What makes water so special? Whether it’s washing dishes after a meal, splashing in our favorite creek or getting a cool drink on a warm day, water is everywhere in our lives, and we use it in many different ways. Through a series of challenge activities, explore the unique properties of water that make life on Earth possible.
Age Level: 5 – Adult
Duration: Set up: 5 minutes; Activities: 15-20 minutes
Supervision Requirements: Set up may require adult assistance
Materials: Water glass, bowl, toothpick, flat glass surface (plate, cutting board, etc.), eyedropper or paintbrush, ruler, penny, small paper clips, hand towel, fork
Starting off with Some Science:
Have you ever heard water called H2O? This name refers to the atoms that make up every water molecule – two hydrogen (which have a slight positive charge (+)) and one oxygen atom (which has a slight negative charge (-)). Because of the way they attach to one another, every water molecule has a positive charge at one end and a negative charge at the other end. When lots of water molecules are grouped together – whether in a raindrop or a bowl of soup or Lake Superior – they stick to each other using a hydrogen bond. This structure and nature of water molecules is what gives water some of its most unique properties.
Cohesion Stretch
Materials
- Glass of water
- Toothpick
- Ruler
- Glass plate (or other flat glass surface)
Introduction
One property of water is that it likes to stick to itself. We call this cohesion. Without cohesion, plants could not suck water up through their roots, puddles would not form, drinking straws wouldn’t work, and it would be difficult for blood to move through our bodies.
Water also likes to stick to other things. We call this adhesion. Without adhesion, raindrops wouldn’t run down your window, minerals wouldn’t stay in the water we drink, and a spilled drink would fall on the floor as one big, jiggly bubble.
Because water exhibits both adhesion and cohesion, it behaves in some pretty unique ways. For instance, you need both properties for a dewdrop to form on a blade of grass! In this activity, we will test whether water sticks more to itself or to the surface it’s on by seeing how far we can “stretch” a drop of water on a glass plate.
Instructions
1. Drip 2 drops of water onto a glass plate or other flat, glass surface. They should form one bubble or puddle of water.
2. Drag the end of a toothpick through the water drop, pulling the water along as far as you can without it separating.
3. Using a ruler, measure the distance stretched from the starting point.
4. Take a closer look: water’s stick-to-itself property of cohesion may have caused the water to transform from a long, stretched line into a row of small, round domed drops. Water’s stick-to-other-stuff property of adhesion may have caused a skinny trail of water to stay stretched out from the starting point to the end. Which property can you observe?
Surface Tension Balance Beam
Materials
- Glass of water
- Eyedropper (don’t have one? Try a paintbrush or a toothpick)
- Penny
Introduction
Water’s cohesive property can be seen by looking at the surface of a still pool of water. Cohesion causes the water’s surface to act like a thin membrane that is always trying to stay stuck together. If something falls into the water, cohesion will try to resist that thing breaking through the surface of the water. We call this force surface tension. This phenomenon is what allows lily pads and water bugs to float on top of a pond, and it’s the reason that a sprinkle of pepper stays on top of a bowl of soup.
In this challenge, we will demonstrate surface tension by seeing how many drops of water will stick to a penny. Surface tension will try to hold the water together as a domed bubble on top of the penny, but gravity will try to pull the water down and off the penny. How many water drops do you think you can fit on a penny?
Instructions
1. Using an eyedropper (or paintbrush, or toothpick), transfer one drop of water from the glass to the penny. It helps to keep the glass and penny close to each other and to drip the water from a low height above the penny.
2. Keep dripping water one drop at a time onto the penny, counting as you go. Be careful not to touch your eyedropper (paintbrush, toothpick) to the penny. That could break the surface tension and spill the water!
3. Continue adding drops until the water collapses over the edge of the penny and spills off.
4. Note: If you’d like to try again, make sure the penny and the surface below it are completely dry before starting over!
Surface Tension Strength
Materials
- Bowl of water
- Several small paperclips
- Fork (optional)
Introduction
Surface tension is important to the survival of many aquatic organisms. Take, for example, the water strider bug – this little creature appears to float like a piece of wood or a boat. Actually, a water strider is more dense than water and would sink without the help of the strong bonds between water molecules that create the phenomenon of surface tension.
In this challenge, we will test our skills and the strength of surface tension by trying to see how many paperclips we can float in a bowl of water.
Instructions
1. Drop a paperclip into a bowl of water. If you let it fall quickly, it will sink! Paperclips are more dense than water. They don’t float!
2. We’re going to get a little help from surface tension to get a paperclip to stay at the surface instead of sinking. This time (using a completely dry paperclip!), set it as gently as you can on the surface of the water. Still sinking? Try using a fork to gently set the paperclip on the surface. Also, make sure you’re setting it down in the position it would sit on a table — flat! It won’t work if you set it on its edge or on its end.
3. Continue adding paperclips. How many can be supported by the surface tension in your bowl of water? When one falls through, do the rest stay at the surface or also sink?
4. Note: If you’d like to try again, make sure the paperclips are completely dry before starting over!
Dive in Deeper
Looking for something more? Try these extra challenges.
- Try these activities with a different scale of size. Can you do a cohesion stretch with a puddle instead of a drop of water? Can you do a surface tension balance beam with a nickel instead of a penny? What about surface tension strength with a bowl twice as big?
- The next time you’re out in nature, look for signs of adhesion and cohesion. Can you find a dewdrop on a blade of grass? Can you find a bug standing on top of a pond?
- Imagine: what would it be like if all the water in the world was replaced by playdough? Pick an action from the list below and perform what it would be like to do:
- Boil pasta in a pot of playdough
- Go swimming in a pool of playdough
- Water your plants with a bucket of playdough
- Brush your teeth with playdough that came out of the faucet
- Filled a goldfish’s bowl with playdough
- Skipped a rock over a lake of playdough
- What else would be different in the world if all Earth’s water was replaced with playdough?