A sink-or-float experiment is a simple way to investigate how materials behave in water. With a careful plan, children can move beyond guessing and collect evidence that helps explain why some objects stay on the surface while others sink.
What You Will Investigate
The main question is:
How can we predict whether an object will sink or float in water?
You can investigate several related questions:
- Does the material affect whether an object sinks or floats?
- Does the size of an object matter?
- Can changing an object’s shape make it float?
- Does adding weight change the result?
- Do objects float in salt water when they sink in fresh water?
For a first investigation, use a variety of small, waterproof objects and test each one in the same container of water. Examples include a plastic bottle cap, wooden craft stick, metal spoon, pebble, plastic toy, coin, leaf, rubber eraser, cork, and a small piece of modeling clay.
Avoid objects that can break, leak chemicals, contain batteries, have sharp edges, or could be a choking hazard for young children. An adult should supervise the activity, especially when small objects or water are involved.
Materials and Safety
Gather:
- A clear bowl, tub, or large measuring container
- Water at room temperature
- A towel or tray to catch spills
- Six to ten clean objects made from different materials
- A pencil and notebook, or a printed results table
- A ruler, kitchen scale, or measuring cup for optional follow-up tests
- Modeling clay for a shape investigation
- Salt and a spoon for an optional salt-water comparison
Place the container on a stable, waterproof surface. Fill it deep enough that each test object can be completely placed in the water without touching the bottom. If an object is too large for the container, choose a smaller example or use a clean bucket.
Wash objects before testing them, and dry them between trials if you are comparing their mass. Do not taste the water or the materials. Empty the water carefully when finished so small objects do not go down a drain.
Make Predictions Before Testing
A prediction is a statement about what you think will happen and why. Predictions should be made before the object goes into the water so the test provides a fair check of your thinking.
Use a table like this:
| Object | Material | Prediction | Reason | Result |
|---|---|---|---|---|
| Craft stick | Wood | Float | Wood often feels light | |
| Coin | Metal | Sink | It is compact and heavy for its size | |
| Bottle cap | Plastic | Float | It is light and may trap air |
Encourage children to describe what they notice rather than relying only on familiar sayings. For example, “This object is heavy” is useful, but “This object is heavy compared with its size” is a better observation. A large sponge may be heavier than a small coin but still float because its material and shape allow it to displace enough water.
Ask children to explain their reasoning in complete sentences:
- “I predict the pebble will sink because it is dense and does not contain trapped air.”
- “I predict the empty bottle will float because it is light and has air inside.”
- “I am unsure about the clay because its shape may affect how much water it pushes aside.”
A prediction does not need to be correct to be useful. It gives the investigation something to test.
Plan a Fair Test
A fair test changes one factor while keeping other important conditions the same. For the basic investigation, the factor being compared is the object. Keep these conditions consistent:
- Use the same water container.
- Use the same type and depth of water.
- Test one object at a time.
- Place each object gently on the water instead of throwing or pushing it.
- Wait the same amount of time before recording the result.
- Use clean objects without extra water clinging to them.
- Repeat any uncertain test.
Define what “float” and “sink” mean before you begin. For example, an object floats if it remains partly or completely above the surface after ten seconds. It sinks if it reaches the bottom. An object that stays suspended in the middle is neither clearly floating at the surface nor resting on the bottom, so record it as “suspended” and discuss why that might happen.
This definition prevents disagreements later. A metal object may briefly remain on the surface because of surface tension, then sink. If the agreed observation time is ten seconds, record what happens at that point and note any later change.
Carry Out the Investigation
Follow these steps for each object:
- Observe the object without testing it. Record its material, size, shape, and any visible air spaces.
- Write a prediction and a reason.
- Place the object gently on the water surface or lower it just below the surface if its shape makes placement difficult.
- Remove your hand without pushing the object down.
- Observe it for the agreed time.
- Record whether it floated, sank, or stayed suspended.
- Remove the object, dry it if needed, and prepare for the next test.
- Repeat tests that produce surprising or unclear results.
Do not stir the water between tests unless it is necessary to reset the container. Wait for ripples to settle before judging a result. If a piece of paper or a leaf floats at first and then becomes soaked, record both observations: “floated initially; sank after absorbing water.” This is more informative than choosing only one label.
Record Evidence Clearly
Good records make patterns easier to see. In addition to the basic table, children can record observations such as:
- Whether the object was fully or partly submerged
- Whether it tilted, rolled, or absorbed water
- How quickly it sank
- Whether bubbles appeared
- Whether the object changed after getting wet
Use simple symbols only if everyone understands them. For example, F can mean float, S can mean sink, and M can mean move or remain suspended. Written notes are helpful when an object changes during the test.
For repeated trials, create extra result columns:
| Object | Trial 1 | Trial 2 | Trial 3 | Same result each time? |
|---|---|---|---|---|
| Cork | Float | Float | Float | Yes |
| Clay ball | Sink | Sink | Sink | Yes |
| Clay boat | Float | Float | Float | Yes |
If the results differ, do not simply discard the unusual trial. Look for a reason. The object may have been placed differently, the water may have been moving, or an air bubble may have changed its behavior.
Explain Why Objects Sink or Float
An object floats when the upward force from the water is enough to support its weight. Water pushes against objects in every direction, including upward. This upward push is called buoyancy.
An object sinks when its weight is greater than the upward support it receives from the water. In many cases, this happens because the object is denser than water. Density describes how much matter is packed into a certain amount of space.
The overall density of an object depends on both its material and its shape. A solid steel marble usually sinks because steel is much denser than water. A large steel ship can float because its hollow shape contains a great deal of air. The ship’s average density, including the steel and the air-filled space, can be less than the density of water.
This is why “heavy things sink” is not a complete rule. A full bottle of water and an empty sealed bottle may have the same outside shape but different masses. The empty bottle may float because it contains air, while the full bottle may sink or sit lower in the water.
Surface tension can also affect very small or carefully placed objects. A needle or paper clip may remain on the surface briefly if the water surface is not disturbed. This does not mean the metal is less dense than water; the surface is supporting it in a different way.
Try the Modeling Clay Challenge
Modeling clay provides a useful demonstration that shape matters. Begin with a small piece of clay. Make a prediction, then roll it into a compact ball and place it in the water. It will usually sink.
Next, remove the clay, dry it, and reshape it into a shallow boat with raised sides. Place it gently on the water. A boat-shaped piece may float because its shape spreads the clay over a larger volume and allows it to displace more water. The hollow space also contains air.
Test how much weight the clay boat can hold:
- Make the boat as evenly as possible.
- Place it in calm water.
- Add small coins, counters, or plastic cubes one at a time.
- Count how many it holds before water enters the boat or the boat sinks.
- Repeat with a redesigned boat.
Keep the amount of clay the same for each design. If one boat is made with more clay, the comparison is no longer fair. Record the boat’s shape, the number of items held, and what caused it to sink.
Optional Extensions
Once the basic investigation is complete, choose one variable to change.
Compare fresh and salt water
Dissolve measured amounts of salt in separate containers of water. Test the same object in fresh water and salt water. Use the same object, container size, and observation time. Salt water is denser than fresh water, so an object may float higher or behave differently. Record the exact amount of salt and water so another person could repeat the test.
Investigate mass and volume
Use a kitchen scale to compare objects with similar sizes but different masses. You can also measure how much water an object displaces by placing it in a measuring container and observing the change in water level. These measurements help connect the results to density, but they are optional for younger children.
Test waterproof and absorbent materials
Compare waxed paper, plain paper, a sponge, plastic wrap, and a leaf. Record whether each object floats at first and whether it remains floating after several minutes. This investigates how water absorption changes an object’s behavior.
Design the best floating shape
Give each group the same mass of modeling clay and ask them to design a shape that holds the most counters. Decide in advance what counts as success. Compare designs using the same water depth, counter type, and testing method.
Troubleshooting Common Problems
The object floats because of trapped air. Check whether it has a hollow space, cap, or pocket. Test it again after filling the space with water if this can be done safely.
The object sticks to the side of the container. Move it to the center and repeat the test. Contact with the side can provide extra support.
The water is too shallow. Use a deeper container so the object can sink without touching the bottom immediately.
The object is pushed under the surface. Place it more gently or use a spoon to lower it without adding downward force.
The results change between trials. Check the placement, water movement, bubbles, and condition of the object. Repeat at least three times and explain the variation.
A paper object floats briefly, then sinks. Record the time or sequence. The material may be absorbing water, changing its mass and shape.
A very small object behaves strangely. Surface tension may be important. Avoid making a broad claim from one tiny object; test several objects and describe the limitation.
What the Results Can and Cannot Show
This experiment can show patterns in how particular objects behave under particular conditions. It can help children connect predictions with evidence and recognize that material, shape, trapped air, and water conditions all matter.
It cannot prove that every object made from one material will behave identically. A hollow plastic object and a solid plastic object may have different average densities. Results can also change with water temperature, salt concentration, object damage, trapped bubbles, or the amount of water absorbed.
A strong final explanation should refer to evidence: “The clay ball sank in all three trials, but the same amount of clay shaped like a boat floated until it held six counters. This suggests that shape and displaced water affected the result.” That kind of statement is more useful than simply saying, “Heavy things sink.”