Tuesday, September 2, 2008

Chapter 9

Thermal Properties Of Matter.


Internal energy
- Particles in a matter(solid, liquid or gas) posses energy called internal energy.
- Comprises of 2 components:
1. Kinetic energy
2. Potential energy

The kinetic component
- Higher the temperature, the more vigorously the particles move.
• Kinetic energy is due to their movement instead of vibrations.

The potential component
- Due to the stretching and compressing of the intermolecular bonds as the particles vibrate.
- Amount of potential energy stored in the bonds depends on the force between the particles and how far apart the particles are.


• An increase in temperature leads to an increase in the kinetic component of the internal energy.


Melting

Heating Curve.

• When a solid changes to a liquid upon heating.
• Pure substances –melting occurs at a definite or constant temperature.

- The temperature goes up the longer the heating continues. However, there are two horizontal flat parts to the graph. These happen when there is a change of state.

• The first change of state is melting (changing from a solid to a liquid). The temperature stays the same while a substance melts.

• The second change of state is boiling (changing from a liquid to a gas). The temperature stays the same while a substance boils.


During melting
- Heat is absorbed to break the attractive force between the particles
- Allows them to break free from their fixed positions and are able to move about.

During boiling
- Heat is absorbed to break the attractive force between the particles
- Allows them to move freely and randomly in all directions.


Solidification.

- Reverse process of melting
- A pure substance will sodify or freeze at a temperature equal to its melting point.


Cooling Curve.


During condensation
-The motion of particles becomes less vigorous in order to form the attractive force between the particles when a gas changes into a liquid.

During freezing (solidification)
-Strong attractive force can be formed between the particles in order to be arranged in a regular pattern at their fixed positions.


Condensation

• A process whereby vapour changes into liquid at the same constant temperature. Heat is given out during condensation.
• During condesation, the temperature remains constant at the condensation point. Thermal energy is released by the substance.


Boiling and Evaporation


• Boiling is a process in which a liquid changes into a gas at the boiling point of the liquid.
• Evaporation is a process in which a liquid changes into a gas temperatures below the boiling point of the liquid.


Difference between boiling and evaporation.

Boiling
- Occurs at constant temperature
- Happens within the liquid
- Bubbles produced
- Rate of process: Fast

Evaporation
- Occurs at any temperature
- Only the surface of the liquid involved
- No bubbles
- Rate of process: Slow


Evaporation

• A cooling process
• Evaporation of a liquid is due to the molecules at the surface with energy greater than the average kinetic energy escaping from the rest of the liquid.


Factors affecting the rate of evaporation.

1. Temperature
- Higher the temperature of the liquid, the higher the rate of evaporation.

2. Humidity of the surrounding air
- Lower the humidity of the surrounding air, the higher the rate of evaporation.

3. Surface area of the liquid
- Greater the surface area of the liquid, the higher the rate of evaporation.

4. Movement of air

5. Pressure
- Lower the pressure in the surroundings, the higher the rate of evaporation.

6. Boiling point of a liquid


>Eureka! Episode 18 - Evaporation and Condensation, Video-YouTube

Posted at 4:07 AM