THE tonoscopy It is a colligative property that causes the vapor pressure of a liquid to be lowered when a nonvolatile solute is added to it.

If we add a non-volatile solvent solute, the vapor pressure decreases and therefore takes longer to evaporate.

The vapor pressure of a pure solvent will always be greater than the vapor pressure of a solution. With the addition of the solute particles, the molecular attractive forces are intensified and the solvent vapor pressure decreases.

Example of pure and solution water vapor pressure:

Pure water (25 ° C) = Pv = 23.76mmHg
1mol / L glucose solution (25 ° C) = Pv = 23.34mmHg
Sucrose 1mol / L solution (25 ° C) = Pv = 23.34mmHg

The greater the amount of particles in a solution, the lower its vapor pressure.

Boiling Pure Liquids

The physical phenomenon where a substance changes from liquid to gaseous state is vaporization.

There are three types of vaporization:
- evaporation
- boiling
- heating

Evaporation happens on the surface of the liquid. It is a slower, calmer vaporization.

Boiling happens inside the liquid. It is a more turbulent, bubble-forming vaporization.

Heating is a very rapid passage from liquid to vapor. Almost instantaneous.

When the vapor pressure equals atmospheric pressure, the liquid boils.

Pv or PO = vapor pressure
Patm = atmospheric pressure (local external pressure)

If the vapor pressure increases, the boiling point (P.E.) also increases.

Atmospheric pressure varies with altitude. With increasing altitude, the atmospheric pressure decreases, decreasing the boiling point, causing the vapor pressure to decrease.

In places where there is less atmospheric pressure, water boils faster. Molecules escape the liquid more easily.

At high altitude, the substances boil at lower temperatures than at sea level. This explains the difficulty of cooking foods such as eggs and rice and making hot drinks such as coffee and tea in places that are at sea level.

See the comparative table between cities of different altitudes and their boiling points:

City / Location

Altitude relative to sea level (m)

Approximate water weight (° C)

Rio de Janeiro



Sao Paulo



Campos do Jordão



Mexico City



La Paz



Mount Everest



The higher the altitude, the lower the P.E., the lower the Patm, less than Pv