Chemistry: classifying chemical and physical changes in various materials/substances

Transcription

1 Chemistry: classifying chemical and physical changes in various materials/substances Nikki Schilling, Ames, St. Paul Mn Based on original activity from Cool Chemistry Concoctions by Joe Rhatigan and Veronika Alice Gunter, p.24 Author Profile Summary In this classroom lab, students will investigate the characteristics of a chemical change. Students will also provide a definition of what constitutes a physical change through observation of several examples. Students will use/create the criteria for identifying a chemical change. Several examples of a chemical change will also be observed for students to compare to physical changes. The students will use the five characteristics that have been created to determine/judge whether or not baking soda, citric acid and water creates a chemical change. Students will write down observations of the reaction and use the five characteristics to explain why it is in fact a chemical reaction (The five characteristics will be listed for use). Learning Goals *Students will apply the five characteristics of a chemical change to real world reactions to determine the type of change that has occurred. *Students will be able to express why a given substance/mixture has had a chemical change. Concepts: -Physical change -Chemical change Key Vocabulary: -Carbon Dioxide Acid -Element Base -Chemical Citric Acid

2 Context for Use This activity would work well in 6th grade but could also be used in lower grades if needed. This would take place in the classroom, which would span 1-2 weeks depending on the number of examples and experiments you are willing to do with your students. This would follow a unit on structure of matter. This will provide students with knowledge in the physical properties of elements. This could even following unit on testing ph samples. It would provide students with an understanding of why acids and bases react to one another. The materials needed for this lesson would range from household materials to specialty items that can be found at health food stores. The desired skills for this lesson would require general knowledge of physical changes, using measuring instruments and writing observations. I would do each experiment in half an hour increments. Subject : Chemistry: General Chemistry Resource Type : Activities: Classroom Activity, Lab Activity Grade Level : Intermediate (3-5), Middle (6-8) Description and Teaching Materials Materials: -Paper -Matches -Vinegar -Milk -Citric acid -Baking soda -Water (ice) -Towel (can be paper) -Large & small plastic bags -Goggles -Lemon juice

3 -Red cabbage juice -Measuring cups or spoons In this activity I would start by comparing the difference between crumpling a piece of paper and burning a piece of paper. In this introduction students will make observations of what they have seen. First they would observe the paper being crumpled. Then their observations will be written on an observation chart. Then I would burn a small piece of paper and write observations down on the chart. I would ask this question, "Which of one was a physical change and why?" Students should be able to identify which one was a physical change by using what they already know about what qualifies as a physical change. We would have discussed why it was. From this point I would introduce the concept of a chemical change. The class would think, pair, share on why the burning paper was a chemical change. On a chart paper the students would list the characteristics of a physical change on one side. The other side would be the characteristics of a chemical change. I would help guide the students in clear characteristics. Ideally I would have the students come to five characteristics: chemical change has color change, temperature change, production of a solid, forms a gas, and change in odor. The students would use their definitions to provided them with evidence of physical and chemical changes while experimenting with the following items: Mixing vinegar and baking soda: 1-tablespoon baking soda and 20 ml of vinegar in a plastic bag (Have students put the vinegar in first then the baking soda, seal bag before mixing) Ice: On a tray observe a piece of ice melt

4 Water being soaked up into a towel: 50 ml of water and a paper towel Students will observe the water can be taken out of the towel Lemon juice and cabbage juice: 10 ml of cabbage juice and 10 ml of lemon juice It will change the colors from purple to pink Citric Acid, baking soda, and water: 1-tablespoon citric acid and 1-tablespoon baking soda in a large plastic zip lock bag Water filled halfway in a small plastic bag Place the unzipped bag with water into your large bag. Have students get as much air out of the bag before closing. Turn the bag so the water spills out. (The water will become cold and CO2 gas will fill the bag) For each experiment the students will make a hypothesis of whether it will be a physical or chemical change. Students will write and draw their observations down. They will use their observations and definitions of physical and chemical changes to decide if their hypothesis was true. After each experiment as a closer, we would have a quick share. What did we find out? Which characteristics were present in the experiment? Does anything surprise you?

5 Source Ref. Rhatigan, Joe & Gunter, Veronika Alice. Cool Chemistry Concoctions. New York: Lark, Teaching Notes and Tips I go over safety when dealing with citric acid. I make sure students understand it may look like sugar, but it is not. I would have the student wear goggles to prevent exposure to their eyes. Citric acid stings just like lemon juice. I remind students with cuts to be extra careful and if it does get into a cut to go wash it out. I also go over the information multiple times and have students repeat back the procedures to insure students have an understanding of expectations and concepts. Ideally I would have groups of two but with limited supplies I have groups of four. In their groups, jobs would be given so no one person would be doing all the work for the group. This activity is different from my past one because of the use of citric acid. The experiment with citric acid would be my last one done because my students could then use their prior knowledge to the vinegar and baking soda experiment to make a better prediction of what will happen. If you don't have time to do all the experiments, I have used the citric acid, baking soda and water to make dragon eyes. If you mix the baking soda and citric acid together in a cup and add about a half-teaspoon of baby oil to the mixture, it sticks together without changing. You can roll them into small balls and students can take them home. It will have the same effect as the experiment. You can even add food color if you like. Assessment I would give a pre- and post test to the students using the experiments. The quiz would describe the reaction and the students would identify what type of change using the characteristics of a physical and chemical change. During the week I would have students turn in their science notebook, which would have the entire student's hypothesizes, observations and conclusions from the experiments. The students will also have the option to write me questions in their notebooks and the class parking lot. I will go over topics that are still unclear using as many examples as possible.