Smithsonian National Air and Space Museum

The Science of Water

Science Goals for Water Exploration

After explorations, students will be able to understand:

  • Water takes the shape of its container

  • Water will always form a flat surface unless it is acted on by another force, such as wind.

  • Objects can sink, float, or stay suspended in water.

  • Water always flows downward, unless a force pushes it upward, such as a pump.

The physics of water can be broken down into four basic components:

Flow, Drops, Sink and Float, and Bubbles. 

The Science of Water Flow

  • Water’s movement is described as flow.

  • Because water is a liquid, it does not have a strong internal structure. It takes the shape of whatever container it is in.

  • Water molecules are sticky! Water will always form a flat surface unless it is acted on by another force, such as wind.

  • Objects can sink, float, or stay suspended in water.

  • Air takes up space in water and will float to the top.

  • In nature, water flows down due to gravity. We see this in rivers, rain, gutters, etc.

  • If we make a force act on the water, we can make it move up!

A turkey baster shows how you can suck the water up into the baster, or squirt the water into the air by pushing it.

The Science of Water Drops

  • Drops stick to each other in a way called cohesion. We can see this in the way water forms droplets and can form a “skin” at the top of a glass.

  • When water molecules stick to other materials, it’s called adhesion. You can think of this also as absorption. We can see this when we pour water onto a paper towel. 

  • Waxed paper does not have good adhesion because it doesn’t absorb much of the water. A water drop on waxed paper stays where it is and keeps its shape, but a paper towel has great adhesion! It absorbs a lot of the water. Water drops on paper towels get sucked into the material and do not keep their shape

The Science of Sink and Float

Objects sink or float based on density: how heavy something is compared to much space it takes up.

  • Sink: objects sink when they are more dense than water. Examples: rocks, your keys, a refrigerator.

  • Float: objects float when they are less dense than water. These solid objects usually have a lot of air in them. Examples: wood, apples, plastic trash bag

Question: How do ships, made of metal, float on top of the water?
Answer: Some shapes let dense materials float on top of the water. The shape of a boat, for example, lets the metal be mostly filled with air, so overall the boat becomes less dense

  • Density (mass/volume) determines if objects sink, float, or stay suspended in water.

  • However, air bubbles can reduce an object’s effective density.

  • Effective shapes for boats distribute the weight of a denser-than-water material in such a way that the air enclosed makes the whole boat less dense.

What Are Air Bubbles?

  • Air (or any gas) takes up space in water, just like solids.

  • Air (or any gas) is less dense than water, so gas bubbles float to the top of water.

  • Air takes up space in containers. When water is poured into the container, it takes the place of the air that was already there.

  • If the air can’t get out of the container, the water can’t go in.

Place some interesting materials to engage students in water exploration. Too many materials can be overwhelming. Too few can lose students' interest.