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These activities give students the opportunity to compare a physical and chemical change that both involve a change in color.
The purpose of conducting these activities together is to give students the opportunity to compare a physical and chemical change that both involve a change in color. Students often generalize that a color change is always an indicator of a chemical change. It is helpful to introduce them to the concept of expected and unexpected color changes. In other words, when red coloring is mixed with water, you expect the water to turn red, when you use a blue marker on paper, you expect the mark on the paper to be blue. These expected changes in color usually indicate a physical change. Unexpected changes in color such as red cabbage juice and ammonia turning green or bromphenol blue turning yellow in the presence of an acid are usually chemical in nature. It is important to reinforce that physical changes are reversible and new substances are not formed. Chemical changes in matter involve the formation of a new substance.
Students will use the Physical or Chemical Change? checklist as they conduct each investigation. It is helpful to make two copies of the checklist so they can compare observations from the two investigations.
In the first activity students mix colors and then use a process called chromatography to separate colors. Color particles (molecules) separate according to size and solubility. Mixing and separating colors is a physical change.
In the second activity, red cabbage juice is used as an indicator to determine if certain substances are acids or bases. The red cabbage juice acts as an acid/base indicator. Red cabbage juice turns different colors in the presence of acids and bases, creating a chemical color change.
1. Use Science Processes and Thinking Skills
5. Understand the Nature of Science
Invitation to Learn
Show students an uncapped dry erase marker and ask them what color of mark it will leave on the whiteboard. Make a mark on the whiteboard and point out that the color they expected appeared on the whiteboard. Repeat the same type of questioning using nail polish. Refer to or repeat the Alka-Seltzer bromphenol blue activity. Was the bromphenol blue and water solution turning yellow in the presence of an acid an expected or unexpected (surprise)color change?
Explain to students that will try to separate the colors that make up the candy shells. Ask, Do you think candy makers use a chemical or physical reaction to color candy shells? If a prompt is necessary, you can ask them to think of a time they may have touched candy with wet hands and what happened? The only materials required for this activity that were not used in Activity 1 are Skittles and table salt.
Hackling, M.W. (Mar2000). Using open investigation for improving scientific literacy. Professional development, 16.1, pp. 2-4.
One of the assessments in this activity involves a student conducting open investigation. In open investigations, it is the student who plans and conducts the investigation. The teacher sets the context and introduces the problem, but the students work in small groups to plan and conduct their own investigations. Open investigations can be scaffolded for students using questions, prompts, and/or report sheets. Report sheets, such as an Experiment Design sheet used in this activity help guide students through the decision making steps of an investigation and elicit from students information about their thinking and doing at each phase of the investigation. Open investigations are useful in gaining insight into students thinking and reasoning.
Marzano, R., Pickering, D., Pollock, J., (2001) Classroom instruction that works, Alexandria, VA.ASCD
This text covers multiple research based strategies for increasing student learning and achievement. Explicitly teaching similarities and differences in relation to what students are learning enhances their learning and ability to use knowledge. As encouraged in this text, these activities were designed to include both teacher and student directed opportunities to identify similarities and differences in chemical and physical changes. Research indicated that classifying is a highly effective form of comparing similarities and differences. Students use the Physical or Chemical Change? checklist to help guide them through the comparison process and eventually classify the reactions they observe.