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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.

Updated 2026-07-03

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

Adding a solute to a solvent, changing pressure, or gaining altitude all change when and how a substance changes phase — and the calculations behind these changes are more connected than they first appear. This guide covers phase behavior in both directions: how pressure and altitude affect pure-substance boiling point, and how dissolved solutes shift boiling point, freezing point, vapor pressure, and osmotic pressure, all as expressions of the same underlying colligative-property principle.

Start with pure-substance behavior, then move into solute-driven effects, ending with the phase rule that explains why these effects are even possible.

Step 1: Calculate Boiling Point Under Different Conditions

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

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

Step 2: Calculate Colligative Effects of Dissolved Solutes

Dissolved solutes shift both boiling point (upward) and freezing point (downward) from the same underlying mechanism — disrupting the solvent's ability to organize into a crystal or escape into vapor — and the size of both shifts depends on solute molality and how many particles each solute unit dissociates into in solution.

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

Step 3: Calculate Vapor Pressure

Vapor pressure — the pressure exerted by a substance's vapor in equilibrium with its liquid — drives evaporation rate and factors into humidity, weather, and storage calculations well beyond just predicting boiling point.

The Vapor Pressure Calculator handles general substances, and the Vapor Pressure of Water Calculator is optimized specifically for water, the most frequently referenced case.

Step 4: Calculate Osmotic Pressure

Osmotic pressure — the pressure needed to stop solvent flow across a semi-permeable membrane — is a colligative property like boiling point elevation and freezing point depression, depending on the number of dissolved particles rather than their specific identity.

The Osmotic Pressure Calculator calculates this pressure from solute concentration, following the same particle-counting logic used throughout Step 2.

Step 5: Reference Standard Conditions and the Phase Rule

Gas calculations often need a fixed reference point — standard temperature and pressure (STP) — to compare measurements taken under different conditions, using the well-known 22.4 L/mol molar volume relationship. Separately, the Gibbs phase rule explains conceptually 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, and the Gibbs Phase Rule Calculator calculates 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 exerted by a substance's vapor 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 change 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 that can be independently changed in a system while maintaining the same number of phases

Frequently Asked Questions

Boiling occurs when a liquid's vapor pressure equals the surrounding atmospheric pressure, and atmospheric pressure drops as altitude increases — so water reaches its (lower) boiling point sooner at high altitude, boiling at 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 this adjusted boiling point for any elevation, useful for cooking and lab work at high-altitude locations.
Boiling point is the temperature a pure substance boils at under given pressure, while boiling point elevation specifically describes how much higher a solution's boiling point becomes when a solute is dissolved in it — adding salt to water, for instance, raises its boiling point slightly above pure water's 100°C. The [Boiling Point Calculator](/boiling-point-calculator/) handles the pure-substance case, and the [Boiling Point Elevation Calculator](/boiling-point-elevation-calculator/) calculates the increase caused by a specific solute concentration.
Both effects stem from the same underlying cause — dissolved solute particles disrupt the solvent's ability to organize into a crystal (lowering freezing point) or escape into vapor (raising boiling point) — so a solute widens the liquid range in both directions simultaneously, rather than working the same direction on both ends. This is why road salt lowers ice's melting point (freezing point depression) while also slightly raising water's boiling point, both from the same dissolved sodium chloride.
Freezing point depression is proportional to solute molality (moles of solute per kilogram of solvent) and depends on how many particles each solute unit dissociates into in solution — a mole of NaCl lowers freezing point roughly twice as much as a mole of glucose, since NaCl splits into two ions (Na⁺ and Cl⁻) in solution while glucose doesn't dissociate at all. The [Freezing Point Depression Calculator](/freezing-point-depression-calculator/) accounts for this dissociation factor when calculating the freezing point change.
Vapor pressure is the pressure exerted by a substance's vapor when it's in equilibrium with its liquid phase at a given temperature, and it's relevant well beyond boiling point prediction — it drives evaporation rate, determines storage requirements for volatile chemicals, and factors into humidity and weather calculations. The [Vapor Pressure Calculator](/vapor-pressure-calculator/) calculates this pressure for a general substance at a specified temperature.
Water's vapor pressure behavior is so frequently referenced — in humidity calculations, weather science, and countless lab contexts — that a dedicated calculator using water-specific reference data (like the Antoine equation coefficients for water) gives faster, more precise results than looking up general substance parameters each time. The [Vapor Pressure of Water Calculator](/vapor-pressure-of-water-calculator/) is optimized specifically for this common case.
Osmotic pressure is the pressure needed to stop osmosis — the flow of solvent across a semi-permeable membrane from a less concentrated to a more concentrated solution — and like boiling point elevation and freezing point depression, it's a colligative property, meaning it depends on the number of dissolved particles rather than their identity. The [Osmotic Pressure Calculator](/osmotic-pressure-calculator/) calculates this pressure from solute concentration, following the same particle-counting logic as the other colligative property calculators.
STP is a fixed reference point (historically 0°C and 1 atm, though IUPAC updated the pressure definition to 100 kPa in 1982) used so gas volumes and properties can be compared on equal footing regardless of actual measurement conditions — one mole of any ideal gas occupies 22.4 liters at the older STP definition, a number frequently used as a stoichiometric conversion factor. The [STP Calculator](/stp-calculator/) converts gas measurements to and from standard conditions.
The Gibbs phase rule (F = C − P + 2) calculates the degrees of freedom in a system — how many variables like temperature and pressure can be independently changed while keeping the same number of phases present — which explains why a pure substance's boiling point is fixed at a given pressure, but why a solution's boiling point can vary at that same pressure depending on concentration. The [Gibbs Phase Rule Calculator](/gibbs-phase-rule-calculator/) calculates degrees of freedom from the number of components and phases present.
No — freezing point is far less sensitive to pressure changes than boiling point, since the volume difference between solid and liquid phases is much smaller than between liquid and gas phases, which is why altitude-adjusted freezing point isn't a common practical calculation the way altitude-adjusted boiling point is. Boiling point elevation from altitude is driven by atmospheric pressure change, a very different mechanism from freezing point depression, which is driven by dissolved solute concentration.
STP is specified purely to fix a common reference point for comparing gas volumes and calculating quantities using the 22.4 L/mol relationship, not because the underlying chemistry changes — it's a bookkeeping convention, similar to how molar mass calculations always reference a mole regardless of the actual sample size used in an experiment. Always confirm which STP convention (older 1 atm or newer 100 kPa definition) a specific problem or the [STP Calculator](/stp-calculator/) is using, since it affects the numeric molar volume slightly.
The Gibbs phase rule explains why a solution's boiling point can vary at fixed pressure (its degrees of freedom include concentration, unlike a pure substance), while boiling point elevation quantifies exactly how much that boiling point shifts for a specific solute concentration — the phase rule provides the conceptual reason the effect is even possible, while the elevation calculation gives you the actual number for a specific solution.

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