How Things are, Part I: Cohesion of Atoms or Molecules

1. How Things are, Part I: Cohesion of Atoms or Molecules • Solids (examples: a penny, ice, glass???) • Molecules have attracted each other • Sometimes in a specific array (crystals) • Sometimes not (glasses) • Hold their own shape • Liquids (example: honey, bath water, glass???) • Molecules have attracted each other • Not in any pattern • Have a surface: adopt shape of bottom of container • Gases (example: air, helium in balloon) • Molecules not attracted to each other • No pattern • No surface: adopt shape of container, period.

2. How things are, Part IIKinetic Theory Solids: molecules or atoms can wiggle about fixed, central location. Liquids: molecules or atoms move along until they bounce into something, and this happens frequently. Gases: like liquids, only collisions not as often.

3. How fast? Depends on molecule. isopentyl acetate, CH3COOCH2CH2CH(CH3)2 About 500 miles per hour—pretty fast! Let’s measure it.

4. Why it took so long to get to back of room: collisions! The isopentyl acetate molecule only goes about 200 nm before hitting something, which then sends it in a different direction. It takes forever for it to spread around the room…..unless there is a draft.

5. Diffusion: 10000 steps forward and 10001 steps backward The “hydrodynamic” world of diffusion—dominated by collisions—is very different from our normal “ballistic” world. An object set into motion comes almost immediately to a stop, rather than coasting as we are used to seeing.

6. Systematic Study of the Major States of Matter, Emphasizing Kinetic Theory & Order Gases first Then normal liquids Then solids, amorphous & crystalline Then special states, like liquid crystals

7. Gases first—pressure matters! Gas particle hits a piston, imparts a force. With a small number of gas atoms in the cylinder, this force would be quite random. With a large number, the force would be quite steady. We usually see almost perfectly steady pressure because there are zillions of particles in any normal sample.

8. Measuring pressure…in principlebalance between force of gravity and force exerted by gases P = F/A = mg/p r2 Mass of bricks 9.8 m/s2 Radius of piston

9. Slightly more practical…using water height to measure pressure • How high will the water rise? • depends on the weather • depends on whether you're in Louisiana • or Colorado • is always a little more than 30 feet • Why? Because that is how far • atmospheric pressure can push it. • The vacuum pump doesn't suck; • the air pushes.

10. Density of the water Yeah, but how far will the water rise? Patm = rgh = 101,325 nt/m2 9.8 m/s2

11. Alternative to the Super Hoover • Fill a hose with water, taking care to introduce no bubbles. • Put the hose in a large tub of water. • Seal one end of the hose. • Drag the sealed end out with a pulley. • When the hose tip exceeds about 34 feet, you will see the water goes no higher! You get an air gap at the top of the hose. • You now have a barometer. As weather changes, the height of the column of water will fluctuate.

12. BOWB = Big Ol’ Water Barometer

13. Announcement BreakWednesday, October 17 Why polarity matters: like dissolves like

14. Remember: P = rghso… h = P/rgDenser liquids mean shorter barometers! Mercury density: 13.6 g/mL

15. mm Hg, Pa, Torr & PSI 760 mm Hg = 101325 nt/m2 = 1 atmosphere = 14.7 PSI Define: one Torr is one millimeter of mercury (named after Toricelli, an Italian)