Tonic water is a carbonated soft drink consisting primarily of water, with quinine added for its distinct bitter flavor. Many assume it will freeze like plain water, solidifying at \(32^\circ\text{F}\) or \(0^\circ\text{C}\). This assumption is inaccurate because the dissolved components fundamentally change the liquid’s behavior when exposed to freezing temperatures. Tonic water does freeze, but the process and the required temperature are different from pure water.
The Definitive Answer: Freezing Point Depression
The short answer is that tonic water does freeze, but it requires a colder temperature than pure water. This phenomenon is governed by Freezing Point Depression (FPD), which describes the lowering of a solvent’s freezing point when a non-volatile solute is introduced.
Pure water freezes at a single, fixed temperature, but tonic water must be cooled further before solidification can begin. Dissolved substances, such as sugars and quinine, interfere with water molecules’ ability to organize into the stable structure of ice crystals. The freezing point of tonic water typically falls within a range of \(28^\circ\text{F}\) to \(31^\circ\text{F}\) (approximately \(-2^\circ\text{C}\) to \(-0.5^\circ\text{C}\)), depending on the concentration of flavorings and sweeteners. The resulting frozen liquid is a mixture of solid water crystals and a concentrated liquid solution.
The Science Behind the Shift
The primary cause of the freezing point shift is the concentration of dissolved particles, a colligative property of the solution. Tonic water contains significant amounts of sugar or high-fructose corn syrup, which acts as the main non-volatile solute. These sugar molecules physically occupy space between water molecules, preventing the formation of hydrogen bonds necessary for the water to solidify into a crystalline lattice.
Quinine and mineral salts also contribute to this effect by adding more dissolved particles to the mixture. Every particle, regardless of its chemical identity, contributes to the overall depression of the freezing point. The greater the concentration of these particles, the lower the temperature must drop for the solution to freeze.
Carbonation, or dissolved carbon dioxide (\(\text{CO}_2\)), plays a minor role in the freezing process. While \(\text{CO}_2\) is a solute, its main effect relates to supercooling, where the liquid drops below its freezing point without solidifying due to a lack of nucleation sites. When tonic water freezes, the \(\text{CO}_2\) is forced out of the solution because it is less soluble in ice than in liquid water.
Physical Effects of Freezing
Freezing tonic water in a sealed container, such as an aluminum can or a glass bottle, creates several notable physical consequences. The most immediate concern is the buildup of internal pressure caused by water expansion. Water is unusual in that it expands by about 9% of its volume when it transitions from a liquid to a solid state.
This volume increase, combined with carbon dioxide being forced out of the solution, exerts tremendous force on the container walls. As ice crystals form, the dissolved \(\text{CO}_2\) concentrates in the remaining liquid and headspace, often leading to the container rupturing or exploding. Glass bottles are particularly susceptible to this pressure.
Freezing also severely degrades the quality of the tonic water. Since solutes are excluded from the forming ice, the remaining unfrozen liquid becomes an extremely concentrated, overly sweet syrup. Upon thawing, the drink is often flat because the carbonation was forced out during the freezing process. The resulting liquid will taste inconsistent and lack the refreshing effervescence.