Stephanie J. Sheehan
Liquids and Solids
Liquids and solids are states of matter, which correspond to the energy and movement of the molecules that make up the materials. A great introductory lesson that will help the students to understand this can be found at http://www.msm.cam.ac.uk/SeeK/solid.htm. This fun lesson will have students acting out the movement of the molecules for different states of matter
The students will learn initially that solids are objects or materials that have a definite shape and do not flow, nor take the shape of their container. A solid typically is a material in which the molecules are close together and usually arranged in a defined pattern. The molecules in a solid vibrate, but do not have space or energy to move around each other or from one place to another
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When you drop a solid, it remains the same shape. In order to change the shape of a solid, you would have to cut it or apply strong pressure. Some examples of solids that the students will examine are apples, cold butter, pickles, carrots, sweet potatoes, hard candies, chocolate bars, ice cubes, and hard boiled eggs, which can all be changed by slicing or cutting them. The students will explore this concept by dropping the objects and then cutting them into new shapes or observing as the teacher does this with a sharp knife. Some of the materials can be changed by applying pressure, or smashing them. The students might find it fun to change the shape of cold butter or chocolate bars by hitting them with a hammer. They should notice that although the solids change shape in these ways, they still do not flow under more gentle forces, like gravity. The students will also examine solids that are too small to easily change in these ways, such as individual grains of rice, sugar, and salt.
Students will initially learn that liquids flow and take the shape of the container they are in. Liquids can also be defined as materials in which the molecules are close together, but not arranged in a definite pattern, and in which the molecules move around and pass each other slowly. When energy is added to a solid material, usually in the form of heat, the molecules move more quickly than the vibrating molecules of a solid, and can spread out. This changes the solid to a liquid and allows the material to flow. Some examples of liquids that the students will examine are syrup, soda, vinegar, orange juice, and milk. The students will move them from one container into containers of various shapes to observe (and enjoy watching) as the liquids change shapes.
Changing Phases of Matter
A material may change from solid to liquid or gas to liquid if the temperature is changed. The temperature of a material determines its state, although different materials change state 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 and separate even more, causing the liquid to change to a gas
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. 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. For example, water will freeze and become solid at 0 degrees Celsius, but salt water freezes and becomes solid at -2 degrees Celsius
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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, the water would boil and begin evaporating at room temperature
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. In the classroom, the students would not be able to observe the effects of differences in air pressure, but could write stories and draw pictures to demonstrate that they understand the teacher's explanation.
Neither Liquid Nor Solid
Several substances defy classification as liquids or as solids, including certain types of solutions and suspensions. The students will create and examine some of these substances including simple suspensions, colloids, and gels, using materials found in the kitchen. They will then discuss and write about how these foods are different from typical solids and liquids.
Suspensions are mixtures between two substances, when a second material is divided into tiny particles and spread out thoroughly within the first substance
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. The first kind of suspension the students will examine is made up of solid particles mixed evenly into a liquid. This cannot easily be classified as liquid or solid, since the observer can detect both solid and liquid parts if looking closely. The particles are often possible to see with the naked eye, and are otherwise visible with the use of a microscope. The solid particles in this type of mixture can sometimes separate out if left to rest long enough and then settle, or if filtered out
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. Examples of common suspensions found in the kitchen are flour mixed in water, orange juice pulp dispersed in orange juice, and milk. If the first two were left to sit overnight, the solid flour particles and orange pulp particles would settle to the bottom of the containers.
One interesting type of suspension is a colloid. A colloid is a mixture that is not truly homogeneous, but appears homogeneous to the naked eye, and cannot be separated or filtered out easily. The particles and the substances they are spread within may be solid, liquid, or gas
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. For the purpose of this unit, the students will observe colloids in which a liquid is permanently or semi-permanently mixed within another liquid that would not normally mix with it to create a solution. One of the liquids will form tiny droplets that will be surrounded by the other. When the two normally immiscible liquids are forced to remain mixed as a suspension, it is called an emulsion. The liquids become inextricably mixed, or emulsified, by incorporating the second liquid a little at a time and/or with use of an emulsifying agent, such as egg yolks. The emulsifying agent is also called a surfactant. The molecules of a surfactant have one end that likes to be attached to the molecules of one liquid, such as oil, and one end that likes to be attached to the molecules of the other liquid, which is usually water
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. Examples of emulsions are mayonnaise and (homogenized) milk. The students will examine and discuss the texture of the mayonnaise emulsion, which they will make. They will see that it does not easily flow, but does not truly keep a definite shape either; the shape depends on how hard you push on it or how long you allow it to sit in its container.
Students will also examine another interesting substance called a gel. The students will create gels in which solid particles are evenly dispersed within a liquid. Examples of gels are gelatin and cheese
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. The students will make and examine a gelatin fruit mold. The prepared gelatin will have similar properties as the mayonnaise, since it neither flows easily nor keeps a definite shape. The gelatin comes closer to the definition of a solid, however, since it will take much longer to flow and will keep a more definite shape until harder pressure is applied. The students will compare and contrast these substances in writing and discussion.
Explanation of Teaching Methods
Literacy Component
Students will read, study and discuss information about states of matter, molecules, and liquids and solids. After the information has been discussed, reading comprehension questions will be modeled, discussed and answered in writing. Some of the reading will be teacher-created, as will the written response questions. This reading material will be on a level the students can read together with the teacher and/or peer support. The written response questions will match the literacy curriculum focus. The students will also read children's science books that pertain to the subject of matter, molecules, and physical changes to matter. A selection of books at the students reading level will be provided for them to read and respond to in writing. A selection of titles for these activities can be found in the Children's Classroom Reading Selections section. A sample written response activity sheet can be found in Appendix 2.
Experimentation
The students will sort and compare various solids, then liquids, and create observation logs that reflect what they notice and learn. The students will compare liquids to solids and try to make generalizations and define these properties. Students will cause changes in the states of matter through traditional experiments and cooking lessons, while utilizing the scientific method. This section will address the New Haven science performance standards for scientific inquiry numbers 1-7.
The Scientific Method
The Scientific Method is a system of steps by which scientists use knowledge and reason to figure things out
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. It consists of observation, questioning, hypothesis, experimentation, discerning the results, and finally discovering the applications of the experiment. Observation means examining information with all the senses and sometimes includes reading or listening to learn what other people have said about their observations. Sometimes observation requires special tools, such as magnifying lenses, microscopes, thermometers, measuring tapes, etc.
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The Scientific Method begins when students use background knowledge and observations to form a question that they want or need to find a scientific answer to. A scientific answer is an objective result that others can replicate and test for themselves, rather than relying on the thoughts and opinions of another person. Next, students use background knowledge to form a hypothesis, which is an educated guess, or the answer they think the experimentation will show. Then, it is important to carefully design (or find) an experiment that will help answer the question. The teacher and/or students devise a specific procedure, which must be written out and followed carefully. The students carefully conduct the experiment and analyze the data from the experiment. Analyzing the data involves observing exactly what happened and recording measurements, time, etc. Then, the experiment has to be examined for limitations, or factors other than the intended variable, that might have affected the results. For example, some experiments might be affected by factors that are difficult or impossible to control, such as the air in the room or the subjectivity of the observer. Therefore, a good experiment should be replicated to check for validity, and may need to be refined by changing the procedure slightly until uniform results can be achieved over several trials.
