Entries in matter (7)
Students have a major misconception that mass determines whether an object floats or sinks. This is an activity that forces students to deal with that misconception and hopefully break it. The following materials are needed:
Have the students read the statement, "Objects that are high in mass, sink in water." Have the students show in some way if they agree with the statement, disagree with the statement, or are somewhat in between. Have them explain why they think that way and give examples. Then present the students with the list of all of the items to be placed into the tank, arranged according to their masses. Do not show the objects to the students. Have them write a hypothesis for each item (on whether they will sink or float) based only the property of mass. Then drop each item one by one, in order of their masses, into the water. Have students record the results. The students will realize that mass has nothing to do with whether an object sinks or floats. Then show the students the objects' densities. Hopefully they see the pattern of those objects with densities less than one float, and those with densities greater than one sink. It is not important to explain what density is at that point, but just that it is a property of matter that is vital in determining if an object floats or sinks.
A major standard our students have to learn is the difference between elements and compounds. Compounds are substances that are made of two or more elements bonded together. The elements that comprise a compound have different properties when they are bonded together than when they are separated. Students have great difficulty with this. I introduce the unit by doing a simple activity on mixtures. Educational Innovations sells a simple Mixture Separation Challenge kit. Students are first forced to separate the mixture into 3 groups. Students usually use the simplest property of color to do it. There are other properties such as relative density or optical properties (opaqueness, transparency, etc.). The main idea is that the substances can be separated by their properties, because those properties do not change when forming a mixture, unlike when a compound is formed.
I found a new way to teach the conservation of mass/matter this past year. Previously I taught it by having steel wool (iron) react with oxygen. Since our periods are shorter this year, I couldn't really do that reaction anymore. Now students mix heavy whipping cream and vinegar in an open system. The vinegar reacts with the casein proteins in the heavy whipping cream to form a solid cheese-like substance (basically it is cheese). It's not a very glamourous reaction, but is also a way to teach students about precipitation. Students have a difficult time understanding the concept of two liquids reacting to make a solid. This is a simple way for students to visualize that type of chemical change.
Students in my classes have to determine if a substance has changed physically or chemically. In order to do that, they need to know if the properties of a substance has changed. We can look at many different properties of a substance such as color, density, boiling point, melting point, taste, texture, hardness, etc. One of the most exciting properties of matter is the color in which they burn. In the video above I show color flame candles and then show a demonstration of two different compounds, strontium chloride and copper sulfate, mixed with denatured alcohol, that produce large colorful flames.