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McREL Standards Activity


Conservation of Matter


Purpose:As a result of this activity, students will show that the mass of a substance does not change when it mixes with another substance; in addition, students will learn the basic methodology involved in scientific investigation.
Related Standard & Benchmarks:
Science
 Standard 8.Understands the structure and properties of matter
   Level II [Grade 3-5]
   Benchmark 2. Knows that the mass of a material remains constant whether it is together, in parts, or in a different state
Science
 Standard 12.Understands the nature of scientific inquiry
   Level II [Grade 3-5]
   Benchmark 3. Plans and conducts simple investigations (e.g., formulates a testable question, plans a fair test, makes systematic observations, develops logical conclusions)
Student Product:Informal science report
Material & Resources:
  • Warm tap water
  • Sugar or salt (about 1/2 measuring cup per pair of students)
  • Lightweight cups (paper or plastic, all the same kind)
  • Small, lightweight containers for holding several tablespoons of sugar or salt (e.g., weigh boats or small Dixie cups)
  • Measuring cups and spoons
  • Spoons or other utensils for stirring
  • Pan balance (see Note to Teacher)
Teacher's Note:If pan balances are not available, this activity could be modified for use with scales. Teachers may want to prepare an assignment sheet that would guide students through their record keeping and provide structure for note-taking.
Activity
Students should already have learned about the conservation of matter in class. Explain to students that this investigation is designed to test that law; outline the steps of the investigation.  Be sure to explain to the students how pan balances work and the importance of balancing scales before weighing items.

Students should work in pairs.  As they work, students should keep a record of what they do (e.g., write a few sentences describing what they did and the results after each step; draw diagrams representing the setup/outcome at each step).  When they are done, students should write a short paragraph explaining their results (i.e., they should use their knowledge of the conservation of matter to explain how, even though you cannot see the sugar or salt anymore, the combined weight is unchanged).  They could also mention any problems they may have had.

If possible, allow time at the end of class for students to investigate related questions on their own.  For example, they may be curious about whether or not there is a change in volume, whether or not it’s possible to "un-dissolve" the sugar or salt, and if there’s a limit on the amount of sugar or salt that can be dissolved in room temperature water.

Procedure:

  1. Each pair of students should label two cups for the water (e.g., "Water 1" and "Water 2") and two small containers for the sugar or salt (e.g., "Salt 1" and Salt 2").
  2. Students should measure out 1/2 cup of warm water into each of the cups; they should use a pan balance to compare their weights and add water to the lighter one until their weights are equal.
  3. Students should measure out two tablespoons of sugar or salt into each of the (empty) small containers; they should use the pan balance to compare their weights and add sugar or salt to the lighter one until their weights are equal.
  4. Students should place the first water cup and sugar or salt container on one side of the balance and the second ones on the other.  If they balanced them correctly in steps 2 and 3, they should be balanced now.
  5. Being careful not to spill any water, sugar, or salt, students should pour the second container of sugar or salt into the second water cup and stir to dissolve.  Be sure to leave the empty sugar or salt container on that part of the balance!
  6. Students should compare the weights of the unmixed substances to the mixed solution; if they have done things correctly and not spilled anything, the two sides should be balanced.