Chemical Changes

(a 6-8 Matter lesson plan)

From the unit: How does a hot air balloon work?

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Abstract
Students observe demonstrations in order to understand chemical reactions.
Standards and Benchmarks
AAAS Benchmarks
  • Atoms and molecules are perpetually in motion. Increased temperature means greater average energy, so most substances expand when heated. In solids, the atoms are closely locked in position and can only vibrate. In liquids, the atoms or molecules have higher energy, are more loosely connected, and can slide past one another; some molecules may get enough energy to escape into a gas. In gases, the atoms or molecules have still more energy and are free of one another except during occasional collisions.
  • There are groups of elements that have similar properties, including highly reactive metals, less-reactive metals, highly reactive nonmetals (such as chlorine, fluorine, and oxygen), and some almost completely nonreactive gases (such as helium and neon). An especially important kind of reaction between substances involves combination of oxygen with something else -- as in burning or rusting. Some elements don't fit into any of the categories; among them are carbon and hydrogen, essential elements of living matter.
Objectives
  • Students will be able to make observations of chemical changes.
  • Students will be able to draw conclusions about the characteristics of chemical changes based on the patterns they detect in the data.
  • Students will understand the characteristics of chemical changes.
Class Time Needed
One forty five minute period
Materials
  • Hot plate
  • Sugar
  • Pan (to heat sugar)
  • Milk
  • Vinegar
  • Alka Seltzer
  • Water
Science Background

What types of changes take place inside the burner to produce the fire?
Most of the hot air balloons used today heat the air by burning propane. The propane is kept in a canister. This canister has: propane in a highly compressed form, hoses, and heating coil. The propane first flows through the hoses to the heating coil. When the balloonist starts the burner, the propane flows out in liquid form and is ignited by the pilot light. As this flame burns it heats up the metal in the heating coil. This heats the propane flowing through the coil, which causes a change in the propane from a liquid to a gas. The gas is ignited and makes a more powerful flame.

Physical and chemical changes are an important part of this process.

Physical changes occur when energy is either added or taken away from an object. Phase changes are examples of physical changes. When heat energy is added to water, for example, the molecules move faster and further apart. They eventually move so fast and so far apart that they escape the water in the gaseous state (steam). This is an example of evaporation: liquid changing to a gas.

No fundamentally new substances are made when a physical change occurs. For example, the steam can be changed back into the liquid form of water.

Chemical changes occur when the atoms of substances are rearranged. Fundamentally new substances are produced when a chemical change occurs. An example of a chemical change is a car rusting. A car rusts because the metal is being exposed to oxygen and water in the air. These products combine chemically to form rust. Rust has different properties from the oxygen and water, thus it is a fundamentally new substance.

Source: How Stuff Works, http://www.howstuffworks.com/hot-air-balloon1.htm

Students' Alternative Ideas

Physical Changes

Alternative idea: Some students may believe that when water evaporates it ceases to exist, changes location but remains a liquid, or that it is transformed into some other perceptible form such as fog or steam.

Scientific idea: When water evaporates the molecules that make up the water spread further apart. Eventually they are so far apart that the water is in its gaseous form and unable to be seen.

Dealing with the alternative idea: The Physical Changes Lesson Plan focuses on the physical changes that substances undergo such as evaporation. Students should be encouraged to think about things in the world around them that undergo physical changes. They should understand that when substances change physically the original substance can always be retrieved.


Chemical Changes

Alternative idea: Some students may not view chemical changes as interactions between the atoms of different substances. Instead they see a chemical change as the result of a separate change in the original substance, or changes, each one separate, in several original substances. For example, some students believe that the smoke being emitted from wood after it is burned has been driven out of the wood by the flame.

Scientific idea: Chemical changes are different from physical changes. Chemical changes occur when two or more substances interact with each other. During these interactions atoms are rearranged causing new substances to form.

Dealing with the alternative idea: The Chemical Changes Lesson Plan will help students understand that these types of changes are due to a rearrangement in the atoms of different substances.


Energy

Alternative idea: Many students use the term energy differently than scientists do. Many think that energy is associated only with humans or movement, can be used up like a fuel, or is something that makes things happen and is given off in the process. Students usually do not believe that energy is measurable and quantifiable.

Scientific idea: Many students are confused by the term energy because most people use the term differently than scientists. For example, students may have heard an adult say, I have no energy to do anything today! Therefore the students believe energy is associated with little movement. This is, of course, not the case. Energy is defined as the ability to do work.

Dealing with the alternative idea: The Energy Transformations Lesson Plan (note: this will take you to another unit) will help students understand how energy is transferred between objects. This will assist them in seeing that the term energy can be applied in many different contexts (and not just to humans) and that it cannot be used up.

Description
  1. Ask students the following questions: Why do cars rust? Once they have rusted, is there anything you can do to get the original metal back? This is an opportunity for students to share what they think and pique their curiosity about the day's lesson. You should not be correcting any alternative ideas at this point.
  2. [?] Why should my students ask and answer questions in science?
  3. [?] How can I help my students ask and answer questions in science?
  4. Explain the task to students: The teacher will be performing several different demonstrations. It is the students' responsibility to write their observations of each of the demonstrations in their science journals. Review with students what makes a good observation: A very descriptive explanation of the object or phenomena (pictures can be used also)- No inferences
  5. Perform the demonstrations. Note: In parentheses is an explanation of what happens to the substances when they are put together. These are NOT observations.
    • Heating sugar (it turns to caramel)
    • Putting together milk and vinegar (it curdles)
    • Putting together Alka Seltzer and water (it fizzes)
  6. [?] Why should students collect evidence to answer questions?
  7. [?] How can I help my students collect evidence?
  8. Place students in pairs. Have them examine their data looking for trends or patterns. What did all of the demonstrations have in common?
  9. Tell students that all of the above demonstrations involved chemical reactions. Ask students whether or not they can develop a list of things that might indicate that a chemical reaction has occurred. These should include:
    • something new is produced
    • the new substance(s) have new properties
    • the original substances cannot be retrieved
  10. Tell students that the process of rusting is a chemical reaction. Ask students: why do some businesses say they can get rid of the rust on cars? How can this be true if an indicator of a chemical reaction is that the original substances cannot be retrieved?
  11. [?] Why should students communicate and justify their findings?
  12. [?] How can I help my students communicate and justify their findings?
Assessment
Have the students explain in their science journals whether or not they think chemical reactions have occurred in the following scenarios. It is important that students give explanations for their answers.
  • salt dissolving in water (physical)
  • a piece of wood on fire (chemical)
  • an ice cube melting (physical)
  • an apple rotting (chemical)
  • a tree growing (chemical)
Images of Inquiry
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This lesson focuses on Explanations & Evidence. (more)

How Eli taught this lesson
Eli usually struggles with lessons where he needs to facilitate a discussion where students share ideas while he guides them to a pre-determined set of concepts. He finds it hard to build on their ideas while guiding them towards his own. Eli also knows that his students might have trouble finding patterns in their observations. While students are working in groups, Eli walks around the room and looks at the observations his students wrote. He jots down Kelly's name because she wrote that "the sugar totally changed - it went from white and grainy to brown and liquid." Eli thoughts this would be helpful for all the students to consider, so he made sure to call on Kelly during the whole class discussion. After he wrote her observation on the board, he added "the new substance had new properties." This helped students think in scientific terms while still valuing their own words. It also helped Eli build from his students' ideas as he led them toward generalizing what makes a chemical change.

Author(s): CASES Team

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