In Grade 4 Curriculum:

3. Liquid Materials:  Investigation 3.3

How do oil and water compare?

4. Make meaning

Discussion 10 Mins
Goals for Productive Discussions and Nine Talk Moves

Use this checklist to plan and reflect.

Available online at inquiryproject.terc.edu

Purpose of the discussion

The purpose of the discussion is for students to use the data they've collected to make and support claims about the investigation question. Return to the investigation question for discussion.

Engage students in the focus question

Now we can see the data more clearly. What claims can we make that the data support?

How do oil and water compare?

  • Look carefully at the differences in weights and in volumes. What do you notice?
  • If the weights are the same, oil has greater volume than water in every case.
  • The heavier the samples, the greater the difference between the volumes.
  • As the weight doubles, the volume difference doubles.

Supporting questions

Note:  The relationship between a material's weight and its volume stays the same no matter how big or small the sample is. It is a property of the material called density. Density is a term we are not introducing to students in this curriculum.
  • What do you think the volume difference for oil and water would be if you had 160 grams of each liquid?
  • How do you think the volumes would compare if you had 5 grams of each? Or just 1 gram of each? Or if the samples weighed tons and tons
  • If instead of starting with equal weights you started with equal volumes: how do you think the weights would compare?
  • If the volumes are the same, oil will weigh less than water.
  • What property of oil or water do you think explains why equal weights of oil and water have different volumes?

The class knows this property from earlier investigations as "heavy for size." We can also say "heavy for volume." Water is heavy for its volume compared with oil, just as sand is heavy for its volume compared with soil.

Summarize the discussion and recap the investigation

Use the same language that students have used to summarize the discussion. Include student comments that address claims and evidence about how oil and water compare.

As you recap the investigation, be sure there is understanding of these two points:

  • If the weights of oil and water are equal, oil will always have a greater volume than water.
  • The amount of space a liquid takes up compared to its weight is the property we call "heavy for size."

Not too small to matter.  Heaviness for size is a property of all materials, independent of sample size. Although it is not very obvious, a single gram of oil has a greater volume than a single gram of water, and 0.1 grams of oil will have greater volume than 0.1 grams of water. If students doubt this, ask if they are saying we might not notice the volume difference any more, or if they think the difference really ends once we reach 1 gram. While our senses are very important in helping us learn about the world, there are times when our senses — or even scientific instruments — cannot detect something, and we have to rely on our understanding. Does it make sense that the pattern we see would suddenly stop being true at 1 gram? No, there is no logical reason for that, so we must rely on our understanding.