter 28 Quiz — Cold and Ice

Multiple Choice (Questions 1–15)

1. The phase of ice that exists in your home freezer is called: - a) Ice II - b) Ice Ih (ice "I-h," for hexagonal) - c) Ice glass - d) Ice IX

2. Why does ice float on liquid water? - a) Ice has more dissolved air than water - b) The freezing process pushes air into the ice, lowering its density - c) Ice's hexagonal hydrogen-bonded lattice is more open than the liquid arrangement, so ice is less dense than liquid water - d) Surface tension at the ice-water interface holds the ice up

3. The freezing point of pure water at 1 atmosphere is: - a) -1°C / 30°F - b) 0°C / 32°F - c) 2°C / 36°F - d) 4°C / 39°F (the same temperature as maximum density)

4. Adding salt to ice on a sidewalk melts the ice because: - a) The salt creates an exothermic reaction that warms the ice - b) The salt causes the ice to chemically decompose - c) Dissolved salt lowers the freezing point of water below the ambient temperature, so the ice can no longer be solid - d) The salt raises the boiling point of the water

5. The formula ΔT = K_f × m × i predicts: - a) The boiling point elevation of a solution - b) The freezing point depression of a solution - c) The osmotic pressure of a solution - d) The vapor pressure of a solution

6. In Pat Hammond's classroom demonstration, the bottle of pure water is below 0°C but still liquid. This phenomenon is called: - a) Subcooling - b) Sublimation - c) Supercooling - d) Solute exclusion

7. The reason supercooling can persist below 0°C in pure water is: - a) The water is too cold to freeze - b) Without a nucleation site (a place for the first crystal to form), the water remains in a metastable liquid state - c) The dissolved gases in the water prevent freezing - d) The bottle is sealed, preventing pressure buildup

8. What is the approximate threshold below which ice crystals are too small for the human tongue to detect as gritty? - a) 5 μm - b) 50 μm - c) 500 μm - d) 5 mm

9. Fast cooling produces creamy ice cream because: - a) Fast cooling produces fewer but larger ice crystals - b) Fast cooling produces many but small ice crystals (high nucleation rate, limited growth) - c) Fast cooling prevents air incorporation - d) Fast cooling caramelizes the sugar

10. Ice cream's three phases are: - a) Cream, sugar, vanilla - b) Liquid, solid, and gas water - c) Ice crystals, fat globules, and air cells - d) Lactose, casein, and butterfat

11. The volume of air incorporated into ice cream is called: - a) Foam factor - b) Aeration index - c) Overrun - d) Gas fraction

12. American premium ice cream typically has an overrun of: - a) 0% - b) 5–15% - c) 20–30% - d) 60–80%

13. Liquid nitrogen boils at approximately: - a) -78°C / -108°F (the temperature of dry ice) - b) -100°C / -148°F - c) -196°C / -321°F - d) -273°C / -460°F (absolute zero)

14. Freezer burn is caused primarily by: - a) Bacterial spoilage during freezing - b) Chemical reactions between fats and freezer chemicals - c) Sublimation of surface ice (direct conversion of solid ice to water vapor) and surface oxidation - d) Slow freezing inside the food

15. The reason boat-frozen fish at -40°C / -40°F can be of higher quality than "fresh" fish that has been on melting ice for three days is: - a) Salt water freezes the fish more deeply - b) Below -18°C / 0°F, enzymatic and bacterial degradation essentially stop, while at +1°C they continue slowly - c) The boat freezer adds preservatives - d) Fish always tastes better frozen

Short Answer (Questions 16–20)

16. Explain in 2–3 sentences why a hand-crank ice cream maker requires both ice and salt in its outer chamber. What does each contribute?

17. Describe the difference between freezer burn and freezing damage. Give one example of each.

18. Why does Italian gelato have lower overrun and lower fat than American premium ice cream — and what trade-offs does this represent?

19. Define Ostwald ripening in your own words and explain why a home freezer (operating around -18°C / 0°F) is worse for long-term ice cream texture than a commercial deep-freezer (-25°C / -13°F).

20. A friend wants to make sorbet at home but the result keeps freezing rock-hard. Diagnose the likely cause and recommend a fix using the science of this chapter.

Answer Key with Explanations

1. b) Ice Ih (hexagonal). Other forms of ice (ice II, III, V, VI, VII, VIII, IX) exist only at extreme pressures or in unusual conditions; ice Ih is the only form found at normal temperatures and pressures, including in your freezer.

2. c) Ice's hexagonal hydrogen-bonded lattice is more open than the liquid arrangement, so ice is less dense than liquid water. This is one of water's most distinctive properties — most substances are denser as solids than liquids. The hydrogen-bond geometry forces a more open structure in the crystal than in the liquid.

3. b) 0°C / 32°F. This is by definition the lower fixed point of the Celsius scale. Pure water's freezing point at 1 atmosphere is the reference.

4. c) Dissolved salt lowers the freezing point of water below the ambient temperature, so the ice can no longer be solid. This is freezing-point depression — a colligative property of solutions.

5. b) The freezing point depression of a solution. ΔT = K_f × m × i is the freezing point depression formula. K_f for water = 1.86 °C·kg/mol; m is molality; i is the van't Hoff factor (number of particles per solute molecule).

6. c) Supercooling. Pure water in a smooth, undisturbed container can persist below its normal freezing point without forming ice; the metastable state is called supercooling.

7. b) Without a nucleation site (a place for the first crystal to form), the water remains in a metastable liquid state. Forming the first ice crystal requires a critical cluster of water molecules to spontaneously align — without dust, scratches, or vibration to nucleate, this is statistically rare.

8. b) 50 μm. Below about 50 μm, the human tongue cannot detect crystals as gritty. This is the texture threshold for ice cream.

9. b) Fast cooling produces many but small ice crystals. Fast cooling triggers a high nucleation rate (many nuclei form before any can grow), so the available water is divided among many crystals, keeping each small.

10. c) Ice crystals, fat globules, and air cells. Ice cream is a three-phase composite with a serum phase (concentrated sugar solution) holding it all together.

11. c) Overrun. Overrun is the volume percentage of air in ice cream. Premium ice creams have lower overrun (denser, more flavorful) and mass-market ice creams have higher overrun (more air, less substance).

12. c) 20–30%. American premium ice cream is typically 20–30% air. Mass-market cheap ice cream can be 50% or higher.

13. c) -196°C / -321°F. This is the boiling point of nitrogen at atmospheric pressure. It is why LN₂ produces the smoothest ice cream — the cooling rate is so high that ice crystals form below 10 μm.

14. c) Sublimation of surface ice and surface oxidation. Freezer burn is not freezing damage; it is dehydration plus oxidation at the surface, caused by water molecules leaving the food as vapor over time.

15. b) Below -18°C / 0°F, enzymatic and bacterial degradation essentially stop, while at +1°C they continue slowly. Boat-frozen fish at -40°C is locked in time; "fresh" fish on melting ice is slowly degrading.

16. The ice itself is not cold enough to freeze cream (ice is at 0°C / 32°F, but cream-and-sugar mix freezes at around -3°C / 27°F). Adding salt lowers the freezing point of the brine to around -18°C / 0°F, far below the freezing point of the cream. The cold brine then draws heat out of the cream rapidly, freezing it. Without salt, you get a slightly cool but not frozen cream; without ice, you have nothing for the salt to depress the freezing point of.

17. Freezer burn is dehydration of the food's surface caused by sublimation of surface ice into the freezer atmosphere over time, often combined with surface oxidation. The food is dry, leathery, off-color on the outside; the inside is fine. Example: an ice cream carton with a dry, off-white skin on top. Freezing damage is rupture of cells inside the food caused by the expansion of ice as it forms during slow freezing. The food is mushy or watery on thawing because ruptured cells leak their contents. Example: a steak that was slow-frozen and now releases significant "thaw drip" when defrosted.

18. Italian gelato is traditionally made with less fat (5–10%) than American premium (14–20%), with less air (overrun 20–30% or lower vs. 30–50%), and served warmer (-12°C vs. -18°C). The lower fat means flavors register more intensely on the tongue (less lipid-coating to mute taste); less air means more substance per spoonful; warmer means more aroma volatilizes. The trade-off is that gelato has less of the rich, slow-melting mouthfeel that high-fat American ice cream provides, and it cannot be stored as long without recrystallization. Each style is a different optimization on the same physics.

19. Ostwald ripening is the slow migration of water molecules from small ice crystals to larger ones over time, driven by the slightly higher chemical potential of small crystals (higher surface-to-volume ratio means higher surface energy per unit volume). Even in frozen ice cream, this process continues as long as the unfrozen serum is mobile. At -18°C, the serum is still above the glass transition temperature for typical ice cream (around -30°C), so ripening proceeds at a measurable rate; at -25°C, the serum is closer to or below T_g and ripening is much slower. The Williams-Landel-Ferry equation predicts that the rate can change by orders of magnitude per degree near T_g — so the 10°C difference between a home and commercial freezer corresponds to a roughly 100× difference in recrystallization rate.

20. Sorbet contains no fat (no fat-globule network) and is essentially a frozen sugar syrup. If it freezes rock-hard, the freezing-point depression is too low — meaning either too little sugar in the formula or sugar that produces too few solute particles per gram. Fixes: (1) increase total sugar content to 25–30% by weight; (2) replace some sucrose with invert sugar (50% glucose + 50% fructose), which gives twice as many solute molecules per gram and depresses the freezing point twice as much; (3) add a small amount of liquor or alcoholic spirit (which lowers freezing point further); (4) add a small amount of stabilizer (gelatin, pectin, guar gum) to thicken the unfrozen serum and slow water release. The right combination of these can take a rock-hard sorbet to scoopable.