A First Course in Physics · Ch. VI, §§ 168–176 · Chapter 6: Molecular Forces
18. Solutions, crystals, and gases dissolved in water
Goal: Your student can explain dissolving and saturated solutions with molecules, grow crystals, and explain why warm water holds less gas and why soda fizzes.
You'll need
- The book, open to these pages
- Salt or sugar, warm tap water, a clear jar, a pencil and string (for growing crystals)
- Food coloring
- A bottle of soda water
- A magnifying glass
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Dissolving as a kind of evaporation into a liquid; different liquids dissolve different things; saturated solutions; more dissolves when hot; crystals from evaporating or cooling solutions (salt, alum, copper sulfate, saltpeter); gases absorbed by charcoal and by water; soda water. Seven problems follow.
The lesson, step by step
Hook question
SayWhy does sugar dissolve so much faster in hot tea than in iced tea?
Read together
Read §§ 168–176. The book compares dissolving to evaporating:
Just as at a given temperature only a certain amount of liquid will evaporate into a closed space, so also there is a definite limit to the amount of a solid which will dissolve at any temperature in a given body of liquid.
Try it: a saturated solution
Stir salt into a cup of room-temperature water a spoonful at a time until no more will dissolve: it is saturated (§ 169). The book says 100 g of water holds about 36 g of salt. Weigh to check, if you have a kitchen scale.
Try it: grow crystals
Dissolve as much sugar or salt as you can in very warm tap water (an adult handles hot water). Pour into a jar, hang a string from a pencil laid across the top, and set it somewhere quiet. Over several days crystals grow on the string. Look at salt crystals with a magnifying glass: they are little cubes (§ 173).
Gases in water
Open a bottle of soda: bubbles appear because the pressure was released (§ 176). Watch a pan of cold water warm on the stove: tiny bubbles of air form on the bottom long before it boils (§ 175). Cooler water holds more gas, which is why fish need cool, fresh water.
Watch only: ammonia and mercury
Watch only: don't try this at home. §§ 174–175 fill tubes with ammonia gas over mercury and use strong chemicals. Describe the results: charcoal soaked up the ammonia, and water absorbed it so fast it rushed up the tube like a fountain. The book's copper sulfate and saltpeter crystals also need a school lab; use salt, sugar, or alum from the spice aisle with an adult.
Say it back
Your student explains why salt dissolves and later comes back as crystals when the water evaporates.
Problems
Work problems 1–7 on book page 128.
Afterwards
Check the crystal jar each day and sketch the crystals. Leave a saucer of salt water in the sun and see what is left behind.
Tip
Never taste or eat crystals grown from anything but plain sugar or salt.
Answers to the book's problems
1. The fish use up the dissolved oxygen. 2. Heating drives the dissolved ammonia gas out of the water, since warm water holds less gas. 3. Once water reached F it began absorbing the ammonia very fast, lowering the pressure in F sharply. 4. Air bubbles appear well below boiling and do not shrink as they rise into cooler water; steam bubbles collapse. 5. 5 (the body loses 60 g less in the lighter liquid: 0.2V = 60, so V = 300 cc and the weight is 1500 g). 6. About 68,300 lb. of lift; about 38,300 lb. of cargo. 7. About 373,800 lb., if the water is 20 ft. deep against the dam (30 × 20 sq. ft. × average depth 10 ft. × 62.3 lb.).
Worked out for this site from the scan. If a number in your copy differs, trust the book.