1. Water, a Liquid:  Investigation 5

What changes and what stays the same when salt dissolves in water?

5. Make meaning

Discussion 10 Mins

Classroom Case

The Role of Explanation Discussions

Available online at inquiryproject.terc.edu

Purpose of the discussion

The purpose of this discussion is for students to connect the investigation question with the weight and volume data they have collected by making claims and describing the supporting evidence.

Claims and evidence: Introduce the scientific practice of supporting claims with evidence. Whenever students make a claim, they are expected to describe the evidence that supports the claim.

Evidence is data that supports or challenges a claim and is often in the form of a measurement or a drawing or photograph.

Sometimes the only way to support a claim is by reasoning. While reasoning is important, it is not always as convincing as actual data.

Example: I see that the scale reads zero when I put a grain of salt on it, but I know that a full container of salt weighs more than an empty container of salt, so I reason that each grain of salt must have some weight.

Students should also start to distinguish between evidence and their own ideas or opinions.

Engage students in the focus question

What changed and what stayed the same when the salt dissolved in water? What evidence supports your claim?

Note: As students address the focus question, be sure they are referring to the data they have collected in their notebooks.

Possible responses include:

  • The weight of the salt remains the same, even after it dissolved and the particles became too small to see. (Evidence: the combined weight of salt and water equals the sum of individual weights of the salt and the water).
  • The salt continues to take up space, even after it dissolved and the particles became too small to see (Evidence: the combined volume of the salt and water is greater than the volume of just the water).
  • Salt remains matter after it has dissolved and can no longer be seen. (Evidence: Dissolved salt has weight and takes up space.)
  • The size of the salt grains changes, becoming too small to see. (Reasoning: The weight data lets us know that the salt is still in the water, and we know we cannot see the particles).
  • The number of salt particles changes. (Reasoning: Data tell us that the weight of the salt remains the same. Since we reason that the particles have become smaller, there must be more of them.

Why do you think the salt is no longer visible? How do you explain this? (If not previously addressed)

  • The salt particles became smaller and smaller as they dissolved until they became too small and too spread out to see. Like the dots on the 315 Dots per Page sheet, when tiny particles (or dots) are clustered they are visible but when they spread out enough, they become too hard to see.

Note: One might claim that the salt particles remain the same size but just become “invisible” in the water. Our sense of touch can provide evidence that the dissolved grains no longer exist at the same size.

Summarize the discussion and recap the investigation

Using the same language students have used, summarize their main ideas. Include the following key ideas:

  • Remained the same: weight of salt, and the fact that the salt still takes up space (although actual volume was not measured)
  • Changed: The salt is no longer visible. The grains became too small to see when they dissolved.

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

  • Students' work in this investigation highlights the importance of accurate measurements.
  • When students make claims, they need to support those claims with evidence or with reasoning.
  • Today's investigation provided evidence that:
    • Weight stays the same as a substance dissolves.
    • Things too small to see have weight and take up space.
  • The concept of matter too small to see continues as a theme throughout the rest of this unit.