🪣 Sink or Float? Testing Matter in Water 🍎🪙

Physical Science Unit, Lesson 3

📚 Grade: 2nd Grade ⏱️ Duration: 45 minutes 🔬 Subject: Physical Science -- Matter Properties 🧰 Materials: Household items only
🪣🍎🪙🫐🪵
Why does a tiny coin sink while a giant apple floats? Let's find out!
📋 NGSS Standard:
2-PS1-1 -- Plan and conduct an investigation to describe and classify different kinds of materials by their observable properties.

🎯 Learning Goals

📦 Materials

🏠 What You Need (All Household Items)

📖 Vocabulary

Predict
To make a smart guess about what will happen before testing
Observe
To watch carefully and notice what actually happens
Float
To rest on top of water without sinking
Density
How much matter is packed into a space -- dense things tend to sink

🌍 Background

Have you ever wondered why a giant cruise ship floats but a tiny coin sinks? It seems backward! The secret is density. Density is how tightly packed the matter is inside an object. A coin is very dense -- lots of metal packed into a small space -- so it sinks. A cruise ship is mostly hollow inside, filled with air, so even though it is enormous, its overall density is lower than water, and it floats.

For 2nd graders, we can think of it this way: if an object is "heavier than water" for its size (meaning denser), it sinks. If it is "lighter than water" for its size (meaning less dense), it floats. Wood floats because wood has lots of tiny air pockets inside its fibers. Metal sinks because the atoms are packed very tightly together with no air gaps.

The most exciting discovery in this lesson is the aluminum foil experiment. A flat sheet of foil floats -- it traps air underneath and spreads its weight over a large area. But crumple the same foil into a tight ball, and it sinks! The amount of foil did not change, but the shape did. The flat sheet has more surface area touching the water, and the trapped air helps it float. The ball has no trapped air and is much denser in its compact form. Shape AND air both matter, not just what something is made of!

🔬 Procedure: The Sink or Float Investigation

Step 1: Fill your large bowl with water. Set all objects beside it. Create a prediction chart with columns: Object | Prediction (sink/float) | Result (sink/float) | Correct? (yes/no).
Step 2: Before dropping each object in, make a prediction and write it down. Remember: a prediction is not cheating -- it is scientific thinking!
Step 3: Gently place the apple in the water. Did it sink or float? Record the result. Compare to your prediction.
Step 4: Test the coin. Then the grape. Then the cork. Then the pencil. Then the paper clip. Then the plastic toy. Record each result.
Step 5 (The Exciting Part): Lay one sheet of aluminum foil flat and gently place it on the water surface. Does it float? Observe carefully.
Step 6: Take the foil out. Crumple it into a tight ball. Drop it in. What happens? Record your observation.
Step 7: Discuss: the foil is the exact same material both times. What changed? (The shape and the amount of trapped air changed!)
Step 8 (Bonus): Can you shape the foil into a boat shape that floats AND holds a small object on top? Try it!
🔑 Key Discovery: A flat sheet of foil floats because it has lots of surface area and traps air underneath. A crumpled foil ball sinks because it is compact and dense with no trapped air. SAME material, DIFFERENT behavior -- because the SHAPE changed!

💬 Discussion Questions

🗣️ Question 1: The foil ball sinks but the flat sheet floats. What changed?

Suggested answer: The amount of foil stayed the same, but the shape changed completely. The flat sheet traps air underneath it and spreads its weight over a large area. The crumpled ball has no trapped air and is very compact and dense. This shows that shape and air content matter for floating, not just what the material is made of.

🗣️ Question 2: Why does wood float but a metal coin sinks, even though the coin is much smaller?

Suggested answer: It is about density, not size. Wood has tiny air pockets inside its fibers that make it less dense than water overall. Metal atoms are packed very tightly, making it denser than water. Even a tiny metal coin sinks while a large wooden log floats!

🗣️ Question 3: How do ships float even though they are made of metal?

Suggested answer: Ships are hollow inside! They are mostly filled with air. Even though the walls are made of dense metal, the overall density of the ship (metal walls plus all the air inside) is less than water. Engineers design ships to be mostly empty inside on purpose -- that is what allows them to float despite being enormous.

🗣️ Question 4: What would happen if you put a ball of clay in the water? What if you shaped it into a bowl?

Suggested answer: A ball of clay sinks because it is dense with no trapped air. But if you flatten it into a bowl or boat shape, it can trap enough air to float! This is the same principle as the foil experiment -- the shape creates a pocket of air that helps the object ride on top of the water.

🔬 Student Worksheet -- Sink or Float?

Name: ________________________________      Date: ________________

Part 1 -- Predictions and Results

Write S (sink) or F (float) for your prediction, then test and record the result!

Object My Prediction (S or F) Result (S or F) Was I right?
Apple
Coin
Grape
Cork
Wooden Pencil
Paper Clip
Plastic Toy
Flat Foil Sheet
Foil Ball

Part 2 -- The Foil Experiment

The flat foil sheet floated because .

The foil ball sank because .

Part 3 -- Draw It!

✏️ Draw the foil as a ball (sinking) and as a flat sheet (floating). Show the water level. Label each drawing.
My most surprising result was:

👩‍🏫 Parent/Teacher Notes