Phase Change Teacher Guide
Physical Science
MS
Latent Heat
Melting Ice
States of Matter
Please check that the computers you plan to use with students run properly ahead of time. Try the activities in advance.
Because this activity uses a model, students can work individually if enough computers are available, or in pairs. Plan ahead to reserve computers and arrange a layout in the classroom for computers and cords.
This activity uses the temperature probe for which students working in pairs is best. Check that the setup is safe. The sensor can cause tall containers to fall over. If possible use plastic beakers with a spout. The sensor can be placed in the spout to balance its handle. The whole experiment can be done on cafeteria trays to minimize spills. Students should wear goggles.
Once again this activity uses a model, so, individual or pair work is possible. Plan ahead to reserve computers and arrange a layout in the classroom for computers and cords.
Share how the models work and how to create a snapshot and use the drawing tools before students begin working, if you think your students will struggle with the directions. Limit the time at each section, if needed. Students MUST read the directions to be successful. Note: In the Latent Heat model, the redder the particles, the higher the kinetic energy.
The ice cube may take 2 to 5 or more minutes to melt, so have additional questions to keep students focused during this time. To save time, pre-measure the salt in small cups. In the Data 3 section, you will need probes placed in the freezer for each group. This section might be easier to perform as a demonstration due to the number of probes required.
Each activity can be completed in a 45 minutes class period if students are familiar with the models and how to use the sensors. If this is not the case, allow another class period. A pre-lab day could be done before the unit to introduce students to the site, look at models, practice with the sensor, and how to take snapshots. Students can go back to the site the next day and finish the activity.
This unit is motivated by the discovery questions:
Students will investigate the three primary states or phases of matter — solid, liquid, and gas — at the atomic or molecular level, and how matter changes from one phase to another. In the activity "Latent Heat", students will use models to investigate patterns of temperature change at the point where phase change occurs. The next activity, "Melting Ice", has students use a temperature sensor to investigate different variables to see if the melting point changes. In the final activity, "States of Matter", students use models to investigate forces and attractions at the molecular level in each state.
Misconceptions about phase change still exist at the middle school level. For the transition from liquid to gas, students may continue have trouble identifying the air as the final location of evaporating water. They may also be challenged by the idea that all materials behave in the same manor, passing through the same phases, though the amount of energy required for phase change varies for different materials. For example, they may have seen solid iron and liquid iron, but not accept the idea that with enough energy (heat) the iron will become a gas.
Students of all ages show a wide range of beliefs about the nature and behavior or particles. They may lack an appreciation of the very small size of particles; believe there must be something in the space between particles; have difficulty in appreciating the intrinsic motion of particles in solids, liquids and gases; and have problems in conceptualizing forces between particles.
What is kinetic energy?
Kinetic energy is the energy of motion that depends on the objects speed and mass. The faster an object moves, the more kinetic energy it has. For a given speed, the more massive an object is, the more kinetic energy it has.
What are the three common (primary) states of matter?
The three common states are solid, liquid and gas (plasma and BEC discussed in additional background).
What is the freezing point of water?
The freezing point of water is 0° C (32° F).
What are the different scales used to measure temperature?
Celsius which most of the world uses and that we usually use in science, Fahrenheit which is commonly used in the US, Kelvin is a scale that starts at absolute zero or 0 K where all atomic movement would stop (for comparison 0° C = 32° F = 273.16 K, the model mentions 300 K which is room temperature).
Which section of the graph that has zero slope (flat horizontal line)?
When the ice is melting (it is changing state or phase) The average kinetic energy of the molecules remains the same as the inter-atomic bonds are broken.
What happens to the temperature as ice melts?
The temperature remains the same since the energy is used to break the inter-atomic forces.
For a single material, in which state of matter are the molecules moving the fastest?
In the gas phase molecules have enough energy to completely break away from each other and move further apart.
Why do the molecules move faster as the temperature increases?
Temperature is a measure of the motion of the molecules, as the temperature increases so does the movement of particles. This represents an increase in kinetic energy. As the temperature increase the atoms jiggle around more vigorously. The higher the temperature, the more thermal energy atoms have and the faster they move around. Thermal energy is transferred to kinetic energy.
Why is there a line flat (no slope) when the ice is melting?
All the heat energy added is being used to break the inter-molecular forces and change the solid to a liquid. Once all the ice has melted, the temperature will increase.
Why is steam more dangerous than boiling water?
Steam has more kinetic energy and thus hotter than boiling water. It is also invisible.
There are 4 common states of matter. Plasma is ionized gas that has enough energy that some of the electrons are free to travel and is the most common state in the universe (an example is the sun), though when introducing states of matter to students we usually focus on solids, liquids, and gases. Solids are formed when the attractive forces between individual molecules are greater than the energy causing them to move apart. Individual molecules are locked in position, and cannot move past one another. The atoms or molecules of solids are always in motion. When the temperature of a solid is increased, the solid retains its shape. In liquids, molecules can move past one another and bump into other molecules; however, they remain relatively close to each other like solids. As the temperature of a liquid is increased, movement of individual molecules increases. As a result, liquids can flow to take the shape of their container. Thus liquids have an undefined shape, but a defined volume. Gases are formed when the energy in the system exceeds all of the attractive forces between molecules. Thus gas molecules have little interaction with each other beyond occasionally bumping into one another. In the gas state, molecules move quickly and are free to move in any direction. As the temperature of a gas increases, the amount of movement of individual molecules increases.
In a gas, particles are not bound together, and are free to move. Particles are far apart and there is free space between them with no real arrangement of particles. The particles assume the shape of a container. The particles vibrate, rotate, are move at high speeds.
In a liquid, particles are free to move relative to each other, have no arrangement of particles have little free space between them, and assume the shape of a container; liquid particles vibrate, rotate, bump, and slide past each other.
In a solid, particles form an ordered arrangement, and do not flow. They have little free space between them, and vibrate in position.
"There are five main states of matter. Solids, liquids, gases, plasmas, and Bose-Einstein condensates (BEC) are all different states of matter. Each of these states is also known as a phase. Elements and compounds can move from one phase to another when specific physical conditions are present. One example is temperature. When the temperature of a system goes up, the matter in the system becomes more excited and active. Scientists say that it moves to a higher energy state. Generally, as the temperature rises, matter moves to a more active state." (American Chemical Society http://www.middleschoolchemistry.com/)
Inter-molecular forces are not as strong as chemical bonds between atoms, but are strong enough to attach neighboring molecules to each other. These forces have different strengths depending on the material. A solid like copper has stronger forces between its particles compared to a liquid like water (both at room temperature). (From: http://www.chem4kids.com/files/matter_states.html)
In the first model, the energy added to the system is absorbed by the atoms and used to break the inter-atomic interactions so that they can leave the condensed state. The kinetic energy does not increase as the potential energy increases. Why?
The Kinetic energy is being used to separate the molecules as it changes state. All the heat energy added is being used to break the inter-molecular forces and change the solid to a liquid. Once all the ice has melted, the temperature will increase. The energy is transferred to change the molecules arrangement, increasing the potential energy as it changes state. The thermal energy being transferred is used to separate molecules so it is not turning into kinetic.
Notice that the Temperature-Total energy graph has fluctuations. Why do you think this is so?
It depends which particle area is being measured at that moment. Most of the molecules are moving at about the same speed and have about the same kinetic energy, but there are always some that are moving slower and some that are moving faster. The temperature is actually a combination, or average, of the kinetic energy of the molecules. If you could place a probe in this animation, it would be struck by molecules going at different speeds so it would register the average kinetic energy of the molecules.
When water cools from 2°C to -2°C, what happens to the motion of the molecules?
The molecular motion decreases as thermal energy is removed. The molecules become locked into a regular pattern as the water passes from the liquid to the solid phase.
When you get out of the water after swimming, you often feel cold as water evaporates off of your skin. That means you must be losing heat energy. As the water evaporates, it is changing from a liquid to a gas. Explain why the process of evaporation should use up heat from your skin.
Heat must flow to cause the molecules to change state. It takes energy to change the state of the water (evaporate it). This energy comes from your skin, you transfer thermal energy from your skin, to the water thus increasing the internal energy of the water and it evaporates. Heat must flow to cause the molecules to change state.
When the ice is first put into the water?
The line has a negative slope (angles down or diagonally down)
While the ice is melting?
The line is flat - horizontal.
After the ice melts completely?
The line starts to slope upward (diagonally up).
Is the temperature of the ice the same as the temperature of the water around it?
No, thermal energy is transferring from the liquid water to melt the ice .The ice should technically be colder until it starts to melt although this might be hard to measure.
Is the ice ever colder than 0°C?
Before it was placed in the water, the temperature of the ice depends can certainly be colder than 0°C, and will depend on the temperature of the freezer in which the ice was made. Once in the water, the ice's temperature would increase reaching 0°C before it melts.
What effect does adding salt have?
Adding salt lowers the temperature of the water. Why does the temperature drop? Energy is required to break the hydrogen bonds that hold the ice together. The melting ice draws that energy from the surrounding solution as heat. Adding salt to the ice/water mix causes a temperature drop that slows the melting rate and increases the freezing rate. The net result is that the ice melts more and more slowly after the initial addition of salt.
Describe the motion of atoms and molecules in a gas.
Particles are free to move and move faster than in liquids or solids of the same material. There is little interaction between particles.
How far did the atoms in a liquid appear to travel?
Not very far apart compared to a gas's particles, but further than in a solid.
How would you describe the movement and arrangement of atoms and molecules in a solid?
A solid's particles are always in motion; they vibrate and bump into each other, but always stay with the same neighbors nearby.
Students can test different liquids' evaporation rates. Place a cotton ball on a thermometer or temperature probe and moisten with several drops of different liquids (water, nail polish remover, perfume, rubbing alcohol). Collect temperature data at regular intervals for for 5 minutes. Throw out the cotton ball and repeat with another liquid. Compare graphs.
Students can collect temperature readings when different solvents (sugar, rock salt, Kool Aid) are added to the ice solution.
Students can design and conduct investigations in which they collect temperature readings as ice melts and water boils.
Students can design and conduct investigations with a heat or cold pack.