What makes a freezing point lower

Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:. Skip to main content. Search for:. Freezing Point Depression. Learning Objective Discuss the effects of a solute on the freezing point of a solvent. Key Points The freezing point depression can be calculated using the pure solvent freezing point and the molality of the solution.

At the freezing point, the vapor pressure of both the solid and liquid form of a compound must be equal. In geographical areas where winter temperatures go below the freezing point of water, using pure water as the coolant could allow the water to freeze. Since water expands when it freezes, freezing coolant could crack engine blocks, radiators, and coolant lines.

What is the boiling point of the solution? We get:. Note: Since sea water contains roughly This page was constructed from content via the following contributor s and edited topically or extensively by the LibreTexts development team to meet platform style, presentation, and quality:. Learning Objectives Explain what the term "colligative" means, and list the colligative properties.

Indicate what happens to the boiling point and the freezing point of a solvent when a solute is added to it. Calculate boiling point elevations and freezing point depressions for a solution. Freezing Point Depression The effect of adding a solute to a solvent has the opposite effect on the freezing point of a solution as it does on the boiling point.

Comparing the Freezing and Boiling Point of Solutions Recall that covalent and ionic compounds do not dissolve in the same way. To compare the boiling or freezing points of solutions, follow these general steps: Label each solute as ionic or covalent. If the solute is ionic, determine the number of ions in the formula. Be careful to look for polyatomic ions. This will give you the total concentration of particles dissolved. Compare these values. The higher total concentration will result in a higher boiling point and a lower freezing point.

Summary Colligative properties are properties that are due only to the number of particles in solution, and are not related to the chemical properties of the solute. Boiling points of solutions are higher than the boiling points of the pure solvents.

Freezing points of solutions are lower than the freezing points of the pure solvents. Ionic compounds split into ions when they dissolve, forming more particles. Covalent compounds stay as complete molecules when they dissolve. Vocabulary Colligative property - A property that is due only to the number of particles in solution, and not the type of the solute.

Boiling point elevation - The amount that the boiling point of a solution increases from the boiling point of the pure solvent. If solving for the proportionality constant is not the ultimate goal of the problem, these values will most likely be given. Molality is defined as the number of moles of solute per kilogram solvent.

Be careful not to use the mass of the entire solution. Often, the problem will give you the change in temperature and the proportionality constant, and you must find the molality first in order to get your final answer. The solute, in order for it to exert any change on colligative properties, must fulfill two conditions.

First, it must not contribute to the vapor pressure of the solution, and second, it must remain suspended in the solution even during phase changes. Because the solvent is no longer pure with the addition of solutes, we can say that the chemical potential of the solvent is lower. Chemical potential is the molar Gibb's energy that one mole of solvent is able to contribute to a mixture. The higher the chemical potential of a solvent is, the more it is able to drive the reaction forward.

Consequently, solvents with higher chemical potentials will also have higher vapor pressures. The boiling point is reached when the chemical potential of the pure solvent, a liquid, reaches that of the chemical potential of pure vapor. Because of the decrease in the chemical potential of mixed solvents and solutes, we observe this intersection at higher temperatures. In other words, the boiling point of the impure solvent will be at a higher temperature than that of the pure liquid solvent.

Thus, boiling point elevation occurs with a temperature increase that is quantified using. Freezing point is reached when the chemical potential of the pure liquid solvent reaches that of the pure solid solvent. Again, since we are dealing with mixtures with decreased chemical potential, we expect the freezing point to change. Unlike the boiling point, the chemical potential of the impure solvent requires a colder temperature for it to reach the chemical potential of the pure solid solvent.