Thermodynamics Jillian Campbell, Karly Johnson, Jared O, Daniel

Evaporation Evaporation: the phase of change from a liquid to gas at the surface of a liquid. Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere. Evaporation from the oceans accounts for 80% of the water delivered as precipitation, with the balance occurring on land, inland waters and plant surfaces. Excited molecules transfer their energy to the other molecules, bumping some out of the liquid and flying into the air. They then become water vapor molecules. When these molecules are bumped into the air, the molecules slow down because of their attraction to the surface of the liquid. When excited molecules bump into each other, the hitting molecule loses the energy that it passes on to the other molecule. This process is known as “billiard-ball physics.” Evaporation does not only occur from a liquid to gas, but from a solid (ice) to gas. This process is called sublimation. An example is dry ice. When exposed to room temperature, it is sublimed into a gas. the rate of evaporation depends upon: Wind speed: the higher the wind speed, Temperature: the higher the temperature, Humidity: the lower the humidity, the more evaporation

Condensation Condensation: the phase of change from a gas to a liquid. This happens when a gas molecule is attracted to the surface of a liquid. It then acquires kinetic energy, crashing into the liquid and becoming part of the liquid. During condensation, there is a warming process. This process is usually seen in steam or water vapor. Since there is steam there is moisture, causing its surroundings to become warm. This amount of water vapor is known as humidity. When that humidity is trapped in the air it is saturated, which occurs when air temperature drops. The drop in temperature causes the gas molecules to slow down and “condense” together, forming a liquid. We usually see this in nature in the form of rain. Condensation is crucial to the water cycle because it is responsible for the formation of clouds.

Condensation in our Atmosphere: Fog and Clouds Humidity = mass of water per volume of air Saturation = air that contains as much vapor as it possibly can. This occurs when the air temperature drops and water-vapor molecules in the air begin condensing. Water molecules generally have high speeds in air, and do not stick together. When slower moving molecules collide, they have a tendency of sticking together, creating water droplets (condensation). Warm air rises, and as it rises, it expands. However, when it expands, it chills, slowing the water-vapor molecules. As it chills, water-vapor molecules begin sticking rather than bouncing off one another. When enough water-vapor condenses in the air and attaches to slow moving particles and ions in the air, a cloud is formed. This can happen when warm breezes blow over and become moist, then moves from warmer to cooler areas to form clouds. When condensation takes place near ground level, we experience fog. There isn’t much of a difference between a cloud and fog, aside from altitude.

Boiling Boiling: evaporation that takes place beneath the surface of the liquid, forming bubbles of water that are buoyed to the surface and that later escape. Boiling point in water is 100 degrees Celsius at ATM If pressure is increased the molecules in the vapor must exert enough pressure to keep the bubble from collapsing Extra pressure can be produced by going deeper below the surface. Extra pressure can also be produces by increasing air pressure above the liquid’s surface. An example of boiling in this way can be seen in a pressure cooker. At high altitudes, water boils at a lower temperature: Denver- boils at 95 degrees Celsius Types of Molecules : the types of molecules that make up a liquid determine its boiling point. If the intermolecular forces between molecules are: – relatively strong, the boiling point will be relatively high. – relatively weak, the boiling point will be relatively low.

Melting The process of going from a solid state to a liquid state. It occurs because of the temperature (heat). As heat is absorbed, the molecules vibrate more and more violently. If enough heat is absorbed, the attractive forces between the molecules will no longer be able to hold them together. The solid then melts. Regulation :Melting under pressure and freezing again when the pressure is reduced. Example: Cutting through a block of ice by hanging weights on the ends of a wire and putting it on the ice to hang. The pressure from the wire crushes the ice molecules causing them to melt. Once the wire is past that point, the molecules reform to freeze again. The wire and the weights go all the way through and you are left with a solid block of ice. Only a property of water.

Freezing The process through which a substance changes from a liquid to a solid. All liquids except helium undergo freezing when the temperature becomes sufficiently cold. Freezing- As energy is withdrawn from a liquid, molecular motion diminishes until finally the molecules, on average, are moving slow enough so that the attractive forces between them are able to cause cohesion. Cohesion means that water is attracted to other water. The oxygen end of water has a negative charge and the hydrogen end has a positive charge. The hydrogens of one water molecule are attracted to the oxygen from other water molecules. The molecules then vibrate about fixed positions and form a solid. An increase of pressure raises the freezing point. As a mixture freezes, the solid that forms first usually has a composition different from that of the liquid, and formation of the solid changes the composition of the remaining liquid, usually in a way that steadily lowers the freezing point. This principle is used in purifying mixtures, successive melting and freezing gradually separating the components. Example: Water changing to ice

Energy & Change of Phase Energy: that which can change the condition of matter. The ability to do work. The phase of matter depends on temperature and pressure exerted on it. Energy input is required for both liquefaction of a solid and vaporization of a liquid. Energy must be extracted from a substance to change its phase in the direction from gas to liquid to solid. Solid must absorb energy to melt and a liquid must absorb to vaporize. Gas must release energy to solidify and a liquid must release energy to solidify. Heat of vaporization is either the energy required to separate molecules from the liquid phase or the energy released when gas condenses to the liquid phase. Heat of fusion is either the energy needed to separate molecules from the solid phase or the energy released when bonds form in a liquid that change it to the solid phase. A refrigerator is a “heat pump.” It transfers heat out of a cold environment. When the process is reversed the heat pump is an air conditioner. In both cases, external energy operates the device.

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