Students will learn about and discuss the behavior of a gas using examples of convection and an optional teacher-led demonstration. Students will then participate in a lab where they will investigate water currents by observing the results of mixing colored warm water with room temperature water. The lab will be followed by a discussion about the molecular activity of the water mixture.
Grade Level
Elementary
Objectives
By the end of this lab, students should be able to:
Understand the effects of heating and cooling.
Explain that warm air and liquids rise because they are less dense than cold air and liquids.
Describe the molecular motion of gases and liquids, and how heat impacts the motion.
Always wear safety goggles when handling chemicals in the lab.
Students should wash their hands thoroughly before leaving the lab.
When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
Teacher Notes
1. Introduction
This topic can be introduced through a teacher-lead demonstration, An Uplifting Experience, found in the AACT Resource Library. However, if time constraints or limitation of materials will not allow for using the demonstration, the following videos and examples will also benefit student understanding of the topic.
Conduct a discussion about hot air balloons through a second video clip which shows the air being heated, allowing the balloon to rise.
Guide student understanding to the idea that warm air rises and is lighter, or less dense, than the surrounding colder air. Also, explain that air molecules are being heated causing their movement to increase in speed and to spread out more.
Depending on the age and experience of your students, you may want them to understand on a deeper level why the hot air balloon floats. Following is information about mass, volume, and density of air and why heating the air in the balloon causes it to float.
2. Density and sinking and floating
The characteristic that determines whether an object floats or sinks when placed in a liquid or gas is whether the object is more dense or less dense than the substance it is placed in. A common example is objects placed in water. An object such as a rock, which is more dense than water, sinks. An object such as wood, which is less dense than water, floats.
But what is density?
A simple definition of “density” is: The measure of how heavy something is compared to its size. Another definition using “mass” and “volume” is: The relationship between the mass of a substance and its volume (the amount of space it takes up). In math terms, the formula for density is: Density = mass/volume or D=m/v.
Here’s an example using wood and clay: Let’s say you have small cubes made of wood and clay, both measuring 1 centimeter on each side. Each cube has a volume of 1cm x 1 cm x 1 cm = 1 cm3. The wood has a mass of .8 grams. The clay has a mass of 2.5 grams.
The wood has a density of .8g/1cm3
= .8g/cm3. The clay has a density of 2.5 grams/1 cm3 = 2.5 cm3. Water (at room temperature) has a density of 1g/cm3. Since the wood has a density less than water, the wood floats. Since clay has a density greater than water, the clay sinks.
3. Applying density and gas to a hot air balloon
Like any other object, the hot air balloon also has mass and volume. Since the air in the balloon is matter and all matter weighs something (has mass), the air in the balloon has a certain mass. The amount of space that the balloon and air inside take up is the volume of the balloon.
So the entire mass of the hot air balloon is made up of the mass of the air inside the balloon, the mass of the material that makes up the balloon, the mass of the basket, and the mass of the person in the basket.
The volume of the balloon is almost all the volume of the giant balloon filled with air. So the density of the balloon is its mass divided by its volume. Since the balloon filled with unheated air is initially on the ground, the density of the balloon must be greater than the density of the air around it.
4. The effect of warming air and why a hot air balloon floats
A hot air balloon on the ground is filled with air that is the same temperature as the air around it. But a powerful burner begins heating the air. When air is heated, the molecules move faster and spread further apart. When the air is heated enough, the molecules spread out so much that they push on the inside of the balloon material and make the balloon expand to a much larger volume. The mass stays the same, but the volume gets bigger and bigger. Since D=m/v a larger volume with the same mass results in a lower density. Eventually the volume becomes so large that the overall density of the balloon, including the basket and its contents is less dense than the surrounding air and the balloon floats.
Convection
Introduce the term and meaning of convection.
Sometimes, if a liquid or gas is being heated from the bottom, the warm liquid or gas rises and transfers its energy to the surrounding area. This makes the heated substance that floated up eventually cool down. As it cools it becomes more dense and sinks to where it can be heated again.
This cycle of heating and rising, and cooling and falling is called convection.
Discuss why we and firefighters are instructed to crawl on the ground to escape hot smoke inhalation if caught in a fire. The hot smoke rises, and the cooler, breathable air is down near the ground.
Do The Demonstration, An Uplifting Experience
Complete the demonstration, An Uplifting Experience. Conclude that the movement of warmer air upwards and colder air downwards is called convection. Also inform students that this also applies to warm and cold water.
Lead the activity to show convection in liquids
Next, students will apply what they learned about air currents and convection to a lab experience about water temperature and currents.
The lab is best managed with students placed in small groups of approximately 4 students per group.
After the lab make students aware that ocean currents are examples of convection. Also, hot springs where the cold rain water falls is heated thermally by Earth’s mantle. The cold rain water sinks and the warm water rises.
This lab lends itself to be used when studying properties of matter, specifically gases and liquids. It may also be used in a unit about weather.
Prepare the lab materials
For the student lab, the teacher should prepare the convection jar lid ahead of time:
Punch two holes in a baby food jar lid and insert straws. Insert one to be as deep as the jar and place the other above the lid. Seal them with clay or hot glue.
As an extension option for the student lab you could create 3 different convection jars containing different colored water samples, at different temperatures. This would allow students to investigate if the temperature of the liquid affects the liquid to rise more, or rise faster.
Below you can see images of anticipated results for this lab. On the left is the convection in action, after the jar has been added to the aquarium. On the right the convection process is near completion, as indicated by the color change in the jar.
For the Student
Lesson
Observing Density of Gases and Liquids
Background
You have learned about the density of gas, and how it is affected by temperature change. You may have observed this through videos and/or a teacher demonstration prior to this lab. In this lab you will investigate how liquids can also be affected by temperature change. This will help you understand temperature changes in the real-world, such as in ocean currents, and natural hot springs.
Pre-lab Questions
In your own words describe the meaning of convection.
Give an example of how you experience convection in your daily life.
Always wear safety goggles when handling chemicals in the lab.
Wash your hands thoroughly before leaving the lab.
Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
Procedure
Fill your convection jar (baby food jar) to capacity with warm water and add several drops of food coloring.
Cover the jar with its lid (the lid should also have 2 secured straws attached to it, prepared by your teacher). Make sure the straws are also filled with warm water—if not, add more warm water to them so it is at maximum capacity.
Place the jar gently on the bottom of the fish tank. Start your stopwatch.
Carefully watch what happens, focusing on the color changes throughout the containers.
Record your observations at the time increments described in the table below.
Observations
Time
Observations
30 seconds
2 minutes
5 minutes
Analysis
How is heating a gas similar to heating a liquid? How are they different? Use examples from what you learned today to support your answer.
If you could see the tiny particles that make up a gas or a liquid, how would cold particles be different than warm particles? Use a sketch to help your explanation.
Can convection be helpful to you? Explain why or why not.
