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Boiling, Freezing, Floating: Phase & Colligative Properties

See how dissolved solutes change boiling point, freezing point, vapor pressure, and osmotic pressure — plus altitude effects and standard reference conditions.

Reviewed by the thecalcu.com team · Last updated August 4, 2026

Free calculators used in this guide

Boiling Point CalculatorBoiling Point at Altitude CalculatorBoiling Point Elevation CalculatorFreezing Point Depression CalculatorVapor Pressure CalculatorVapor Pressure of Water CalculatorOsmotic Pressure CalculatorSTP CalculatorGibbs Phase Rule Calculator

Overview

Add a solute to a solvent, change the pressure, or gain altitude, and you change when and how a substance shifts phase. These calculations connect more than they first appear to. This guide covers phase behavior from both directions: how pressure and altitude affect a pure substance's boiling point, and how dissolved solutes shift boiling point, freezing point, vapor pressure, and osmotic pressure, all expressions of the same colligative-property idea.

Start with pure-substance behavior, move into solute-driven effects, then finish with the phase rule that explains why these effects happen at all.

Step 1: Calculate Boiling Point Under Different Conditions

A pure substance's boiling point isn't fixed. It depends on the surrounding pressure, which is why water boils at a lower temperature at high altitude, where atmospheric pressure drops.

The Boiling Point Calculator handles the general pure-substance case. The Boiling Point at Altitude Calculator adjusts specifically for elevation.

Step 2: Calculate Colligative Effects of Dissolved Solutes

Dissolved solutes push boiling point up and freezing point down, and both shifts trace back to the same mechanism: the solute particles interfere with the solvent's ability to organize into a crystal or escape into vapor. How big either shift gets depends on solute molality and how many particles each solute unit breaks into once dissolved.

The Boiling Point Elevation Calculator and Freezing Point Depression Calculator work out these two related, opposite-direction shifts from solute concentration and dissociation behavior.

Step 3: Calculate Vapor Pressure

Vapor pressure is the pressure a substance's vapor exerts when it sits in equilibrium with its liquid. It drives evaporation rate and feeds into humidity, weather, and storage calculations, well beyond predicting where something boils.

The Vapor Pressure Calculator handles general substances. The Vapor Pressure of Water Calculator is tuned for water specifically, since it's the case you'll hit most often.

Step 4: Calculate Osmotic Pressure

Osmotic pressure is the pressure needed to stop solvent flow across a semi-permeable membrane. Like boiling point elevation and freezing point depression, it's colligative: what matters is how many dissolved particles are present, not their identity.

The Osmotic Pressure Calculator works out this pressure from solute concentration, using the same particle-counting logic from Step 2.

Step 5: Reference Standard Conditions and the Phase Rule

Gas calculations often need a fixed reference point, standard temperature and pressure (STP), so measurements taken under different conditions can be compared using the 22.4 L/mol molar volume relationship. Separately, the Gibbs phase rule explains why a solution's boiling point, unlike a pure substance's, can vary at fixed pressure depending on concentration.

The STP Calculator converts gas measurements to and from standard conditions. The Gibbs Phase Rule Calculator works out a system's degrees of freedom from its number of components and phases.

Key Terms

  • Colligative property: a property of a solution (boiling point elevation, freezing point depression, vapor pressure, osmotic pressure) that depends on the number of dissolved particles, not their identity
  • Vapor pressure: the pressure a substance's vapor exerts in equilibrium with its liquid phase at a given temperature
  • Molality: a concentration unit (moles of solute per kilogram of solvent) used in colligative property calculations because it doesn't shift with temperature
  • Osmotic pressure: the pressure required to stop solvent flow across a semi-permeable membrane from a less concentrated to a more concentrated solution
  • STP (Standard Temperature and Pressure): a fixed reference point used to compare gas measurements taken under different conditions
  • Degrees of freedom (Gibbs phase rule): the number of variables you can change independently in a system while it keeps the same number of phases

Frequently Asked Questions

Why does water boil at a different temperature depending on altitude?
Boiling happens when a liquid's vapor pressure matches the surrounding atmospheric pressure, and atmospheric pressure drops as you climb. That means water reaches its boiling point sooner at altitude, around 95°C (203°F) at 5,000 feet compared to 100°C (212°F) at sea level. The [Boiling Point at Altitude Calculator](/boiling-point-at-altitude-calculator/) estimates the adjusted boiling point for any elevation, which is handy for cooking and lab work in mountain towns.
What's the difference between boiling point and boiling point elevation?
Boiling point is the temperature a pure substance boils at under a given pressure. Boiling point elevation describes something narrower: how much higher a solution's boiling point climbs once you dissolve a solute in it. Add salt to water and its boiling point rises slightly above the pure water value of 100°C. The [Boiling Point Calculator](/boiling-point-calculator/) handles the pure-substance case, and the [Boiling Point Elevation Calculator](/boiling-point-elevation-calculator/) works out the increase from a specific solute concentration.
Why do dissolved solutes raise boiling point but lower freezing point? Shouldn't they push in the same direction?
They come from the same root cause. Dissolved solute particles get in the way of the solvent organizing into a crystal, which lowers the freezing point, and they also get in the way of solvent molecules escaping into vapor, which raises the boiling point. So a solute widens the liquid range at both ends instead of shifting things one direction. Road salt lowers ice's melting point through freezing point depression while also nudging water's boiling point up, both effects coming from the same dissolved sodium chloride.
How much does dissolved solute concentration actually affect freezing point?
Freezing point depression scales with solute molality (moles of solute per kilogram of solvent) and also depends on how many particles each solute unit breaks into once dissolved. A mole of NaCl lowers the freezing point roughly twice as much as a mole of glucose, because NaCl splits into two ions (Na⁺ and Cl⁻) in solution while glucose stays intact. The [Freezing Point Depression Calculator](/freezing-point-depression-calculator/) factors in that dissociation behavior when it works out the freezing point change.
What is vapor pressure, and why does it matter beyond just predicting boiling point?
Vapor pressure is the pressure a substance's vapor exerts when it's in equilibrium with its liquid at a given temperature. It shows up well beyond boiling point prediction: it drives evaporation rate, sets storage requirements for volatile chemicals, and feeds into humidity and weather calculations. The [Vapor Pressure Calculator](/vapor-pressure-calculator/) works out this pressure for a general substance at whatever temperature you specify.
Why does water get its own dedicated vapor pressure calculator instead of the general one?
Water's vapor pressure comes up constantly, in humidity calculations, weather science, and countless lab settings, so a calculator built on water-specific reference data (like the Antoine equation coefficients for water) gets you a faster, more precise answer than pulling general substance parameters each time. The [Vapor Pressure of Water Calculator](/vapor-pressure-of-water-calculator/) is built around exactly this common case.
What is osmotic pressure, and how does it relate to the other colligative properties here?
Osmotic pressure is the pressure needed to stop osmosis, the flow of solvent across a semi-permeable membrane from a less concentrated solution into a more concentrated one. It's a colligative property just like boiling point elevation and freezing point depression: what matters is how many dissolved particles are present, not what they are. The [Osmotic Pressure Calculator](/osmotic-pressure-calculator/) works out this pressure from solute concentration, following the same particle-counting logic as the other calculators here.
What are standard temperature and pressure (STP), and why do they matter for gas calculations?
STP is a fixed reference point, historically 0°C and 1 atm, though IUPAC updated the pressure definition to 100 kPa back in 1982, so gas volumes and properties can be compared on equal footing no matter what the actual measurement conditions were. One mole of any ideal gas occupies 22.4 liters at the older STP definition, a number used constantly as a stoichiometric conversion factor. The [STP Calculator](/stp-calculator/) converts gas measurements to and from standard conditions.
What does the Gibbs phase rule tell you that isn't obvious from a phase diagram alone?
The Gibbs phase rule (F = C − P + 2) works out the degrees of freedom in a system, meaning how many variables like temperature and pressure you can change independently while keeping the same number of phases present. It explains why a pure substance's boiling point sits at one fixed value at a given pressure, while a solution's boiling point can shift at that same pressure depending on concentration. The [Gibbs Phase Rule Calculator](/gibbs-phase-rule-calculator/) works out degrees of freedom from the number of components and phases present.
Does altitude affect freezing point the same way it affects boiling point?
Not really. Freezing point is far less sensitive to pressure changes than boiling point, because the volume difference between solid and liquid phases is much smaller than the volume difference between liquid and gas. That's why altitude-adjusted freezing point isn't something you calculate in practice the way altitude-adjusted boiling point is. Boiling point elevation from altitude comes from a change in atmospheric pressure, a completely different mechanism from freezing point depression, which comes from dissolved solute concentration.
Why would a chemistry problem specify STP if it doesn't change the actual chemistry involved?
STP exists purely to fix a common reference point for comparing gas volumes and calculating quantities using the 22.4 L/mol relationship. The underlying chemistry doesn't change; it's a bookkeeping convention, similar to how molar mass calculations always reference a mole regardless of how much material an experiment actually uses. Check which STP convention, the older 1 atm or the newer 100 kPa definition, a problem or the [STP Calculator](/stp-calculator/) is using, since it shifts the numeric molar volume slightly.
How would I use the Gibbs phase rule and boiling point elevation together in a real problem?
The phase rule explains why a solution's boiling point can vary at fixed pressure in the first place: its degrees of freedom include concentration, unlike a pure substance's. Boiling point elevation then gives you the actual number, how much that boiling point shifts for a specific solute concentration. One tells you the effect is possible; the other tells you how big it is for your solution.

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