Close-up Look at Change

This section contains six slides related to evaporation for students to discuss.

Slide 1:

Building on the particulate views of mixtures from the previous section, this section starts by considering three phases of water. These three phases should be very familiar to students at this grade level.

Show the three particulate illustrations on this to the students. Ask them to consider what happens to water when it changes from one phase to another?

Slide 2:

Ask students to think through how to label the arrows that represent the changes of water between phases.

• Click on "reveal" to show answers.

Slide 3:

Read and discuss:

• For chemists, one thing that distinguishes a mixture from a compound is that the ingredients in a mixture can be taken apart by a physical process, while the ingredients of a compound can only be taken apart by a chemical reaction. You can get solid salt out of a saltwater mixture by evaporating the water molecules away, a physical process requiring no chemical reaction. This evaporation might be caused by, for example, sunlight heating the water, so that many of the water molecules get enough energy to escape the salt water and fly into the air.

Slide 4:

Tell students that the image #1 is a saltwater solution with air above. Ask students to consider what the images #2 and #3 show.

Paraphrase:

• When seawater evaporates, only the water molecules evaporate into the air. The sodium and chlorine atoms do not; they remain in the seawater. As more water evaporates, students should notice that the sodium and chlorine form crystals of sodium chloride, just as the powder from pink lemonade recrystallizes when the water evaporates in the Science Scene.

Slide 5:

Show students the photograph of the salt pan worker. This is how we get sea salt to put in our food. Ask them if this is a natural process, a human-engineered process, or both.

Answer:

• Both

Ask students to read the lines from the Coleridge poem. Ask them if they’ve ever seen a movie or read a book where the character(s) were at sea and needed fresh water. What did the characters try to do to get freshwater?

(Some students may have read the novel or seen the movie Life of Pi, in which the protagonist captures fresh water from evaporated seawater using a floating “solar still.”)

Paraphrase:

The three particulate illustrations shows air that has lots of water vapor in it. When the concentration of water vapor in the air gets high enough (relative to the temperature—it has to be cold enough), the water molecules clump together into liquid droplets. This transition from gas to liquid is called condensation. As more water condenses, droplets join together into larger drops and can fall as rain.

The water cycle diagram shows the processes of evaporation and condensation at work on a massive scale, in the water cycle.

Ask students to discuss the role that temperature plays in this process.

Answer:

• In general, colder temperatures with high water vapor concentration in the air promote condensation. Higher temperatures and lower water vapor concentration work against concentration.

Paraphrase:

This illustration of the laboratory equipment shows how we can replicate the distillation process seen in the water cycle.

Underscore for students that the cold water circulating in and out of the condenser is only there to transfer heat away from the steam. None of that cold water passes into the tube where the steam is. All of the water that drips into the collection flask is condensed steam from the original salt water.

Ask students to label the equipment with the corresponding natural phenomena.

Answers:

• Bunsen burner—sun
• salt water—the ocean
• condenser—cloud formation
• dripping condensation—rain
• collected fresh water—rivers and lakes

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