Why does vapour pressure not increase linearly

Volatile substances have low boiling points and relatively weak intermolecular interactions; nonvolatile substances have high boiling points and relatively strong intermolecular interactions.

The experimentally measured vapor pressures of liquid Hg at four temperatures are listed in the following table:. Safety note: mercury is highly toxic; when it is spilled, its vapor pressure generates hazardous levels of mercury vapor.

Given: vapor pressures at four temperatures. A The table gives the measured vapor pressures of liquid Hg for four temperatures. We therefore select two sets of values from the table and convert the temperatures from degrees Celsius to kelvin because the equation requires absolute temperatures. Substituting the values measured at At what temperature does the liquid have a vapor pressure of 2. As the temperature of a liquid increases, the vapor pressure of the liquid increases until it equals the external pressure, or the atmospheric pressure in the case of an open container.

Bubbles of vapor begin to form throughout the liquid, and the liquid begins to boil. The temperature at which a liquid boils at exactly 1 atm pressure is the normal boiling point of the liquid. Although we usually cite the normal boiling point of a liquid, the actual boiling point depends on the pressure. Hence the molecules must have greater kinetic energy to escape from the surface.

Typical variations in atmospheric pressure at sea level are relatively small, causing only minor changes in the boiling point of water. For example, the highest recorded atmospheric pressure at sea level is mmHg, recorded during a Siberian winter; the lowest sea-level pressure ever measured was mmHg in a Pacific typhoon.

At high altitudes, on the other hand, the dependence of the boiling point of water on pressure becomes significant. At an elevation of only ft, for example, the boiling point of water is already lower than the lowest ever recorded at sea level.

Conversely, pressure cookers, which have a seal that allows the pressure inside them to exceed 1 atm, are used to cook food more rapidly by raising the boiling point of water and thus the temperature at which the food is being cooked. As pressure increases, the boiling point of a liquid increases and vice versa.

Asked for: corresponding boiling point and pressure. Ethylene glycol is an organic compound primarily used as a raw material in the manufacture of polyester fibers and fabric industry, and polyethylene terephthalate resins PET used in bottling.

Because the molecules of a liquid are in constant motion and possess a wide range of kinetic energies, at any moment some fraction of them has enough energy to escape from the surface of the liquid to enter the gas or vapor phase. This process, called vaporization or evaporation , generates a vapor pressure above the liquid. Molecules in the gas phase can collide with the liquid surface and reenter the liquid via condensation. Eventually, a steady state is reached in which the number of molecules evaporating and condensing per unit time is the same, and the system is in a state of dynamic equilibrium.

Under these conditions, a liquid exhibits a characteristic equilibrium vapor pressure that depends only on the temperature. We can express the nonlinear relationship between vapor pressure and temperature as a linear relationship using the Clausius—Clapeyron equation.

This equation can be used to calculate the enthalpy of vaporization of a liquid from its measured vapor pressure at two or more temperatures.

Volatile liquids are liquids with high vapor pressures, which tend to evaporate readily from an open container; nonvolatile liquids have low vapor pressures. When the vapor pressure equals the external pressure, bubbles of vapor form within the liquid, and it boils.

The temperature at which a substance boils at a pressure of 1 atm is its normal boiling point. Modified by Joshua Halpern Howard University. Learning Objectives To know how and why the vapor pressure of a liquid varies with temperature.

To understand that the equilibrium vapor pressure of a liquid depends on the temperature and the intermolecular forces present. To understand that the relationship between pressure, enthalpy of vaporization, and temperature is given by the Clausius-Clapeyron equation. The vapor pressure is a measure of the presure force per unit area exerted by a gas above a liquid in a sealed container.

Vapor pressure is a property of a liquid based on the strength of its intermolecular forces. A liquid with weak intermolecular forces evaporates more easily and has a high vapor pressure. A liquid with stronger intermolecular forces does not evaporate easily and thus has a lower vapor pressure. For example, diethyl ether is a nonpolar liquid with weak dispersion forces.

Water is a polar liquid whose molecules are attracted to one another by relatively strong hydrogen bonding. Vapor pressure is dependent upon temperature. When the liquid in a closed container is heated, more molecules escape the liquid phase and evaporate. The greater number of vapor molecules strike the container walls more frequently, resulting in an increase in pressure. The Table below shows the temperature dependence of the vapor pressure of three liquids.

Notice that the temperature dependence of the vapor pressure is not linear. Use the link below to answer the following questions:. Skip to main content. States of Matter. Search for:. Vapor Pressure Learning Objectives Define vapor pressure. Describe the relationship between the intermolecular forces in a liquid and its vapor pressure. Describe the relationship between the vapor pressure of a liquid and temperature. Examples What causes this toy to move?

You can see the drinking duck in action in the video at the link below: The motion of the duck illustrates a physical principle called vapor pressure. Figure