Core Lab: Energy Changes During Melting and Evaporation

Transcription

1 Science 1206 Name: Core Lab: Energy Changes During Melting and Evaporation Background: When a substance melts, it changes from a solid to a liquid. When a substance evaporates (or vaporizes), it changes from a liquid to a gas. Heat energy is needed to cause these phase changes and the energy taken in by the substance during melting and evaporation is known as Latent Heat. Latent Heat of Fusion & Vaporization: Energy Needed for Phase Change Between Solid, Liquid, & Gas The latent heats of fusion and vaporization tell us how much energy is needed for transitions between the liquid and solid phase or between gas and liquid phases. Scientists use latent heats to find how much energy is required for something to change from one phase to another. The common phases (states) of matter are solid, liquid, and gas. Water freezing to ice or evaporating to steam are examples of changing phase. When any material changes between liquid and solid or between liquid and gas, it is changing phase. These phase changes occur at specific temperatures. Transitions between the solid and liquid phase occur at either the freezing or melting point temperatures. They are usually the same temperature. Transitions between the liquid and gas phases also occur at a specific temperature known as the boiling point. Latent Heats: The energy required for a kilogram of some type of material to change its phase is called the latent heat. Latent heats are usually measured in joules per kilogram. To find the total energy required for a phase change, multiply the mass of the object changing phase by the appropriate latent heat. Many textbooks have tables of latent heats for various substances, which are measured experimentally. The latent heat of fusion refers to transitions between the liquid and solid phase. The latent heat of vaporization refers to transitions between the liquid and gas phase

2 Introduction: A beaker of crushed ice has been heated on a hot plate. The ice melts to water which eventually boils. Students have read and recorded the temperature at one minute intervals (See the data in table #1) The purpose of the lab is to help students understand energy transformations associated with phase changes. Hypothesis: State a hypothesis in the space below. Purpose: To investigate the amount of heat put into a system, to the energy needed to bring about a change of phase, that is, melting and evaporation. Materials: 500 ml beaker graph paper hot plate thermometer ring stand crushed ice Procedure: Set up the materials so that a beaker of crushed ice is placed on a hot plate. Turn the hot plate to its high setting. Using a thermometer to gently stir the crushed ice, read and record the temperature at one minute intervals and record this data in table #1. Temperature readings should be recorded until the water starts to boil, at this point record three additional readings and then stop to analyze your data. Construct a line graph using the data recorded in Table #1. Place time on the horizontal axis and temperature on the vertical axis. (This has been supplied for you)

3 Observations / Data (Table #1) Time (min) Temperature ( C)

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5 Discussion: 1. Explain what happens during the time when the temperature being recorded is at; i) 0 C; and ;ii) 100 C 2. When did the greatest temperature change occur? What phase was the material during this temperature change?

6 3. Discuss the shape of your graph with respect to the slope of the line at different points along the graph. Explain why the graph displays different slopes.

7 Science 1206 Post lab Handout Core Lab: Energy Changes During Melting and Evaporation Your graph should be similar to the one above. As you can see there are periods during the graph where as time passes heat is being added but the temperature doesn t rise; ``solid melts`` and ``liquid boils``. Remember latent means hidden. As this heat is being added this energy goes into changing the phase of the substance. This energy can be considered as stored. When at a later time the phase change is reversed, this same energy is released.