The Fairy-Land of Science · Lecture IV, part 2 · Part 4: Lecture IV: A drop of water on its travels
11. Dew, frost, and growing crystals
Goal: Your child can explain how dew forms on cold grass, and can grow salt crystals by letting salty water dry slowly.
You'll need
- A glass of ice water
- A dish towel
- Two tablespoons of salt dissolved in half a cup of warm water
- A dark-colored saucer or plate
- A magnifying glass
- Science notebook and pencils
What's in the book
Pages 81 to 88. Rain soaked up by plant roots; the invisible vapor in the air that shields us from the sun by day and keeps warmth in at night; how dew forms on cold grass but not under a cloth, under cobwebs or on gravel; hail and snow; and crystals of salt, sugar-candy and gems, with little bar magnets to show how the bits might line up.
The lesson, step by step
Hook question
SayOn a summer morning the grass is wet, but it didn't rain. Where did the water come from?
Let your child guess.
Try it: make dew
Wipe the outside of the glass of ice water dry and set it on the towel. Wait five minutes. The outside gets misty, then wet.
Read page 84.
I have here a bottle of ice which has been kept outside the window. When I bring it into the warm room a mist forms rapidly outside the bottle.
SayThe water came out of the air, not through the glass. The cold glass chilled the air next to it, and the invisible vapor turned to drops.
Dew on the grass
Read pages 83 to 85. On a clear night the thin grass blades lose their warmth fast and get cold, like our glass. Under a cloth spread on sticks there is hardly any dew, and there's none on a gravel path.
you will see plenty of drops on the cobwebs themselves sparkling like diamonds
Buckley also says the vapor in the air is a blanket that keeps the Earth from freezing at night.
Try it: grow salt crystals
Read page 86.
If you melt salt in water and then let all the water evaporate slowly, you will get salt-crystals
Pour a thin layer of your salty water onto the dark saucer and set it somewhere warm and still. Over the next few days, little square crystals will appear. Look at them with the magnifying glass. Look at some sugar and table salt, too.
Crystals everywhere
Read or tell page 87: cave icicles of lime, diamonds, quartz and many gems are all crystals. The tiny bits inside line up in a regular pattern. Buckley used small magnets to show how bits with two different ends might line up neatly. (She also grows crystals of nitre under a microscope; plain salt or sugar is the safe choice at home.)
Tell it back and check
Your child tells back how dew forms. Then ask:
1. Where did the water on the outside of the cold glass come from?
2. Why is there less dew on a cloudy night?
3. What shape are salt crystals?
Draw it
Draw the cold glass with drops on it, and a blade of grass with dew drops. Later in the week, draw your salt crystals as you see them through the magnifying glass.
Afterwards
Notice it this week: go out early one clear morning and look for dew on grass, cars and spider webs. Is there any on the sidewalk? Check the salt saucer every day.
Tip
Salt crystals grow best when the saucer is left completely alone. Put it on top of the fridge or a high shelf.
Answers to the book's problems
1. Out of the air: the cold glass chilled the air, and the invisible vapor in it turned into drops. 2. The clouds send warmth back down, so the grass doesn't get cold enough. 3. Little cubes (squares).
Worked out for this site from the scan. If a number in your copy differs, trust the book.