=28. Absorption of Gases by Solids and Liquids.=--If a piece of heated charcoal is placed in a test-tube containing ammonia gas, inverted in mercury, the ammonia is seen to disappear, the mercury rising to take its place. The ammonia has been absorbed by the charcoal, the gas molecules clinging closely to the solid. The charcoal being very porous presents a large surface to the action of the gas. This experiment indicates that attraction exists between gas molecules and other molecules. Many porous substances have this power of absorbing gases. We have all noticed that butter has its flavor affected by substances placed near it. That _liquids absorb gases_ is shown by slowly heating cold water in a beaker. Small bubbles of air form on the sides and rise before the boiling point is reached. Ammonia gas is readily absorbed in water, the bubbles disappearing almost as soon as they escape into the water from the end of the delivery tube. _Household ammonia_ is simply a solution of ammonia gas in water. On warming the solution of ammonia the gas begins to pass off; thus, warming a liquid tends to drive off any gas dissolved in it. _Soda water_ is made by forcing carbon dioxide gas into water under strong pressure. When placed in a vessel open to the air the pressure is lessened and part of the gas escapes. The dissolved gas gives the characteristic taste to the beverage. Important Topics 1. The solution of solids is increased by heating. 2. The solution of gases is decreased by heating. 3. Pressure increases the quantity of gas that can be dissolved in a liquid. 4. The attraction (cohesion) of molecules of a dissolved solid for each other is shown by crystallization. Exercises 1. How do fish obtain oxygen for breathing? 2. Why does warming water enable it to dissolve more of a salt? 3. Why does warming water lessen the amount of a gas that will stay in solution? 4. Will water absorb gases of strong odor? How do you know? 5. Name three solvents. Give a use for each. 6. What liquids usually contain gases in solution? Name some uses for these dissolved gases. 7. What is the weight of a cubic meter of water? 8. Name three substances obtained by crystallization. 9. How is maple sugar obtained? 10. Name five crystalline substances. (5) EVIDENCE OF MOLECULAR FORCES IN SOLIDS =29. Differences between Solids and Gases.=--In studying gases, it is seen that they behave as if they were composed of small particles in rapid motion, continually striking and rebounding, and separating to fill any space into which they are released. This action indicates that there is practically no attractive force between such molecules.