LS1a F08 Lect 3 Lecture Notesr-1

LS1a F08 Lect 3 Lecture Notesr-1 - page 38 Water solid H 2...

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Unformatted text preview: page 38 Water solid H 2 O (ice) liquid H 2 O (water) gaseous H 2 O (steam) You now have a partial framework for understanding the demonstration that I showed you in the last lecture. As all of you know, water has three states. Solid water (ice) is very ordered and structured. The intermolecular forces between the water molecules keep them at their Fxed positions. If you know the position of one molecule, you know the position of the other molecules. In other words, we can fully describe the solid state. In the gaseous state, molecules are so far apart that they do not really interact with each other. They have no way of communicating with each other, and we say that the gas has no structure. In the liquid state, molecules are further away from each other than in a solid, but nearer to each other than in a gas. There are intermolecular forces between the molecules, but they are always forming and breaking (i.e. transient). To interconvert between these states, you need to make or break hydrogen bonds. We ve just learned that in order to break a bond (whether covalent or non-covalent), you require energy. So to go from solid state to liquid state of water, we need to put in some energy. To go from the liquid state to the gaseous state, we need to put in more energy. That s why you need to heat water to make it boil. page 39 Water solid H 2 O (ice) liquid H 2 O (water) gaseous H 2 O (steam) To understand the demonstration, you need to know what the boiling point of a substance is. In order for liquid water to become gaseous water, the water molecules must attain enough energy to break free from each other. At a temperature below the boiling point of water, only some of the water molecules have the energy to break free into the gas state. Only the water molecules at the gas-liquid interface can break free and move into the gaseous state because the other water molecules have too many intermolecular interactions with other molecules holding them in a liquid state. As we increase the temperature of the water to the boiling point, all the molecules now have enough energy to break free from each other. Because of that, you start to see bubbles (pockets of gaseous water) being generated from within the water mass (and not just from the surface). By deFnition, then, boiling takes place when the molecules in the liquid state have enough energy to break free from each other and become (energetically) identical to the molecules in the gaseous state just above it. page 40 Water solid H 2 O (ice) liquid H 2 O (water) gaseous H 2 O (steam) So what happened in the demo? If you remember, we started the experiment by heating the water to boiling. We then removed the Fask from the heater and stoppered it in order to prevent the water molecules from escaping. The water stopped boiling when we removed it from the heat, but it started boiling again when we placed it into the bucket of ice water. What happened?...
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LS1a F08 Lect 3 Lecture Notesr-1 - page 38 Water solid H 2...

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