Water, aka dihydrogen monoxide (two hydrogen atoms and one oxygen atom), is one of the most common substances on planet Earth. We can find it in the ground, as underground lakes (aquifers), rivers, and more! We can find it in the air, in clouds, falling as rain or snow, or just hanging out (this is what we call humidity). Everywhere you look, it is likely that you will find water as a pure substance (only water) or in a mixture (with other things).
Something that makes water so unique is it's "strength". Ever wonder how bugs can walk on water? Or why things can float on water? We have to understand surface tension! So let's dive into this activity to introduce you to this amazing physical property.
What you'll need:
penny or any coin
water
spoon, syringe, or eye dropper
optional add-ons: salt, sugar, oil, food dye, toothpicks, paperclips
Time to Explore!
Setup the coin: First, clean coin with soap and water, it is important to remove any oils from surface of coin in order for this to work properly. Coin does not need to be completely free of dirt.
Place clean coin onto a flat surface, to protect work area place a plate or towel under the coin to control where water goes.
Create your hypothesis: how much water can you add to top of coin before it spills? This is where syringe or eye dropper are preferred, as you can track and count the number of drops added.
Examples:
Only one spoonful of water can fit onto coin before it spills.
No water can be added before it spills, it will not stay on top of coin.
30 drops of water can be added to top of coin before it spills.
Test your hypothesis: Using spoon, syringe, or eye dropper; carefully add water to the top of coin. If using syringe or eye dropper, try your best to count how many drops are added.
Did your experiment support your hypothesis? If not, what can you do to change experiment and try again? Remember, a good scientist always repeats an experiment to make sure they get the same results each time. If after at least 3 attempts and your hypothesis is still not supported, then maybe your hypothesis needs to change. This is what we call the scientific process: hypothesize, test, re-test and/or re-design test, share results or re-do hypothesis.
The Science behind it!
Surface tension is a term used to describe how well a substance "sticks" together while at rest. We learned in the last blog, Candy Colors, that temperature affects how fast a substance moves. But also that the pepper did something very interesting in the presence of soap. But back to that change in temperature, it also affects the surface tension of a substance! Think of it this way, when playing a game of tag do you want to run or walk to get away from the one who is it? Now think of temperature as a running speed; the slower (colder) you are, the easier you are to get tagged (or stuck together). But if you are running fast, you are hotter, and it is harder to get tagged (or stuck together). So, as temperature increases, surface tension decreases since it is harder for the molecules to find each other. Once they find each other, it is like they are holding hands through cohesive and intermolecular forces, we can measure how strong this is using special tools and give it a value and unit. For example, speed is usually miles per hour and temperature uses degrees Celsius or Fahrenheit. For surface tension we use millinewtons per meter (mN/m), a newton is unit of energy or force and a meter is a specific surface area. The value varies depending on nature of the liquid (what it is: water, acetone, gasoline) and the purity of the liquid (is there anything dissolved in it?), each solution differs in surface tension depending on what it is.
Essentially, the surface tension of water allows 40+ drops of water to bead up on surface of a penny. As water temperature increases or decreases, the amount of water that can be added will decrease or increase, respectively.
SAFETY - very minimal, excellent activity to do with any age level!
Disposal: pour all liquids down drain and dry off coins to prevent corrosion.
For more fun!
So many ways you can experiment further with this activity, it is best to just let children use their imagination and let curiosity lead the way! Just make sure it is supervised exploration...
Challenge Accepted - who can add the most water to surface of their coin before it spills? This is best done when everyone has a eye dropper or syringe so that drops can be tallied.
Surface area - As the size of coin changes, can more water be added? Compare a penny, nickel, quarter, or any souvenir coin. Use a ruler to measure how wide the coins are, and take it a step further by plotting it on a graph!
Purity of water - what happens if you use a mixture instead of pure water? Try adding salt, sugar, oil, soap, vinegar, etc... to the water and see how much can be added to coin. Did it increase or decrease, meaning that surface tension was increased or decreased by the addition of another chemical - creating a mixture!
Objects floating on liquid - so long as the object doesn't rupture the "skin", the surface of liquid behaves like an elastic membrane that can resist the objects weight. This is how small insects can walk on water! It's also how you can float a paperclip, toothpick, or other small objects on water!
Laundry & Dishwashing Science (hot water, soaps, and detergent )- hot water has higher surface tension than colder water, which makes it ideal for getting clothes wet. Soaps and detergents reduce surface tension even more, so that water can enter further into materials and soiled parts - helping get them even cleaner!
Liquid droplets - many solutions can form round droplets because of the cohesive forces pulling it into a spherical shape
Surface tension disinfectants - solutions of low surface tension that spread out on cell walls of bacteria and disrupt them, reducing the likelihood of getting sick from these illness causing organisms
Water resistant/proof materials - fabric can be woven so tightly so that water molecules cannot penetrate because of water's high surface tension, however touching the fabric will break the surface tension and water will seep through. Fabric and other materials can also have a coating applied that prevents water from penetrating inside, but this is mainly because of polarity differences (revisit our blog on static electricity to learn more!)
For older kids (and parents too!) - continue (or start) using your science journal and take notes to record your observations from each experiment you do. “Remember kids, the only difference between science and screwing around….is writing it down!” Adam Savage, MythBusters.
Keywords: surface tension, temperature, intermolecular forces, hypothesis, scientific process
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