Water exists as a liquid because of the strong attraction that its molecules have for each other – this force of attraction is called cohesion. At the surface of a liquid, the molecules have nothing attracting them from above. Consequently, all of the molecules are pulled or attracted toward the molecules below, causing an invisible film on the surface – this film is called surface tension.
Cohesion and surface tension are just two of the amazing properties of water that can be demonstrated through natural science experiments. Try the following two easy science projects to learn more about water and how it reacts to outside stimuli.
Easy Science Project – Can You Make a Needle Float on Water?
- Clear drinking glass
- Sewing needle
- Small piece of newspaper (a little wider and longer than your needle)
- Fill the drinking glass with water.
- Float the small piece of newspaper on top of the water.
- Carefully place the sewing needle on top of the paper.
- Use the toothpick to slowly push the edges of the paper, and then the entire paper, into the water.
- Does the needle float?
- Now push the needle with the toothpick.
- What happens now?
Observation: If you proceed carefully and slowly enough, you will cause the paper to sink, leaving the needle floating on top of the water. The needle floats on what appears to be a film on the water's surface. When you touch the needle with the toothpick, you cause it to break through the surface of the water, and gravity causes it to sink quickly to the bottom of the glass.
Explanation: Surprisingly, the needle floats, not because of buoyancy, but because of cohesion and surface tension. The surface tension of the water at the top of the drinking glass forms an invisible film that is able to support the needle.
Perform the experiment again after drying the needle thoroughly with a towel, but this time, look carefully at how the needle is resting on the water. You will observe that the surface of the water is bent under the weight of the needle!
Easy Science Project – Can You Keep Water Inside a Holey Bottle?
- Plastic bottle with a screw-on top
- Sharp scissors or nail
- An assistant
- Carefully use the tip of the scissors or nail to poke a small hole in the bottle's side near the bottom. (Ask an adult to help with this step.)
- Place the bottle under the faucet in the sink.
- Cover the hole in the side of the bottle with your finger.
- Ask your assistant to fill the bottle to the very top with water, leaving no room for air.
- Ask your assistant to screw the top onto the bottle.
- Move your finger to uncover the hole in the side of the bottle.
- What happens?
Observation: After few water escapes from the hole, the water stops leaking out of the bottle.
Explanation: Cohesion and surface tension are two of the forces at work here. Because the hole you made in the bottle is a small one, the water's tendency to cling together (cohesion) is able to form an invisible skin (surface tension) at the site of the hole, helping to keep the water inside.
Other Forces at Work in This Experiment: Air Pressure and Gravity
Because the bottle has been filled to the very top with water and then covered, the upper surface of the water is not affected by air pressure. However, the water at the site of the hole is acted on by the air pressure outside the bottle, which helps to keep the water from leaking out.
Now see what happens when you remove the screw-on top. Because air pressure is now acting on the water's upper surface as well as on the water where the hole is, air pressure is canceled out, and gravity takes over, causing the water to pour out of the hole. Surface tension and air pressure are both weaker forces than gravity, so gravity is the ultimate winner!
Science for Kids – Fun Experiments With Water
Isn't science fun and interesting? Now you know how to make a needle (which is not buoyant...) stay on top of water. You also know how to keep water from leaking out of a bottle with a hole in it – and why the water stays inside until you're ready to let it out.