The Nature of Liquids

A Model for Liquids According to the kinetic theory, both the particles that make up gases and liquids have motion. While particles in gases are not attracted to each other, particles in liquids are.

A Model for Liquids The attractive forces between molecules are called intermolecular forces. The forces allow the particles in liquids to slide past one another.

A Model for Liquids Most of the particles of liquids do not have enough kinetic energy to overcome the intermolecular forces and escape into the gaseous state. Liquids take the shape of its container.

A Model for Liquids The interplay between the disruptive motions of particles of a liquid and the attractive forces between them causes liquids to flow and have definite volumes.

A Model for Liquids Intermolecular forces also reduce the amount of space between the particles in a liquid, which makes them more dense than gases. Increasing the pressure on a liquid has hardly any effect on its volume. So, liquids and solids are known as condensed states of matter.

Evaporation The conversion of a liquid to a gas or vapor is called vaporization. When it goes from a liquid to a gas at the surface of a liquid that is not boiling, the process is called evaporation.

Evaporation vs. Boiling

Evaporation In evaporation some molecules in the liquid break away and enter the gas or vapor state. Only those molecules of the liquid with a certain minimum kinetic energy can break away from the surface.

Evaporation Added heat increases the average kinetic energy of the liquid’s particles. The energy enables more particles to overcome the attractive forces keeping them in the liquid state. As evaporation occurs, the particles with the highest kinetic energy tend to escape first.

Vapor Pressure When a partially filled container of liquid is sealed, some of the particles in the liquid vaporize. These particles collide with the walls of the sealed container and produce a vapor pressure, or a force due to the gas above the liquid.

Vapor Pressure As time passes, the number of particles entering the vapor increases and eventually some particles will return to the liquid, or condense. After a time, the number of vapor particles condensing will equal the number of liquid particles vaporizing and the vapor pressure will remain constant.

Vapor Pressure In a system at constant vapor pressure, a dynamic equilibrium exists between the gas and the liquid. Within the system, the rate of evaporation of liquid equals the rate of condensation of vapor.

Vapor Pressure and Temperature Change An increase in the temperature of a contained liquid increases the vapor pressure. The vapor pressure of a liquid can be determined by means of a device called a manometer.

Vapor Pressure Measurements In a simple manometer, one end of a U- shaped glass tube containing mercury is attached to a container. The other end of the tube is open to the surrounding atmosphere.

Vapor Pressure Measurements When a liquid is added to the container, the pressure in the container increases due to the vapor pressure of the liquid. The vapor pressure of the liquid pushes the mercury on the container side of the U-tube; the levels of mercury in the U-tube are no longer the same.

Boiling Point When the liquid is heated to a high enough temperature, many of the particles throughout the liquid have enough kinetic energy to vaporize. At that point, boiling (or vaporization throughout the liquid) occurs.

Boiling Point The boiling point (bp) is the temperature at which the vapor pressure of the liquid is just equal to the external pressure. Bubbles of vapor form throughout the liquid, rise to the surface, and escape into the air as the liquid boils.

Boiling Point and Pressure Changes The boiling point of a liquid varies with the external pressure. At high altitudes, the atmospheric pressure is lower than it is at sea level which allows the liquid to boil at a lower temperature.

Boiling due to Altitude

Boiling Point and Pressure Changes Boiling point decreases at lower pressure and increases at higher pressure. At lower pressure, the boiling point decreases because the particles need less kinetic energy to escape the liquids. At higher external pressures, a liquids boiling point increases because the particles in the liquid need more kinetic energy to escape.

Boiling Point and Pressure Changes The temperature of the boiling liquid never rises above its boiling point. If heat is supplied at a greater rate, the liquid only boils faster. The vapor produced is at the same temperature as that of the boiling liquid.

Normal Boiling Point Because a liquid can have various boiling points depending on pressure, the normal boiling point is defined as the boiling point of a liquid at a pressure of kPa.