Engage (10 minutes):
Show students a piece of paper and say there are many ways to change it. Some involve physical changes (rip paper into pieces) that change the form of the paper without changing it into a different substance, and some involve chemical changes (burn one of the pieces over a metal bowl) where the particles of paper change into different substances (smoke and ash).
Ask the essential question: When matter changes, is the total amount of matter increased, decreased, or does it stay the same? Today we will experiment to find out.
Have each student write his name on a yellow sticky note and post it on the class chart to make a bar graph of the students’ predictions. (View a sample graph here. Students will have the opportunity to change their answers based on evidence from their investigations at the end of the lesson.)
Explore (20 minutes):
Have students use the chart on the note-taking guide or create a chart in their notebooks with columns for Material, Weight Before Change, Type of Change, Prediction, Weight After Change, and How Weight Changed. Have students work in groups of four to collect materials and make changes.
Materials needed for each group:
- Small (6” - 10” square) piece of cardboard
- Ice cube in a Ziploc bag
- One paper or plastic cup filled halfway with water
- Sugar packet
- 10 – 12 linking cubes or Lego bricks
- Balance
- Gram weights
Within their groups, have students discuss different ways they could change each of the materials. For example, they can let the ice in the plastic bag melt or hit it with a hammer to break it into pieces. They can rip or fold the cardboard. They can assemble the cubes or Legos to make many different structures.
Have each group predict whether changes made to the materials will increase, decrease, or not affect the total weight of the matter. (Students may also predict that different changes will affect the total weight in different ways.)
You may allow students to choose which changes they want to make with each type of matter, or you may guide them to make the following physical changes:
- Melting ice
- Tearing cardboard
- Building cube or Lego structure
- Dissolving sugar in water
As students investigate, circulate to ask them how they are making sure they are weighing all the matter involved in the original change and none from outside the change. For example, students will weigh the sugar packet and cup of water before they dissolve the sugar in water. After the sugar is dissolved, the final weight must include the weight of the cup with the solution and the empty sugar packet. Since the paper packet has some weight that is included before the change, it must also be included as part of the system after the change. Condensation that appears on the outside of the bag of melted ice should be wiped off before weighing the bag of water since this water comes from the air and not the ice cube.
Students should record information in their data tables as they investigate. After completing the four changes, have students make generalizations about the weight changes during changes in matter (the total weight does not change). Introduce term conserved (which means the total quantity stays the same). Have students write down this definition and use the word in a sentence to explain how this term applies to their investigations (The total weight was conserved during all the changes.)
Explain (30 minutes):
Have students weigh the different pieces in one of the part-to-whole changes (the ripped cardboard or the individual Lego blocks or cubes.) Ask students how the weight of these parts relates to the weight of the whole piece of cardboard or block structure. (The total weight of a system is equal to the sum of the parts of the whole.) Have students work together in their groups to answer these questions:
- How can we represent this as an equation? Weight of Part 1 + Weight of Part 2 + Weight of Part 3 + Weight of Part 4 = Total Weight After Change
- How can we show this in a bar graph? Model how to construct a bar graph in which each the measurements for each part are stacked to show the total weight. (Click here for a sample graph of the part-to-whole relationship.)
Most students will understand that these physical changes do not change the total amount of matter present, but is the same true for chemical changes?
Mix vinegar and baking soda in a water bottle and have students observe what happens. (It fizzes.) Have students explain what causes the bubbles. (As the two substances mix, a gas is released.) Ask students whether they think the total weight of the matter changed in this reaction. Tell them they will investigate to confirm or refute their predictions.
Explain the procedure: In order to figure out whether the total amount and weight of matter changes during a change, we must have a closed system. A system is a group of parts that works together to form a whole. Refer students to the burning paper demonstration. What were the parts of this system? (The paper, the lighter, and the bowl.) How did these parts work together to make a change? (The lighter made a flame that ignited the paper, and the bowl caught the ashes the paper burned.) Was this a closed system? (No, because the smoke and gases released went into the air; it was not enclosed in anything.) Why do you think it is important for the system to be closed when we are trying to find out if the total weight changes? (If some of the matter escapes into the air, then we can’t measure it all.)
Ask students: How can we create a closed system to include the gas released by the vinegar and baking soda reaction? (Put a balloon over the top of the bottle to trap the gasses.)
Model how to mix baking soda and vinegar to capture the gas released by the reaction. Measure the weight of the balloon, and then pour one teaspoon (5 mL) of baking soda into a balloon using a funnel. Measure the weight of the balloon filled with baking soda. Ask students how we could figure out the weight of the baking soda. Why don’t we just weigh the baking soda by itself? (It would make a mess; we might spill some of it when pouring it in the balloon and then our measurements would not be accurate.)
Measure ¼ cup (approx. 120 mL) of vinegar and pour it into the water bottle using the funnel. Measure the weight of the water bottle and vinegar. Carefully stretch the open end of the balloon around the top of the water bottle, making sure to hold the balloon so no baking soda falls into the bottle. Measure the weight of the entire system. Write the weight of the total system as sum of the parts (weight of water bottle with containing vinegar + weight of balloon containing baking soda = weight of total system.) Do you think the weight will remain the same once you lift the balloon to mix the baking soda falls into the vinegar? Have students test their predictions in small groups and discuss the following questions:
- Is more matter present now than before we mixed the vinegar and baking soda? Why?
- Has the weight of the matter (which includes the bottle, balloon, and all contents) changed? Why not?
- What generalizations can we make about changes in weight during changes in matter?
- What do you think would happen to the weight of the water bottle and baking soda if we did not trap the gas released in a balloon?
Have students make bar graphs to show the sum of the weight of the parts of the vinegar and baking soda bottle system equals the total weight of the system both before and after the reaction. (See sample.)
Elaborate (30 minutes):
Have students plan their own experiment to test the weight and graph the changes in another chemical reaction: mixing an effervescent antacid tablet with water in a plastic bag. Students can reuse the bag of water from the melted ice cube. They should measure the weight of the tablet and the bag of water, noting the weight of each of these parts before allowing them to mix. They should hold the tablet in the corner of the bag and zip it closed. Once the bag has been sealed, students can drop the tablet into the water and observe the bag inflating due to the gasses released during the reaction. Have them measure the final weight of the bag and all its contents following the reaction. Students should graph the results and use their data as evidence to support the claim that matter is not created or destroyed in a closed system, so the total weight of the reactants stays the same.
