Science · 2nd Grade · Matter and Its Mysteries · Weeks 1-9

Reversible Changes: Melting and Freezing

Students will conduct experiments to observe and explain reversible changes like melting ice and freezing water.

• Common Core State Standards
• 2-PS1-4

About This Topic

This topic gives students direct experimental experience with reversible changes by focusing on two of the most visible phase transitions in daily life: melting and freezing. Students observe ice turning into water and water turning back into ice, gathering evidence that the material itself does not change even though its form does. This directly addresses NGSS 2-PS1-4 and supports early understanding of states of matter.

In the US K-12 curriculum, this topic sits at an important conceptual bridge. Students have observed water in its different forms throughout their lives, but rarely have they thought systematically about why those changes occur or why they can be undone. Structured experimentation helps students move from casual observation to evidence-based explanation.

Active learning is highly effective here because the experiments are quick, visual, and satisfying. Students can observe a complete reversible change within a single class period, which is rare in science and particularly motivating for young learners building scientific confidence.

Key Questions

1. Analyze why melting and freezing are considered reversible changes.
2. Compare the properties of water in its liquid and solid states.
3. Design an experiment to demonstrate a reversible change in a material.

Learning Objectives

• Explain why melting and freezing are reversible changes using evidence from experiments.
• Compare the properties of water in its liquid and solid states, such as shape and ability to flow.
• Design and conduct a simple experiment to demonstrate a reversible change in a common material.
• Identify materials that undergo reversible changes when heated or cooled.

Before You Start

Observing and Describing Properties of Objects

Why: Students need to be able to observe and describe basic properties like shape and texture to compare states of matter.

Introduction to Solids and Liquids

Why: Students should have a foundational understanding of what solids and liquids are before exploring how they change between states.

Key Vocabulary

Reversible Change	A change that can be undone, returning the substance to its original state.
Melting	The process where a solid turns into a liquid due to an increase in temperature.
Freezing	The process where a liquid turns into a solid due to a decrease in temperature.
Solid	A state of matter that has a definite shape and volume, like ice.
Liquid	A state of matter that has a definite volume but takes the shape of its container, like water.

Watch Out for These Misconceptions

Common Misconception

Students often believe that melting water means it is ruined or disappears.

What to Teach Instead

Weighing ice before melting and comparing to the weight of the resulting water on a scale shows that the amount stays exactly the same. Students are often surprised the scale reads identically, which makes the conservation idea memorable and personal.

Common Misconception

Many children think that freezing water makes it lighter.

What to Teach Instead

Water has the same mass whether frozen or liquid. Weighing equal volumes of liquid water and ice from the same source shows identical results. This can also introduce the interesting side note that ice is slightly less dense than water, which is why it floats.

Active Learning Ideas

Inquiry Circle: The Freezing Experiment

Pairs fill identical cups with the same amount of water, mark the water line with a marker, and place one cup in a freezer. When retrieved the next day, students compare the ice level to the original water line mark, weigh both cups on a balance, and discuss whether any water was lost or gained during the change.

40 min·Pairs

Think-Pair-Share: Can We Undo It?

Show images of a melted candle (reversible), a cooked egg (irreversible), and a melted ice sculpture (reversible). Students categorize each with a partner and explain their reasoning, identifying what evidence would let them know whether a change can be undone.

15 min·Pairs

Gallery Walk: States of the Same Thing

Post photos of water in solid form (ice), liquid form (puddle), and gaseous form (steam from a kettle) at three stations. Students walk to each station and write one observation on a sticky note about what changed and one thing that stayed the same across all three forms.

20 min·Individual

Real-World Connections

• Ice cream makers use the principles of freezing to turn liquid ingredients into a solid dessert, often using salt and ice mixtures to lower the freezing point and speed up the process.
• Chefs and bakers utilize melting and freezing in recipes, such as melting chocolate for sauces or freezing dough to preserve it for later use.
• Construction workers use the melting of ice and snow to manage water runoff and prevent flooding in colder climates, sometimes employing salt to accelerate melting on roads and sidewalks.

Assessment Ideas

Exit Ticket

Provide students with two pictures: one of ice melting and one of water freezing. Ask them to write one sentence explaining why each is a reversible change and one property that changes (e.g., shape, ability to flow).

Quick Check

Show students a video clip of butter melting in a pan. Ask: 'Is this a reversible change? How do you know?' Have students give a thumbs up if they agree it's reversible and a thumbs down if they don't, then ask a few students to explain their reasoning.

Discussion Prompt

Pose the question: 'Imagine you have a block of ice and a glass of water. What would you need to do to change the ice into water, and then change the water back into ice? What evidence shows that the water itself didn't change into something new?'

Frequently Asked Questions

What is the simplest explanation of a 'reversible change' for second graders?

A reversible change is one where you can make the material go back to exactly what it was before. Melting ice is reversible because you can freeze the water back into ice. Nothing new was made and nothing was destroyed, just rearranged into a different form.

How can I do this lesson without a classroom freezer?

Use an insulated bag with ice and salt to freeze small cups of water quickly, which takes about 15 minutes. Alternatively, bring pre-frozen samples from a school freezer to observe melting in real time during the period. The observation of melting is just as effective as the freezing step.

How does active learning help students understand reversible changes like melting and freezing?

Experiments that students run themselves, where they observe before and after states of the same material, give them ownership of the evidence. When students see their own water freeze and melt back to water, the concept of reversibility moves from abstract to personal and concrete, and they can cite their own data.

Does this topic connect to the water cycle?

Yes, directly. Melting (ice to water) and freezing (water to ice) are part of how water moves between Earth's surface and its solid forms in glaciers and snowpack. This is a natural preview for Earth science work students will encounter in later grades.