The Nature of LiquidsThe Nature of Liquids

Prentice-Hall Chapter 13.2Prentice-Hall Chapter 13.2

Dr. YagerDr. Yager

ObjectivesObjectives

IdentifyIdentify factors that determine the physical factors that determine the physical properties of liquidsproperties of liquids

DefineDefine evaporation in terms of kinetic energy evaporation in terms of kinetic energy

DescribeDescribe the equilibrium between a liquid and the equilibrium between a liquid and its vaporits vapor

IdentifyIdentify the conditions at which boiling occurs the conditions at which boiling occurs

Model for LiquidsModel for Liquids

All particles have kinetic energy. All particles have kinetic energy.

Liquids flow and conform to the shape of Liquids flow and conform to the shape of their container.their container.

Liquid particles are attracted to each other.Liquid particles are attracted to each other.

Substances that can flow are referred to as fluids. Both liquids and gases are fluids.

The interplay between the kinetic energy of a liquid’s particles and the attractive forces between them determine the liquid’s physical properties.

Key IdeaKey Idea

In an open container, molecules that evaporate can escape from the container.

EvaporationEvaporation vaporizationvaporization - the conversion of a liquid to gas - the conversion of a liquid to gas

evaporationevaporation - vaporization at a surface without - vaporization at a surface without boilingboiling

Both occur because particles have the minimum Both occur because particles have the minimum kinetic energy to break away from the surface.kinetic energy to break away from the surface.

Some particles rebound back to the liquid.Some particles rebound back to the liquid.

Both increase with heat. Both increase with heat.

In a closed container, the molecules cannot escape. They collect as a vapor above the liquid. Some molecules condense back into a liquid.

Vapor PressureVapor Pressure

Vapor pressureVapor pressure is the force exerted by a gas is the force exerted by a gas above a liquid.above a liquid.

Some particles collide and condense back to Some particles collide and condense back to liquid.liquid.

Key IdeaKey Idea

In a system at constant vapor pressure, a In a system at constant vapor pressure, a dynamic equilibriumdynamic equilibrium exists between the exists between the vapor and the liquid. The system is in vapor and the liquid. The system is in equilibrium because the equilibrium because the rate of evaporation of rate of evaporation of liquid equals the rate of condensation of vaporliquid equals the rate of condensation of vapor..

Vapor Pressure and Temperature ChangeVapor Pressure and Temperature Change

An increase in temperature causes an increase An increase in temperature causes an increase in vapor pressure. in vapor pressure.

The particles in the warmed liquid have The particles in the warmed liquid have increased kinetic energy. As a result, more of increased kinetic energy. As a result, more of the particles will have the the particles will have the minimumminimum kinetic kinetic energy necessary to escape the surface of the energy necessary to escape the surface of the liquid.liquid.

Vapor Pressure and Vapor Pressure and Temperature Change Temperature Change

ManometerManometer

Vapor Pressure MeasurementVapor Pressure Measurement

Boiling PointBoiling Point

As heating increases so does the kinetic As heating increases so does the kinetic energy of the particles.energy of the particles.

Boiling occurs when the liquid has enough Boiling occurs when the liquid has enough kinetic energy kinetic energy throughoutthroughout the liquid to the liquid to vaporize.vaporize.

Bubbles of vapor form throughout the liquid.Bubbles of vapor form throughout the liquid.

Boiling PointBoiling Point

boiling pointboiling point: the temperature where the : the temperature where the vapor pressure of the liquid just equals the vapor pressure of the liquid just equals the external pressure.external pressure.

At a At a lower external pressurelower external pressure, the boiling , the boiling point point decreasesdecreases..

At a At a higher external pressurehigher external pressure, the boiling , the boiling point point increasesincreases..

Sometimes in a chemistry lab we put glass Sometimes in a chemistry lab we put glass boiling beads in a liquid as we heat it. Why?boiling beads in a liquid as we heat it. Why?

The beads absorb some of the kinetic The beads absorb some of the kinetic energy by bouncing around and prevent the energy by bouncing around and prevent the liquid from boiling too quickly.liquid from boiling too quickly.

Altitude and 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 101.3 kPa.

Boiling Versus EvaporationBoiling Versus Evaporation

Both are cooling processes (remove kinetic energy).Both are cooling processes (remove kinetic energy).

During both the highest kinetic energy particles During both the highest kinetic energy particles escape first.escape first.

Increased heat increases Increased heat increases raterate of boiling. of boiling.

Boiling liquid temperature Boiling liquid temperature nevernever rises above the rises above the boiling point.boiling point.

More heat means more kinetic energy.More heat means more kinetic energy. Vapor can have more heat (potential energy) than the liquid.Vapor can have more heat (potential energy) than the liquid.

Evaporation and SweatingEvaporation and Sweating

As liquids evaporate the temperature of the As liquids evaporate the temperature of the remaining liquid decreases.remaining liquid decreases.

When you perspire water molecules absorb When you perspire water molecules absorb heat from your body when they leave your heat from your body when they leave your skin surface, making it cooler.skin surface, making it cooler.

Evaporation and CoolingEvaporation and Cooling

1.1. In liquids, the attractive forces areIn liquids, the attractive forces are

a.a. very weak compared with the kinetic very weak compared with the kinetic energies of the particles.energies of the particles.

b.b. strong enough to keep the particles strong enough to keep the particles confined to fixed locations in the liquid.confined to fixed locations in the liquid.

c.c. strong enough to keep the particles from strong enough to keep the particles from evaporating.evaporating.

d.d. strong enough to keep particles relatively strong enough to keep particles relatively close together.close together.

1.1. In liquids, the attractive forces areIn liquids, the attractive forces are

a.a. very weak compared with the kinetic very weak compared with the kinetic energies of the particles.energies of the particles.

b.b. strong enough to keep the particles strong enough to keep the particles confined to fixed locations in the liquid.confined to fixed locations in the liquid.

c.c. strong enough to keep the particles from strong enough to keep the particles from evaporating.evaporating.

d.d. strong enough to keep particles relatively strong enough to keep particles relatively close together.close together.

2. Which one of the following is a 2. Which one of the following is a process that absorbs energy?process that absorbs energy?

a.a. freezingfreezing

b.b. condensationcondensation

c.c. evaporationevaporation

d.d. solidifyingsolidifying

2. Which one of the following is a 2. Which one of the following is a process that absorbs energy?process that absorbs energy?

a.a. freezingfreezing

b.b. condensationcondensation

c.c. evaporationevaporation

d.d. solidifyingsolidifying

3. In a sealed gas-liquid system at constant 3. In a sealed gas-liquid system at constant temperature eventually temperature eventually

a.a. there will be no more evaporation.there will be no more evaporation.

b.b. the rate of condensation decreases to zero.the rate of condensation decreases to zero.

c.c. the rate of condensation exceeds the rate of the rate of condensation exceeds the rate of evaporation.evaporation.

d.d. the rate of evaporation equals the rate of the rate of evaporation equals the rate of condensation.condensation.

3. In a sealed gas-liquid system at constant 3. In a sealed gas-liquid system at constant temperature eventually temperature eventually

a.a. there will be no more evaporation.there will be no more evaporation.

b.b. the rate of condensation decreases to zero.the rate of condensation decreases to zero.

c.c. the rate of condensation exceeds the rate of the rate of condensation exceeds the rate of evaporation.evaporation.

d.d. the rate of evaporation equals the rate of the rate of evaporation equals the rate of condensation.condensation.

4. Where must particles have enough 4. Where must particles have enough kinetic energy to vaporize for boiling kinetic energy to vaporize for boiling to occur?to occur?

a.a. at the surface of the liquidat the surface of the liquid

b.b. at the bottom of the containerat the bottom of the container

c.c. along the sides of the containeralong the sides of the container

d.d. throughout the liquidthroughout the liquid

4. Where must particles have enough 4. Where must particles have enough kinetic energy to vaporize for boiling kinetic energy to vaporize for boiling to occur?to occur?

a.a. at the surface of the liquidat the surface of the liquid

b.b. at the bottom of the containerat the bottom of the container

c.c. along the sides of the containeralong the sides of the container

d.d. throughout the liquidthroughout the liquid

5.5. The boiling point of a liquid The boiling point of a liquid

a.a. increases at higher altitudes.increases at higher altitudes.

b.b. decreases at higher altitudes.decreases at higher altitudes.

c.c. is the same at all altitudes.is the same at all altitudes.

d.d. decreases as the pressure increases.decreases as the pressure increases.

5.5. The boiling point of a liquid The boiling point of a liquid

a.a. increases at higher altitudes.increases at higher altitudes.

b.b. decreases at higher altitudes.decreases at higher altitudes.

c.c. is the same at all altitudes.is the same at all altitudes.

d.d. decreases as the pressure increases.decreases as the pressure increases.