Float or sink: testing objects in water

The idea

Whether an object floats depends on what it is made of and, for a hollow shape, on the air that shape holds, not on how big the object is: objects of the same material behave the same way, and one piece of foil floats as a boat and sinks as a crumpled ball.

What you need

• A clear tub with 10 L of tap water
• Test objects: a cork, a wooden cube, a metal spoon, a plastic bottle cap, a marble, an apple, a coin, a paper clip
• Two equal squares of household kitchen foil about 15 cm on a side: one pressed into an open boat with its sides turned up, the other crumpled into a ball by hand
• Towels and a tray; a two-column recording sheet (float, sink) with a prediction column

How to do it

1. Hold each object and predict float or sink. Record the prediction.
2. Lower each object gently onto the water. Let go. Record what it does after ten seconds.
3. Push the cork under and let go. Watch what happens.
4. Test the foil boat and the crumpled foil ball. Then open the ball out flat, press it into a boat and test it again, so the same foil is seen doing both.
5. Sort the objects by material and look for a pattern: which materials float, which sink.

What you should see

Cork, wood, the plastic cap and the apple float; the marble, coin, spoon and paper clip sink. The foil boat floats until water comes over its side and fills it, and the crumpled foil ball sinks. The ball sinks although it is the same foil, and the same weight of foil, as the boat: the folds of a ball crumpled by hand are open channels that water runs into, not sealed pockets of air, so the water the ball pushes aside is little more than the metal itself. The boat's turned-up sides hold a pocket of air the water cannot enter, so the boat pushes aside far more water than its foil alone would. Opening the ball out and pressing it back into a boat makes it float again, which is the check: the shape changed and the material did not. The cork springs back up when pushed under. A paper clip laid very gently flat can rest on the water's surface, which is a different effect; dropped in end first it sinks. The learner knows it worked when the objects of the same material behave the same way, and the same piece of foil floats as a boat and sinks as a ball.

What changes

What you change

material of the object; shape of the foil

What you measure

float or sink; how much of a floater sits below the surface

What you keep the same

• same water
• objects released, not pushed
• ten-second wait

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• Big things sink and small things float (a big log floats, a small coin sinks).
• Heavy things sink (a heavy apple floats; a light paper clip sinks).
• Air-filled boats float because they are light, not because of the water they push aside.
• A ball of foil crumpled by hand floats because air is caught in its folds (water runs into the folds of a hand-crumpled ball, so it sinks).

Safety card

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Science and Technology K-6 Syllabus (2017), NESA. Current syllabus; NESA's timeline is 2026 plan and prepare, 2027 start teaching the 2024 syllabus. Code read from the official syllabus document on 2026-09-22.STe-4MW-STSTe-5PW-STSTe-1WS-S
• Science and Technology K-6 Syllabus (2024), NESA. Implementation from 2027, so this code describes the future syllabus. Code read from the outcomes page on 2026-09-22.STE-SCI-01STE-PQU-01
• Australian Curriculum v9AC9SFU03AC9SFU02AC9SFI01AC9SFI02AC9SFI03AC9SFI04

Sources

The pages the author read to write this activity.

1. www.scootle.edu.au/ec/search?accContentId=AC9SFU03
2. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
3. primaryconnections.org.au/v84-sequences/whats-it-made
4. hyperphysics.gsu.edu/hbase/pbuoy.html