According to the kinetic theory, both particles in gases and the particles in liquids have kinetic energy. This energy allows the particles in gases and liquids to flow past one another.
Substances that can flow are referred to as fluids.
The interplay between the disruptive motions of particles in a liquid and the attractions among the particles determines the physical properties of liquids.
Liquids and solids are known as condensed states of matter.
The ability of
liquids to flow
allows them to
conform to the
shape of their
The conversion of a liquid to a gas or vapor is called vaporization.
When this conversion occurs at the surface of a liquid that is not boiling, the process is called evaporation.
During evaporation, only those molecules with a certain minimum kinetic energy can escape from the surface of the liquid.
As evaporation occurs, the particles with the highest kinetic energy tend to escape first. The particles left in the liquid have a lower average kinetic energy than the particles that have escaped.
As evaporation takes place, the liquid's temperature decreases. Therefor, evaporation is a cooling process.
A measure of the force exerted by a gas above a liquid is called vapor pressure.
In a system at constant vapor pressure, a dynamic equilibrium exists between the vapor and the liquid. The system is in equilibrium because the rate of evaporation of liquid equals the rate of condensation of vapor.
At equilibrium, the particles in a system continue to evaporate and condense, but no net change occurs in the number of particles in the liquid or vapor.
Vapor Pressure & Temperature Change
An increase of temperature in a liquid leads to increased vapor pressure.
This happens because the particles in the warmed liquid have increased kinetic energy.
As a result, more particles will reach minimum kinetic energy to escape the surface of a liquid.
The particles escape the liquid and hit the walls of a container with greater frequency.
Vapor Pressure Measurements
The vapor Pressure of a liquid can be determined by a device called a manometer.
As the vapor Pressure increases, so does the difference of the two levels on the manometer.
The rate of evaporation of a liquid from an open container increases as liquid is heated.
Heating allows most particles at the liquids surface to overcome the attractive forces that keep them in the liquid state.
The average kinetic energy of particles in the liquid increases and the temperature of the liquid increases.
When a liquid is heated to a temperature at which particles throughout the liquid have enough kinetic energy to vaporize, the liquid begins to boil.
Boiling point is the temperature at which the vapor Pressure of a liquid is equal to external pressure on liquid.
Boiling Point and Pressure Changes
Liquids don't always boil at the same temperature.
A change in altitude affects the boiling point of a liquid.
Because atmospheric pressure is lower at higher altitudes, boiling points decrease at higher altitudes.
Boiling is a cooling process, similar to evaporation.
The vapor being produced is at the same temperature as the boiling liquid.
Normal Boiling Point
The normal boiling point is defined as the boiling point of a liquid at a pressure of 101.3 kPa.