Opening up the world of the states of matter: liquids – part 1

There are many states of matter.  The most common ones are solids, liquids and gases. These are also the three states of matter within easy reach of a home, parent and child. Previously we studied solids. This time we focus on liquids. In liquids the atoms are able to move and are no longer contained in the structure of the solid. In a liquid the forces that hold the molecules together are still strong, but the molecules have enough energy to move and the structure is mobile. In other words it flows, moves and the shape of the liquid is determined by its container (or the lack thereof, like the floor). To move from solid to liquid, one needs to add heat. When a solid is heated above its melting point, it becomes a liquid. Basically what happens is that the atoms in the solid gain energy and with that added energy they break free of the structure that keeps them in place. If even more heat is applied to the liquid, it starts to become gas, but more on gases later.

Teilchenmodell_Flüssigkeit.svg

Structure of a classical monatomic liquid. Atoms have many nearest neighbors in contact, yet no long-range order is present. Picture from Wikipedia. CC BY-SA 3.0

Liquids have many ways in which to study them and many important concepts to study such as buoyancy, density, viscosity and surface tension. First, however, we looked at some liquids (milk and water). I poured them in various shapes of containers and we watched how they behave (they take the form of the container). Then we had a lot of fun with water balloons. They are an excellent way to study the shape of liquids and kids (and adults too I think) find it a lot of fun to play with these. First the balloon is filled with water and the water takes the shape of the balloon. We used standard balloons, which are not exactly meant for water and became enormous and very heavy. I did not fill them all in, so when I dropped it on the bathroom floor, it just bounced. I had to use quite a bit of force to throw it on the floor before it burst. At that point, or more precisely shortly after, the water takes the form of the floor.

Picture of a water balloon and a picture of it breaking on the floor. Experimenting with the shape of liquids.

Experimenting with the shape of water with water balloons. First the water takes the shape of the balloon and later the shape of the floor

Next we looked at viscosity. Viscosity as a term refers to a state of being thick, sticky and semi-fluid in consistency due to internal friction. This friction resists movement, so viscosity is friction in liquid. Previously we have studied friction in solids here and in gases (air resistance) here. I still had some of the sugary liquid used in the crystallisation experiment when we made rock candy, so we used that. That was very viscous. We also used a glass of water, because the viscosity of water is very low, so they are easy to compare.

Four images of water (low viscosity) and sugar water solution (highly viscous) and what happens when they are moved by mixing and blowing air into them

Experimenting with viscosity using water and sugar-water-solution from rock candy experiments. Top left showing the liquids. Top right showing what happens when air is blown into water and bottom two images showing what happens when air is blown into a viscous fluid

We had two glasses of liquids. I put a stick into both of them and asked my daughter to mix them. As she was doing this I asked her which moved more easily. She said water. Then I took two straws and put them into the liquids and asked her to blow into them. In the water the bubbles came easily and made the entire thing move quickly everywhere. In the sugar-water-solution it took more effort to make the bubbles and they took more time to get to the surface and pop. I told how this is due to viscosity, which resist movement and reminded her about our earlier experiments on friction and air resistance.

Next we looked at density and buoyancy. Density is a measurement of how much stuff is in the item or liquid. The same amount of water and sugar water solution (1dl for instance) hold different amounts of stuff in them. Water is less dense than the sugar-water-solution. Buoyancy is an upward force, which affects submerged objects, making them rise if they are less dense than the fluid they are in. If they are more dense, they will sink and if they are equally dense, they will remain in at the same level.

First we took some items, like a magnetic letter, a screw, a raisin and a pacifier. We plopped them into the water to see which would float and then I remarked upon the density of each. The screw and raisin went to the bottom as they are more dense than water. In the magnetic letter the magnet wanted to be at the bottom but the rest of the letter did not, so there were two different densities in the same item. The pacifier floated. Then, the next time daughter took a bath, we experimented with a small bottle. We experimented with how filled it had to be in order for it to sink. It had to be filled pretty completely to sink, but it was fun to experiment with.

Two experiments with density: floating different items in water and altering the density of raisins

Experiments with density. Top: floating different items in water. Bottom: altering the density of raisins and creating movement up and down as raisins dance

Finally we experimented with changing the density of raisins as we were studying buoyancy. I bought some soda water (carbonated water) and dropped some raisins into it. The raisins in and of themselves are more dense than water so they sink. During this process though, as they are very wrinkly, some air remains in the creases of the raisin and that air aids in the creation of larger gas bubbles, which, when there are enough of them, start to lift the raisin up to the surface. Density is always about the whole item, so air+raisin is less dense than water, so it floats. At the surface the raisin releases its gas and regains its previous density and sinks. And so they go up and down and up and down for some time until the gas from the water has been released.

Next was surface tension and capillary action of water.  The surface tension of water is very strong. This is why water likes to be in droplets and when possible tries to create a round shape. Raindrops are never the shape they are most often drawn because the surface tension of water would not allow a small sharp point to be created. Some things float because of this, like water striders and some plant leaves. Capillary action is the thing that is responsible for the growth of trees and plants. Plants have small tubes inside of them. They are so small that as water is evaporated from the leaves of the plant, surface tension is enough to raise more water from the ground. This is capillary action (or capillary movement). So capillary action of water can raise water up against gravity.

First I took a small flat object and put a string in it so I could lift it from the surface. I then tried to lift it and asked my daughter to look at whether the water came up with the item. Some did, even though it is not very easy to detect. I then asked her to lift it and asked her if she could feel when the water let go. She is four, so she could not, but that is ok. We then put some soap into the water and repeated the experiment. She loved to mix the soap into it, but she did notice that now the water did not follow. I explained that this is why soaps work. They break up the surface tension of water and allow it to enter the cloth better to remove the stains. Simplified, but then again, she is now just five.

A picture of raising a flat object from the surface of water to test surface tension

Studying surface tension with a flat object, water and soap.

Secondly I took three different cloths and put the bottom of them into water. We then left the water to rise. Later we returned to measure which cloth could bring the water highest. While we did this, I talked about capillary action in plants and cloths.

Finally we created a water flower. To do this you need to cut a circle of paper. Then make incisions into the circle to create petals (see image). Then you fold the petals into the center and place into water. Water starts to enter the paper and fills it up (capillary action) opening up the petals. And the entire thing floats due to surface tension. Daughter loved this one. Luckily we had many flowers.

Surface tension_flower

Studying surface tension and capillary action with a flower, which opens up due to water rising up in the petals

More liquid experiments to follow…

If you try these experiments at home and your child asks questions, please let me know what they ask in the comment section so I can further develop these instructions. If you have questions about these experiments or instructions, leave me a comment and I will answer and also improve these instructions. Also, please remember like and share if you find this useful.

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