This unit will help the students understand the scientific principles that affect matter. The students will use scientific inquiry, experiments, and observations to understand different states of matter. The experiments included in this unit will be designed to challenge and expand student comprehension by demonstrating the phase changes that occur around us.
Everything in our world is comprised of tiny particles called atoms. These atoms move around in different ways in solids, liquids and gases. In solids, the particles are packed so tightly together that they can barely move. This allows them to keep their shape. Solids have a definite shape, are more or less rigid and have a definite volume. A steel ball has a distinctive shape and unless force is applied, it retains that shape. The amount of space that the steel ball takes up also remains constant. The particles of a solid are tightly packed together so increasing pressure barely compresses the solid to a smaller volume.
1
Solids also have properties such as elasticity, hardness, malleability, brittleness, and tensile strength. Elasticity is the ability of a solid to regain its shape and size after having been stretched, compressed or squeezed, or otherwise distorted, a classic example is a rubber band; hardness is the capability of a solid to resist being scratched or dented, such as a diamond; malleability refers to the capability of a solid to be hammered into thin sheets or specific shapes, with the metals gold, iron, and aluminum as examples; brittleness is how easy it is to break a solid by bending it, such as glass or ceramics; tensile strength is an indicator of how much force is required to break a solid by stretching it. These sub-categories can help students learn to sort solids into more specific characteristic groupings.
In the liquid phase, the molecules of a substance can move around more than those in a solid. This allows them to change shape easily. The liquid molecules are not held in a regular arrangement, although they are still very close to each other. Liquids, like solids, cannot be compressed and have a definite volume but because they have just enough room to flow around each other. However, liquids have an indefinite shape: A liquid will change shape to conform to its container.
2
Liquids flow at different rates. Viscosity is the term used to measure the tendency for a liquid to resist flow. The tendency of a liquid to flow easily is called fluidity. Syrup is more viscous than water, and water is more fluid than syrup. Additional properties include surface tension and miscibility. Surface tension is the tendency of a liquid’s molecules to cohere to other which makes it hard to penetrate the surface of a liquid. Surface tension causes a falling drop to assume the shape of a ball, or allows an insect such as a water strider to walk on water. The ability of one liquid to mix with another is called miscibility.
Gas particles have a great deal of space between them, have high kinetic energy and can spread out indefinitely. If confined, a gas will expand to fill its container. Gas molecules have enough kinetic energy to overcome intermolecular forces that hold solids and liquids together, thus a gas has no definite volume and no definite shape.
3
Most gases are invisible to us, like the air we breathe.
Although this unit does not include investigation or understanding of a fourth state of matter, it bears mentioning to support a complete overview. Plasmas are a lot like gases, but the atoms are different because they are made up of free electrons and ions of an element such as neon (Ne). Natural plasmas are not found that often, but we are familiar with man-made plasmas most everywhere. Fluorescent light bulbs are an example of this state of matter. They are not like regular light bulbs – inside the long tube is a gas. Electricity flows through the tube and acts as an energy source, ionizing the gas. This charging and exciting of the atoms creates glowing plasma inside the bulb.
Another important concept to understand is that substances can change from one form to another when energy is either added or taken away. When water loses energy, it cools down. When liquid water reaches the freezing point (0 degrees Celsius or 32 degrees Fahrenheit), the particles stop moving and it turns into a solid – ice. When water is heated, adding energy, the molecules move faster and spread our farther. This is called evaporation. Clouds are made when water evaporates. However, when the air gets colder and loses energy, the gas, called water vapor, turns back into the liquid we call rain. This process is called condensation. On Earth, frozen water is always melting, liquid water is constantly evaporating to the sky, and water vapor is falling back down again as rain or snow. This process is called the water cycle. It is important to remember that matter can change from a solid to a liquid to a gas and still be the same matter.
4
A general way to present this process is to fill several ice cube trays with water and add a drop of food coloring to each cube the day before science class. Freeze the cubes until they're solid, and then place one in a zip-top bag, enough for each student to have one bag. Students can identify what state of matter the ice cubes show, and then tape the bags to your classroom window. Over the course of the day, have students observe what's happening in the bags. First, the ice cubes will change from a solid to a liquid, and then the liquid will start forming water vapor, which is a gas.
One way to show second-graders how gas -- in this case water vapor -- changes to a liquid by placing a glass of ice water on the desk. Over the next few minutes, the ice will cause water to condense on the sides of the glass. Show the process of liquid changing to a gas, called evaporation, by leaving the glass of water on the desk for several days. Have the students observe how the water level drops as passes. If it rains, your students can also witness evaporation in action by watching the puddles dry up as the sun comes out and the rain stops.
Changing Phases
Phase describes a physical state of matter. Things move from one phase to another by physical means, such as been mentioned, if energy is added (temperature increase) or taken away (temperature decrease), a physical change can be created.
Phase changes occur when certain temperatures are reached. Sometimes a liquid will transform into a solid, and sometimes it is the other way around. Scientists use something called a freezing point or melting point to measure the temperature at which a liquid turns into a solid, or the temperature where a solid turns into a liquid. There also are physical effects that can change the melting point. Pressure is one of those effects. When the pressure surrounding a substance increases, the freezing point and other special points also go up. It is easier to keep things solid when they are under greater pressure. Generally, solids are denser than liquids because their molecules are closer together. The freezing process compacts the molecules into a smaller space. Water is a bit of an exception. It is special on many levels. It has more space between its molecules when it is frozen. The molecules organize in a specific arrangement that takes up more space than when they are in the liquid state. Because the same number of molecules takes up more space, solid water is less dense than liquid water, and therefore ice cubes float.
A material may change from solid to liquid or gas to liquid if the temperature is increased. The temperature of a material determines its state, with different materials changing states at different temperatures. The temperature corresponds to the movement of the molecules within a material. The higher the temperature gets, the faster the molecules move. When enough heat is applied to the vibrating molecules of a solid, they vibrate faster, eventually moving farther apart and breaking the forces that hold them together. When this happens, the molecules move freely and the solid becomes a liquid. If enough additional heat energy is applied, the molecules move faster still and separate even more, causing the liquid to change to a gas.
5
The reverse will happen if heat is removed and a material is cooled to the temperature that causes its molecules to slow down. The temperature at which molecules slow down or speed up enough to change the state of a particular material depends upon the type of substance being heated or cooled. As noted earlier, water will freeze and become solid at 0 degrees Celsius, but ethylene glycol, an odorless, colorless, syrupy liquid used in antifreeze in cars and other engines, freezes at -13 degrees C.
6
A material may also change state if the amount of pressure exerted on the molecules changes. For example, in order for liquid water to become gas, or evaporate, the water molecules must move fast enough to move away from each other and to move into the air molecules above. Therefore, the air pressure has an effect on the water molecules. At sea level, a square inch column of air straight up to outer space weighs about 14.7 pounds. At sea level, this column contains a lot more air than it does at a point much higher up and closer to the atmosphere, such as the top of a tall mountain. Therefore, there is more air pressure pushing on the water molecules at sea level than there is on top of a mountain. With less pressure, the molecules are able to move more freely at lower temperatures. In fact, at 90,000 feet above sea level, water would boil at room temperature.
7
When a solid become a gas, the process is called sublimation. The most common example of sublimation is dry ice. Dry ice is solid carbon dioxide (CO
2
). When dry ice is left out in a room, it just turns into a gas. Coal is another example of a compound that will not melt at normal atmospheric pressures. It will sublimate at very high temperatures.
Deposition occurs when a gas becomes a solid without going through the liquid state of matter (this is the opposite of sublimation). In this area of the country, there is often frost on winter mornings. Those frost crystals on plants build up when water vapor from the air becomes a solid on the leaves of plants.
8
The list below summarizes these processes:
|
Fusion/Melting
|
Solid to Liquid
|
|
Freezing
|
Liquid to Solid
|
|
Vaporization/Boiling
|
Liquid to Gas
|
|
Condensation
|
Gas to Liquid
|
|
Sublimation
|
Solid to Gas
|
|
Deposition
|
Gas to Solid
|
When studying matter, both physical and chemical properties need to be examined. Physical properties can be observed without altering the composition of matter – color, shape and tendency to flow are examples of physical properties. By contrast, chemical properties can be observed only when the chemical composition of matter is changed – for example, iron and oxygen interact, rusting occurs and the chemical properties of the iron are changed.
Density is a measure of how tightly things are packed together. It is given by the mass found in a certain amount of space or volume (D=m/V). Imagine a solid cube that is one centimeter tall, one centimeter wide, and one centimeter long. If that cube is filled with Styrofoam, it is light. If it is filled with lead (Pb), it is heavy. The lead is heavier because it has a higher density than Styrofoam.
Investigations, discussions and questions to incorporate with instruction as the students develop understanding: Why do ice and wood float, but rocks and metals sink? Why do helium balloons and hot air balloons float, but normal balloons that you blow up yourself do not? This could also lead to the distinction between mass (or weight) and density. Low density objects like wood fall just as fast as high density objects such as rocks which weigh more (feathers and Styrofoam fall more slowly due to air resistance and not due to their low density).
