What Is Freezing Point Definition Formula and Factors Affecting It
The particular temperature at which a liquid transforms into a solid is known as its Freezing Point. Like the melting point, the freezing point also rises when there is an increase in pressure.
When we talk about mixtures and specific organic compounds, their freezing point is lower than their melting point. When these mixtures begin freezing, the solid they form in the start has a different composition than that of the liquid.
This formation substantially changes the composition of the remaining liquids, this usually happens in a manner that lowers the freezing point slowly.
We can apply this same treatment in successive melting, purifying mixtures, and freezing.
Let us study the different types of freezing points, the factors affecting them, the supercooled liquid, and other concepts.
Freezing Point - Basics
As we discussed that freezing is the process where a substance changes its state from liquid to solid, we understand that in this process a substance is transforming from one state of matter into another.
We will call a point a freezing point if its solid and liquid states exist at the same time in the given equilibrium.
Freezing Point Defection
The freezing point is the temperature at which a liquid changes into solid, at normal atmospheric pressure. A more precise definition of a freezing point is the temperature at which liquid and solid phases coexist in the equilibrium.
How Does Freezing Occur? - The Process
As the liquid freezes, it turns from a liquid state into a solid-state. This phenomenon occurs when a substance’s molecules are loosely bound. The intermolecular forces of attraction between the molecules are less than that of the solids.
In liquids, water molecules are always moving. They continuously bump into each other and are always in motion. This is the thermal energy between these molecules and this energy cools down when it freezes, as a result, these molecules come closer and turn the liquid-solid.
When a liquid like water freezes, its molecules settle down in one place, the forces attracted to one another hold these molecules together and solid crystals begin to form.
During this freezing process, the temperature of the substance remains the same. The particles in this liquid substance turn into crystalline solids, as the particles lose their energy when they are turning into solids, this energy gets released.
Fun Fact
A liquid's freezing point is the same as its melting point in the solid-state. For example, water’s freezing point is 0° Celsius (32° Fahrenheit), but the melting point of water in its solid state is also 0° Celsius.
Factors Affecting the Freezing Point
Types of molecules: If the intermolecular forces between the molecules of a liquid are strong, its freezing point also becomes high.
In contrast, if the intermolecular forces of attraction between the molecules are weak, then its freezing point becomes low.
By observing these two facts we can say that the intermolecular forces of attraction are directly proportional to its freezing point.
Types of Changes in a Freezing Point
We observe that there are two kinds of changes, chemical and physical. These changes can affect a substance’s freezing point. We can sometimes also change the freezing point and melting point of a certain substance by mixing another soluble substance with it.
You can also obtain a lower freezing point by altering the pressure.
Supercooled Liquid
What if a liquid is cooled to an extreme but it is still a liquid substance?
This process is called supercooling a liquid, where we chill the liquid even beyond its freezing point and melting point without turning its state to solid.
Now in theory we know this is not possible, as the freezing point and melting point of a substance has to be the same. It applies to most substances.
However, there are few substances that have a slight difference between their melting and freezing points. These kinds of substances can get cooled beyond their freezing point and still stay in their liquid state. These are substances that are known as Supercooled Liquids.
The most well-known example of a supercooled liquid is the clouds at high altitudes that are nothing but a collection of supercooled water droplets that are below their freezing point.
FAQs on Freezing Point in Chemistry Explained Clearly
1. What is freezing point in chemistry?
The freezing point is the temperature at which a liquid changes into a solid at a given pressure, usually 1 atm. At this temperature, the liquid and solid phases are in equilibrium. For example, the freezing point of pure water is 0°C (273.15 K) at 1 atm. Freezing occurs when particles lose kinetic energy and arrange into a fixed, ordered structure.
2. What is the freezing point of water?
The freezing point of pure water is 0°C (273.15 K) at standard atmospheric pressure (1 atm). At this temperature, H2O(l) changes to H2O(s). The value can change if pressure changes or if impurities (like salt) are dissolved in the water.
3. What is freezing point depression?
Freezing point depression is the decrease in the freezing point of a solvent when a non-volatile solute is added. It is a colligative property, meaning it depends on the number of solute particles, not their identity. For example, adding NaCl to water lowers its freezing point below 0°C, which is why salt is used to melt ice on roads.
4. What is the formula for freezing point depression?
The formula for freezing point depression is ΔTf = iKfm. In this equation:
- ΔTf = decrease in freezing point
- i = van 't Hoff factor (number of particles formed)
- Kf = molal freezing point constant of the solvent
- m = molality of the solution (mol/kg)
5. How do you calculate the new freezing point of a solution?
The new freezing point is calculated by subtracting the freezing point depression from the pure solvent’s freezing point. Steps:
- 1. Calculate molality (m) = moles of solute / kg of solvent.
- 2. Use ΔTf = iKfm.
- 3. New freezing point = normal freezing point − ΔTf.
6. Why does adding salt lower the freezing point of water?
Adding salt lowers the freezing point because dissolved ions interfere with the formation of the solid crystal lattice of ice. When NaCl(s) → Na+(aq) + Cl−(aq), the ions disrupt hydrogen bonding between water molecules. This requires a lower temperature for water molecules to organize into solid ice, causing freezing point depression.
7. What is the difference between freezing point and melting point?
The freezing point is the temperature at which a liquid becomes a solid, while the melting point is the temperature at which a solid becomes a liquid. For a pure substance at 1 atm, the freezing point and melting point are the same temperature. For example, water freezes and ice melts at 0°C under standard conditions.
8. What factors affect the freezing point of a substance?
The freezing point of a substance is affected mainly by pressure and the presence of impurities. Key factors include:
- Intermolecular forces (stronger forces usually mean higher freezing point)
- Pressure (changes can slightly alter freezing temperature)
- Impurities or solutes (cause freezing point depression)
9. What is the freezing point constant (Kf)?
The freezing point constant (Kf) is a property of a solvent that shows how much its freezing point decreases per molal concentration of solute. It is expressed in units of °C·kg·mol−1. For water, Kf = 1.86 °C·kg·mol−1, meaning 1 mol of solute per kg of water lowers the freezing point by 1.86°C (assuming i = 1).
10. How is freezing point used to determine molar mass?
Freezing point depression can be used to determine the molar mass of an unknown solute by rearranging ΔTf = iKfm. Steps:
- 1. Measure the decrease in freezing point (ΔTf).
- 2. Calculate molality (m = ΔTf / iKf).
- 3. Find moles of solute using molality and mass of solvent.
- 4. Molar mass = mass of solute (g) / moles of solute.
