27. Liquids and air

An article with 30 demonstrations with a glass of water is available from the author and in GIREP proceedings . These demo’s cover mechanics, liquids, optics, waves, and even electricity.

27.1. Liquids: Anybody with nail polish, coca cola, or something else that is liquid?

Whatever container or bottle is there, use it to show that the liquid surface is always horizontal; look at the edges of the meniscus: adhesion, cohesion; look at waves on the surface when disturbed with a pencil, study how easily the liquid goes down when the bottle is turned over, or is it slow and sticky?

27.2. Floating and sinking

Take all kinds of materials and objects and see whether they float or sink in a glass of water. Students would have lots of objects in their pockets. Some materials like clay or aluminum foil would sink, but if folded might float. From aluminum foil one could fold boats that can carry a load and still float. A Kindergarten class tried to fold pieces of foil of fixed area such that they would hold the maximum number of tiny St. Nicholas presents (tiny blocks or Lego pieces). This could be a teacher led demonstration, but also a brief activity by student pairs.

27.3. Floating and sinking

Anybody brought a tangerine? Will it sink or float in water? Why? Try it out. Then peel it and try again (figures 1 and 2). Try other kinds of fruits and explain the results.

Fig. 27.1 A tangerine floats and sinks…

27.4. Adhesion in a glass of water

See how the meniscus of water in a glass stands up to the wall, adhesion, that is attraction between two different substances, in this case glass and water.

27.5. Adhesion on a penny

Rinse a penny or other coin in tap water and dry completely and put it then on a paper towel or paper handkerchief. How many drops will fit on the penny? Record some predictions on the board. Then add drops one by one with a dropper until any amount of water runs over the edge of the penny. If you have more droppers, students can do this in their seats [@Hammack2016].

27.6. Adhesion and crumbs

Another example is picking up crumbs of cookies or bread from a plate, we lick our finger first and then the crumbs stick to it. That is an example of adhesion between water and crumbs. In class demonstrate this with a wet finger and very small pieces of paper like those coming from a perforator or -tastier- try cookie crumbs.

27.7. Capillarity 1

If you do have a very thin glass tube, then stick it into a glass of water and see the water move up … capillarity. Otherwise, take a piece of paper and dip in one end, the water will move upward in the paper. Try different kinds of paper. With the hair of a paintbrush or human hair, the same phenomenon occurs, “capilla” is the Latin word for hair. Capillarity can be seen as a tug-of-war between adhesion and gravity [@rogers2011physics, p95]. Water creeps up the glass wall due to adhesion and is pulled down by gravity. In very thin tubes the gravitational pull on the water creeping up the wall is smaller compared to adhesion than in wider tubes. Adhesion scales with surface area and gravity with volume. Capillary tubes have a large surface area compared to their volume.

27.8. Capillarity 2

Bring a sugar cube and a cup of coffee from the teacher room. Dip the edge of the cube in the coffee. Coffee will quickly climb into the cube, capillarity!

27.9. Cohesion

Fill a glass all the way to the top with water but such that the surface of the water is still concave. Then collect coins from your students. How many coins can I add without letting the water overflow? Then carefully insert the coins. Lots of them can be inserted in the water. In the end the water will bulge higher than the rim of the glass but it will still not flow down along the sides of the glass: cohesion!

27.10. Adhesion and cohesion

Use a straw or other means to make a drop of water on the table. Use different materials as surface (glass, wood, metal, plastic, paper). Rub some fat or wax on the table. Does that make a difference in the shape of the drop? Add some soap or detergent to the water. Does it make a difference in the shape of the drop? On which kind of surface is cohesion at a maximum? At what kind of surface is cohesion at a minimum?

Why is a drop of water round and the surface of a lake flat? This is the contest between surface forces and gravity. Think of a cube with side \(a\), surface area 6\(a^2\), and volume \(a^3\). Let \(a\) increase, then the surface area thus also surface forces scale as \(a^2\). The volume thus also volume forces like weight scale as \(a^3\)! With large amounts of water the volume forces dominate and so the surface of a lake is flat. With small amounts (a drop and smaller) the surface forces dominate such as with surface tension and capillary phenomena. A beautiful discussion can be found in [@rogers2011physics, p92].

27.11. Floating on convex and concave surfaces

Fill a glass of water until just below the edge (concave surface) and then fill another glass carefully until the water level is over the edge of the glass but the water does not drip down the outside of the glass. Draw the two situations on the blackboard or whiteboard. We are going to put a ping-pong ball (or anything that floats) on top of the water surface. Which way will it position itself, in the center or against the edge of the glass? Let students draw their prediction in their notebooks. Walk around to take a look and ask a few students for a reason. Then drop some floating material on the surface such as a ping-pong ball or pieces of cork or anything that floats. And then let students explain, perhaps first in small groups of two or three. In the end draw some explanations on the board and discuss. The ping-pong balls or other floating objects will go to the highest position and displace water downward. Objects that sink will displace water upward.

Fig. 27.2 Meniscus below the rim (concave, hollow).

Fig. 27.3 Meniscus above the rim (convex).

27.12. Chromatography

Take a strip of paper, make a big dot with a black marker. Then hang the edge of the paper in water. The water will move up (capillary motion), pass the black dot, and beyond that one will see a separation of colors. The different pigments in black ink move at different speeds in the water. That is how pigments can be separated. The same principle is used with chromatography which can be executed with different liquids and even different gases. Google on chromatography in the classroom and you will see many experimental possibilities.

27.13. Air pressure and liquids

Take a straw and a glass of water or better tea for visibility. Dip the straw in the water, close the top with a wet finger, then lift the straw out of the water while keeping the top closed. Why does water remain in the straw? This is a way to transfer small amounts of water. In a traditional pipette, one can suck up a liquid and measure exact amounts using the markings.

Fig. 27.4 Water stays in the straw when your finger closes the lid.

27.14. Air bubbles

Now close the top of the straw with your finger before inserting it. Now the water level in the straw will be below the water level in the glass. The latter will be clearer when we have a basin with water and push a glass upside down into it. There is something which prevents the water from coming in, air. The air in the glass can be compressed, but not all the way. Students can be reminded of their experiences when washing dishes. Try to pass air bubbles from one glass to another in the basin. Bubbles will move to the highest point available. Or stick a straw up to the bottom and blow air through it. The bubbles will move straight up in the water.

27.15. Transferring air bubbles under water

Take two glasses under water in a basin. One is filled with water, the other is pushed upside down into the water thus is filled with air (see previous demo). Try to transfer the air to the other glass. How would you do that? Play around a bit with the air bubbles, it is fun.

27.16. Air occupies space

Take a basin with water, a glass with a piece of paper handkerchief at the bottom, turn the glass upside down and push it into the basin (figure 7). Does the handkerchief get wet? What prevents it from getting wet?

27.17. Carbonated drinks

There always is a student with a carbonated drink in the bag. At the factory the CO\(_2\) is mixed with the drink. As CO\(_2\) has a lower density than water or whatever drink, it will tend to escape from the liquid and from the bottle. When opened, the pressure releases. When put in a glass we see bubbles rising. Why don’t bubbles rise when the bottle is still closed? There seems to be an equilibrium between the CO\(_2\) above the liquid and the dissolved CO\(_2\). Once opened the pressure in the top of the bottle is reduced and more of the dissolved CO\(_2\) rises to the surface.

27.18. Pressure, bubbles, straws

At the 1998 GIREP meeting Leon Jablko (1998) presented a series of pressure experiments with straws and glasses of water. The series could be teacher demo’s, or synchronized teacher and student demo’s, or it could be a student lab investigation.

27.19. Atmospheric and liquid pressure, cohesion and surface tension

Shouldn’t there always be a glass of water in the room? If so, then first have somebody help you to spread your clean handkerchief horizontal and pour water from the glass through the handkerchief to water the plants. Then put the handkerchief over the top of the glass with the remaining water (have a refill up to ¾). Make sure the wet part of the handkerchief covers the open end of the glass. Then turn over (figures 9 and 10). Surprise, a little water comes out, the rest stays. While the glass is upside down, walk around the room to show. Point to the shape of the part of the handkerchief that “supports” the water. Explain by applying Boyle’s Law to the trapped air and by the need for the sum of pressures/forces on the handkerchief to be zero. \(P_{air outside} = P_{air trapped} + ρgh\) where \(ρ\) is the density of water and \(h\) is the height of the water column. The cohesion and surface tension of water helps to make the handkerchief impenetrable. I have done it with a strainer also, more spectacular but it requires more preparation as not all strainers work. Of course, if students bring any drinks to class, one could try which liquids work and which do not and the instructor can have a good time. Anybody with beer?

27.20. Projectile motion with water

Many students bring plastic water bottles to class. With a pin or your knife make a little hole in the side of the bottle, just above the bottom. Water will come out and make a beautiful parabola.

27.21. Liquid pressure

Same bottle, screw the top tight. Water will still come out of the hole, but for how long? Does it stop? What happened to the pressure inside the bottle while the water was still going out? Consider a drop of water near the hole, draw the forces on the drop.

27.22. Liquid pressure and free fall

Bottle without top but with the hole. Water will come out, now drop the bottle. Did water come out while the bottle was falling? Why not? Repeat with the lid on (same result). What will happen in the space station if you turn a bottle upside down with the lid off?

27.23. Emptying bottles

This experiment requires a basin unless you hold the bottle outside an open classroom window. Turn de bottle with water upside down. The water will come out in bursts. It is difficult for air to come in and replace the water. Then swirl it to make the water rotate. Now the water will flow out continuously as air can flow in through the center to replace the water. You can also just tilt the bottle such that air can flow in while water is flowing out. If you have a basin, you can hold a competition between the vertical bottle without swirl, one with swirl, and the tilted bottle. Which one is empty first?

27.24. Emptying beakers/glasses

Put 2 or 3 pairs of students in front of the class, each with a table with a small bottle of water half full, a straw, and an empty glass. The task is to empty the bottle without lifting it. Some students will cleverly think that the straw could be used as a siphon. By blowing through the top of the bottle (with the straw siphon) one could even reinforce the working of the siphon (by blowing increasing the pressure above the water surface), however, just sucking up the water into the mouth and then spitting it into the glass works faster. Use this opportunity to ask students again to explain the physics of a straw and of a siphon.

27.25. Not free fall of stones in water

In air stones fall with the same acceleration g. But how is that in water? Take stones with a different surface area to mass ratio and drop them in a basin with water. Which one will sink faster?

27.26. Liquid versus vapor balance, relative humidity, saturation, open versus closed container:

Take two containers of water, one with an airtight lid and one without, put them on the windowsill in the sun. Figures 11 and 12 show the differences. In figure X we see condense on the inside of the glass above the water. The air above the water is saturated, 100% humidity, so water that evaporates will condense again. In figure Y we do not see this, any water that evaporates goes into the air, so the air right above the water is unlikely to reach 100% humidity unless the room is extremely humid.

27.27. Bernoulli and a candle flame

Light a candle, why does the flame point upwards? The heated air above the flame expands and becomes less dense. The burning gases along the wick move towards the location with the lowest density (lowest pressure). When I blow softly at the flame without extinguishing it, which way does the flame go? In the direction of the blow. And when I blow through a straw on one side of the candle? Then the flame bends towards that side, as there is the lowest pressure, a nice illustration of the Bernoulli effect. Explanation for a younger audience: I blow some air away, the burning gas is pushed to the place with least air.

27.28. Lift water by blowing

Use two transparent straws or cut and bend a straw as in figure 16. Take a glass of water. Position part of the straw in de glass, bend the rest (with a partial cut) horizontal. Then blow in the direction of a vertically held piece of paper. It gets wet. Use some ink or food coloring to make it more visible. Explanation? Fast moving air has a lower pressure, thus the water is sucked up through the vertical part of the straw and sprayed onto the paper. Practice a little bit before the lesson.