Science · Grade 2 · Properties of Liquids and Solids · Term 2

Gases: The Invisible State

Students will explore the concept of gases, demonstrating that they take up space and have mass, even if invisible.

• Ontario Curriculum Expectations
• 2-PS1-1

About This Topic

Physical Changes focuses on how heating and cooling can alter the properties of materials. Students observe how solids can turn into liquids (melting) and liquids into solids (freezing), and how these changes can sometimes be reversed. This topic is a key component of the Ontario Grade 2 curriculum, as it helps students understand the relationship between temperature and the state of matter. It also introduces the idea that some changes, like burning or cooking an egg, are permanent, while others, like melting an ice cube, are not.

By exploring these changes, students learn to make predictions and record observations over time. This topic is particularly effective when students can engage in hands-on modeling and real-time observation. When students work in groups to see which material melts fastest or how a liquid changes as it cools, they are directly experiencing the cause-and-effect nature of science. These active experiences help them grasp that 'change' is a process that can be measured and described.

Key Questions

1. Explain how we know air is a gas even though we cannot see it.
2. Design an experiment to show that air takes up space.
3. Predict what would happen if a balloon was filled with a different gas.

Learning Objectives

• Demonstrate that gases occupy space by using a balloon and a container.
• Explain why a balloon inflates when air is blown into it, showing gases have mass.
• Design a simple experiment to show that air has mass.
• Predict how the volume of a balloon changes when filled with different amounts of air.

Before You Start

Properties of Solids and Liquids

Why: Students should have prior experience observing and describing the properties of solids and liquids to build understanding of a third state, gas.

Basic Observation Skills

Why: This topic requires students to observe changes and infer properties of invisible substances, building on their ability to make careful observations.

Key Vocabulary

Gas	A state of matter that has no fixed shape or volume, spreading out to fill its container. It is often invisible.
Mass	The amount of 'stuff' or matter in an object. Even invisible things like air have mass.
Volume	The amount of space that an object or substance takes up. Gases take up space.
Invisible	Unable to be seen. Many gases are invisible, making them hard to detect without experiments.

Watch Out for These Misconceptions

Common Misconception

Melting and dissolving are the same thing.

What to Teach Instead

Students often say sugar 'melts' in water. Use a side-by-side comparison: an ice cube melting (heat only) and sugar dissolving (requires a liquid). Peer discussion helps clarify that melting is a change of state, while dissolving is mixing.

Common Misconception

Everything that melts can be turned back into a solid.

What to Teach Instead

While many physical changes are reversible, students might think all heat-related changes are. Discussing 'cooking' versus 'melting' helps them see that some heat changes the material's nature permanently.

Active Learning Ideas

Inquiry Circle: The Melting Race

Small groups are given an ice cube, a piece of chocolate, and a crayon. They place them in a sunny spot and predict which will melt first, recording the time it takes for each to change state.

30 min·Small Groups

Think-Pair-Share: Reversible or Not?

Show images of a melted popsicle, a burnt piece of toast, and a folded paper. Students think about which can be changed back, pair up to explain their reasoning, and share with the class.

20 min·Pairs

Stations Rotation: Temperature Effects

Stations include 'Warm Water vs. Cold Water' for dissolving, 'Modeling Clay' (softening with hand heat), and 'Freezing Juice' (observing results from the previous day). Students record how heat changed each item.

40 min·Small Groups

Real-World Connections

• Hot air balloon pilots use their understanding of gases and temperature to control lift and direction, navigating through the sky for recreational flights or transportation.
• Scuba divers must manage the volume and pressure of gases they breathe underwater, ensuring they have enough oxygen and can safely return to the surface. This involves understanding how gases behave under pressure.

Assessment Ideas

Quick Check

Provide students with a clear plastic bag and a straw. Ask them to blow air into the bag and then seal it. Ask: 'What did you put inside the bag? How do you know it's there even though you can't see it?' Observe their explanations about the bag expanding.

Exit Ticket

Give students a card with the question: 'If you put a deflated balloon on a scale, and then blew it up, what would happen to the weight? Explain your answer.' Students write their prediction and a brief reason.

Discussion Prompt

Pose the question: 'Imagine you have two identical balloons. One is filled with air, and the other is empty. If you held them both, would they feel different? Why or why not?' Guide students to discuss the concept of air having mass.

Frequently Asked Questions

How does this topic connect to the Ontario environment?

The change of seasons in Ontario is a perfect example. Discussing the freezing and thawing of local lakes or the melting of snow in spring makes the concept of physical change highly relevant to their lives.

What is the best way to demonstrate reversible changes?

Water is the gold standard. Freezing it into ice and then melting it back into water is a clear, safe, and repeatable demonstration that students can easily understand.

How can active learning help students understand physical changes?

Active learning, such as the 'Melting Race,' allows students to see the process of change as it happens. Instead of just seeing the 'before' and 'after,' they observe the transition, which helps them understand that temperature is the driving force behind the change.

Why do some things melt faster than others?

Explain that different materials have different 'melting points.' Some things, like ice, only need a little bit of heat, while others, like metal or even some plastics, need a lot of heat to turn into a liquid.

From the Blog

15 Active Learning Strategies for Elementary School: A Teacher's Guide to Engagement

Discover 15 research-backed active learning strategies for elementary school that boost engagement, retention, and critical thinking in K-5 classrooms.