Will It Sink or Will It Float? A Simple Science Experiment

Drop a rock into a pond, and it sinks. Toss a dry stick into the same pond, and it floats. The difference comes down to density and buoyancy. An object sinks if it is denser than the fluid it sits in, and it floats if it is less dense. This simple principle is the core of a classic science experiment you can do with household items.

What Makes an Object Float or Sink?

The basic rule is straightforward: an object will float if its density is lower than the density of the water (or any liquid) it is placed in. Density is how much mass is packed into a given volume. Water has a density of about 1 gram per cubic centimeter (g/cm³). An object with a density below 1 g/cm³ will float; one above 1 g/cm³ will sink.

But shape also matters. A lump of clay sinks because its density is higher than water. If you shape that same lump of clay into a wide, hollow boat, it floats. You did not change the clay’s density, but you changed its overall volume. The boat shape traps air, increasing the total volume of the object (clay plus air). That lowers the average density of the whole structure below 1 g/cm³.

This is the same reason a steel ship floats. Steel is much denser than water, but the ship’s hull encloses a large volume of air. The average density of the ship (steel plus air) is less than water. The ship displaces a weight of water equal to its own weight before it sinks below the surface. That is the principle of buoyancy, first described by Archimedes.

How Buoyancy Works

Buoyancy is the upward force that a fluid exerts on an object placed in it. The force is equal to the weight of the fluid that the object displaces. If the buoyant force is greater than the object’s weight, the object rises and floats. If the buoyant force is less, the object sinks.

You can feel this force when you try to push a beach ball underwater. The water pushes back hard. That push is buoyancy. For a floating object, the buoyant force exactly balances the object’s weight.

A Surprise with an Orange

Try this experiment with a whole orange. It floats. Now peel the orange and try again. The peeled orange sinks. Why?

The peel of an orange is porous and contains tiny pockets of air. This air lowers the average density of the whole orange enough to make it float. Without the peel, the fruit inside is denser than water, so it sinks. This is a good example of how small changes in an object’s structure can change its buoyancy.

Why Salt Water Changes the Results

If you repeat the experiment with salt water, you will get different results for some objects. Dissolving salt in water increases the water’s density. Seawater has a density of about 1.025 g/cm³, compared to fresh water’s 1.0 g/cm³.

An egg sinks in fresh water but floats in a glass of very salty water. The denser salt water provides a stronger buoyant force. This is why it is easier to float in the ocean than in a lake. You can try this yourself by adding table salt to a glass of water until an egg floats Science Buddies. The amount of salt needed depends on the water volume, but roughly 2-3 tablespoons per cup of water is enough to make a noticeable difference.

Real-World Applications of Buoyancy

The same principles you test in your kitchen apply to large-scale engineering and nature.

Ships and Submarines

A ship’s hull is designed to displace a large volume of water. The shape pushes water aside, creating a buoyant force that supports the ship’s weight. A submarine can control its buoyancy by taking water into ballast tanks (making it denser to sink) or blowing the water out with compressed air (making it less dense to rise). This is a direct application of the sink-or-float principle NASA.

Fish and Marine Animals

Many fish use a swim bladder, an internal gas-filled organ, to control their buoyancy. By adjusting the amount of gas in the bladder, a fish can change its average density to match the water around it. This lets the fish stay at a certain depth without swimming constantly. Sharks do not have swim bladders; they rely on their large, oil-filled livers (oil is less dense than water) and their forward motion to stay off the bottom.

Hot Air Balloons

Buoyancy works in air, not just water. A hot air balloon rises because the air inside the balloon is heated, making it less dense than the cooler air outside. The surrounding air pushes the balloon upward, just like water pushes a boat upward. The balloon sinks when the air inside cools and becomes denser.

Key Takeaways

• An object sinks if it is denser than the fluid. It floats if it is less dense.
• Shape matters. A flat sheet of metal sinks, but the same metal shaped into a bowl with trapped air can float.
• Adding salt to water increases its density, making it easier for objects to float.
• Buoyancy is the upward force that pushes against an object’s weight.
• The same scientific principles explain how ships, submarines, fish, and hot air balloons work.