Density Curriculum

Section 4—Lesson 14: Layering Liquids: How Does Density Affect Sinking or Floating?

Lesson Plan

Materials

Prep Step

Analyze Thinking

Step 1: Extending Relational Causality to Liquids

Ask the students to predict (either in a class discussion or in their journals) what some of the outcomes might be if you put two liquids into a beaker or jar. (They might mix, or mix initially and then separate, one liquid might rise to the top of another, etc.)

Ask, "In the case of liquids that do not stay mixed, what determines which liquid ends up on top, for example oil floating on water?"

Collect whatever ideas the students have. Some of them may extend the learning about Relational Causality that they did in the last lesson to reasoning about what happens with liquids. Leave the question open while they do the activity and come back to it at the end of the lesson.

RECAST Thinking

Step 2: RECAST Activity: What do Liquid Layers Tell Us About Density?

Pass out the sheet Sinking and Floating Fluids to pairs or small groups of students. Have each group collect the materials that they need. Explain that the sheet guides them through a series of questions and activities to help them think about why some liquids sink and some liquids float on other liquids. The activity asks students first to compare the relationship between liquid pairs. It then asks them to find the density of the liquids. After the densities are found, students are asked to consider what would happen if all three liquids were placed in one jar. After testing their predictions, and drawing another model, students are asked again how they think the density of the liquids determine their placement in the test tube.

Have the students work through the question on the sheet. Circulate while they are working to see how they understand what they are doing.

Explore Causality

Step 3: Relational Causality and Liquid Layers

Remind the students of the discussion that you had at the start of the lesson. What do they think now? How have their thoughts changed? As a group, read and discuss the sheet entitled, Relational Causality and Layering Liquids.

Review, Extend, Apply

Step 4: Applying Relational Causality to Liquid Layers

Pass out the sheet, Liquid Layers: How Does Density Affect Sinking and Floating?. Ask the students to use what they have learned about density and Relational Causality to answer the questions.

Step 5: Making Connections: Layers in the Atmosphere

Explain to the students that what they learned extends to all fluids. Gases are also fluids. Pass out or put up an overhead of Layers of the Atmosphere. Where would they predict that they would find the densest gases in the atmosphere? The least dense gases? Pass out or put up an overhead of Layers of the Atmosphere Showing Air Density. What do they predict would happen if a gas that was denser than the gases we breathe was released into the atmosphere in abundance? (See box below for a real-world example.)

KILLER LAKES OF CAMEROON

Gases with different densities often form layers in the atmosphere, and these layers can have very important, and sometimes life-threatening effects in the real world. One example is the so-called "killer lakes" of Cameroon. Lake Nyos is located in a part of Cameroon that is volcanically active. Carbon dioxide gas from this underground volcanic activity built up at the bottom of Lake Nyos over time, like the dissolved carbon dioxide in a can of soda. In August of 1986, a disturbance in the lake, possibly from a landslide, caused the carbon dioxide to be released, the way the bubbles in a can of soda rise to the top when shaken. Because pure carbon dioxide is denser than air, the carbon dioxide gas settled in a layer on the ground around the lake, suffocating more than 1700 people who lived in the area. Since then, scientists have been continuously pumping smaller amounts of carbon dioxide out from the bottom of Lake Nyos to prevent this kind of density-driven natural disaster from happening again.

Source: http://www.biology.lsa.umich.edu/~gwk/research/nyos.html. Information from a description of the research of George W. Kling, Department of Ecology & Evolutionary Biology, University of Michigan.