Journal 9/26/16 Objective Tonight’s Homework The picture here shows water sitting on a penny. What's going on with the water on this penny? Shouldn't it be flowing off the penny? Objective Tonight’s Homework To learn about the special properties of water Finish answering demo questions

Surface Tension and Capillary Action You guys already know what liquids are. However, you may not know about some of the special properties some liquids have.

Surface Tension and Capillary Action Many liquids have a property called “surface tension”. Surface tension holds liquid molecules together. Think of it like a room full of people where everyone has ropes tying them to the people next to them. Each person can still move, but they also tug on each other.

Surface Tension and Capillary Action If we put water in a glass tube, we see a special effect. Near the edges, the water starts to bend up. Why? If we could zoom in really close, we’d see water molecules piled up. These molecules are pulling on each other just enough to let us get a bit of a pile instead of it being totally smooth.

Surface Tension and Capillary Action The same thing happens with our penny. A lot of the water molecules want to spill over the edge, but they can’t because the ones near them are holding them back. Of course, if we add enough drops, the ones that want to spill over will be too many and it will all flow.

Surface Tension and Capillary Action This also explains how we can make a paperclip float on water. The paperclip is trying to push through the water molecules, but at first it can’t because the molecules are grabbing on to each other just strong enough to hold up the paperclip.

Surface Tension and Capillary Action Not everything works the same way as water, though. Mercury has such a strong surface tension that its atoms bend the other way in a tube. It really wants to stick together. Some other liquids have almost no surface tension at all and will lie flat and not support a paperclip.

Surface Tension and Capillary Action Related to this is something called “capillary action”. Without capillary action, straws wouldn’t work and fish tanks would be in trouble. So what is it? Let’s look at a tube and imagine we give some of the top molecules a good yank. As they move up, they’ll pull the ones below them as well, which pull the next below etc. This sucks all the water up and keeps it flowing.

Surface Tension and Capillary Action One of the easiest ways of explaining this is with this short video of polymer beads doing something very similar.

Surface Tension and Capillary Action This capillary action explains tons of things. If you suck on one end of a tube in liquid and get the liquid flowing, it will keep going on its own as each molecule is pulled by the one in front and pulls the one behind. When doing this with a tube, it’s specifically called siphoning.

Demos on Surface Tension and Capillary Action Mr. C is going to do a number of demos up front to demonstrate these concepts. As we go, answer the questions as they come up.

Demos on Surface Tension and Capillary Action 1) You can see the paperclip floating, but what would happen if we tried something a little bigger? 2) Why do you think the paperclip sank the instant we added a drop of soap? 3) Why do you think even the lightest things won’t float on the rubbing alcohol? 4) Compare the differences between the water, alcohol, and salt water graduated cylinders. Explain it using what we’ve learned today. 5) For your homework, write this up nicely and find 1 thing in real life that uses this capillary action.

Exit Question Would it work if you tried sucking on a straw on the space station? a) Yes b) No c) It depends on how big the straw is