Chapter 8 Those incredible water molecules Water is the most important substance on earth and in any living organism. Why?

8.1 Water molecules form an open crystalline structure in ice When water freezes, the water molecules arrange themselves in a six-sided crystalline structure. Increased structural orderliness usually means increased density. Solids and crystals have high structural orderliness and are thus denser than liquid and non-crystalline materials. But water is exception. Why?

8.2 Freezing and melting go on at the same time Hydrogen bonds form during freezing of water, releasing heat. They break during melting, absorbing heat. Adding salt to water decreases the melting point of water.

Water is densest at 4℃ When the temperature of a given is increased, its molecules vibrate faster and move farther apart. So the substance expands. This is the case in all three phases of water, apart from the liquid water from 0-4℃. The reason is “ice-like” crystal structure starts to form at 4℃. Question: What is the temperature of water at the bottom of sea?

The fact that water is densest at 4℃ is of great importance in nature

8.3 The behavior of liquid water is the result of the stickness of water molecules Cohesive force (内聚力): forces of attraction between molecules of a single substance. Adhesive force (粘合力): forces of attraction between molecules of two different substances.   Coin resting on the water surface, Why? Surface tension (表面张力) Surface tension (表面张力): The tendency of the molecules on the surface of a liquid to go into the liquid. The surface tension also accounts for the spherical shape of liquid drops, especially in the weightless environment.

Question 1: What is the main factor influencing the surface tension of a material? Question 2: What would happen if a coin was placed on the surface of oil?

Water has very high surface tension because of the strong hydrogen bond, which prevents it from wetting materials with nonpolar surfaces, such as waxy leaves, umbrellas. The addition of detergent (or) soap can reduce the surface tension and increase the wettability of water.

How to decide the wettability of a solid by a liquid?

Capillary action (毛细管作用) results from the interplay of adhesive and cohesive forces Question: 1. What would happen if the liquid is oil and tube is made of glass? 2. What would happen if the liquid is water and tube is made of plastics? 3. What would happen if the tube is wider?

Surface Energy: results from the unstability (higher energy) of the molecules on the surface. The particle size and surface energy? Surface energy and nano technology Surface energy and migration of materials to the surface

8.4 Water molecules move freely between the liquid and gaseous phases

Evaporation (蒸发) is a cooling process Evaporation (蒸发) is a cooling process. Condensation (凝聚) is a warming process.   A large amount of energy is released during the condensation of water vapor. Household radiator Hurricanes (The formation of 1 inch of rain over an area of 1 square mile yields the energy equivalent of about 32000 tons of exploded dynamite.) Question: Why do hurricanes and typhoons usually happen in summer?

Boiling is evaporation beneath a liquid surface

8.5 It takes a lot of energy to change the temperature of water Water has a very high specific heat capacity (热容) because of the strong hydrogen bond. Specific heat capacity is defined as the quantity of heat required to change the temperature of 1 gram of substance by 1℃.

Global climates are influenced by water’s high specific heat capacity.

8.6 A phase change requires the input or output of energy Any phase change involves the breaking or forming of molecular attraction and thus the release or absorption of energy. Solid to liquid: heat of melting (positive: increase the potential energy) Liquid to solid: heat of freezing (negative: reduce the potential energy) Liquid to gas: heat of vaporization (positive) Gas to liquid: heat of condensation (negative)

Summarization