Evaporation and Condensation I

Objectives

• Observe the cooling effects of evaporation.

• Know that evaporation occurs as a result of high energy particles escaping from the liquid.

• Be able to describe and explain heating and cooling processes involving evaporation and condensation in terms of the energy of the particles involved.

Task 1 - Recap

Remind the person next to you:

- How the particles are arranged in a solid, in a liquid, and in a gas.

- What we call a change of state from a solid to a liquid, or a liquid to a solid.

- What we call a change of state from a liquid to a gas, or a gas to a liquid.

Task 2

The particles in this liquid have energy.

As the particles vibrate there are many collisions between them. This causes some particles to have more energy and other particles to have less energy. We say there is a "varying distribution of energies".

Think of the particles as being like bumper cars at the fairground.  Sometimes two or more bumper cars will collide and stop altogether, and sometimes your bumper car is "bumped" from behind and you end up travelling much faster than you were before!

Because of the collisions, and the resulting "distribution of energies", some of the particles will gain enough energy to escape from the top surface of the liquid. Your teacher will give you some notes and a copy of the diagram below:

Stick the notes in your exercise book and write each of the following labels in the correct place on the diagram:

some particles escape from the liquid completely.

particles with a high enough energy can escape from the surface of the liquid.

some particles fall back into the liquid.

Task 3

Your teacher will provide you with a small beaker of propanone. Use a pipette to drop a small amount onto the back of your hand. Now blow on the propanone - you should notice a distinct cooling effect!

Warning: Propanone can be very damaging to eyes.  You must wear safety glasses at all times while carrying out this task.

Now complete the text on your sheet to explain what you have just seen:

The _____________ depends on the __________ thermal energy of the particles in the liquid. The particles with the most energy will ___________from the ______________ of the liquid. If the most ____________ particles escape the average energy of the particles ___________. This means that the fluid _________ as evaporation takes place.

escape          cools          energetic          temperature          decreases          surface          average

Task 4

Condensation is the opposite of evaporation.  Particles in a vapour (gaseous) state lose energy and condense back into a liquid state.  This process of condensing usually happens when the particles encounter a cool surface.

Your teacher will give you a copy of the diagram and notes above to stick into your exercise book.

Now explain, in as much detail as you can, why condensation forms on the mirror, windows and walls when you run a hot bath.  You may want to use this planning skeleton to help you.

Use as many of the following terms as you can in your explanation:

evaporate          evaporation      liquid      collision     gain energy          energetic      particles      escape      vapour     average thermal energy      temperature      cool          lose energy          distribution          surface          condense          condensation