Liquids

Underwater Fireworks

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Fireworks are fabulous! We love watching the colors explode and gently float down through the air, but we can’t watch fireworks all the time. Underwater fireworks are something we can watch any time, even in our kitchen!

The Experiment

Supplies: Water, vegetable oil, food coloring (any color), a large clear glass, a second smaller glass, a fork

What to do: Fill the large glass almost to the top with room-temperature water. Pour 2 tablespoons of oil into the other glass. Add 2 drops of food coloring to the glass with the oil. Vigorously stir the oil into the food coloring using a fork. Stop once you break the food coloring into little drops. Pour the oil and coloring mixture into the tall glass. Now watch! The food coloring will slowly sink in the glass, with each droplet expanding outwards as it falls.

What is happening: Food coloring dissolves in water, but not in oil. This has to do with the incompatibility of the molecular structures of the water and the oil. When you pour in the food coloring/oil mixture, the oil will float at the top of the water because it is less dense. The food coloring will begin to dissolve once it sinks through the oil and into the water.

Take It Further

Variations on this demonstration:

  • Try mixing in one drop each of two different food colorings.
  • What happens if you omit the oil and drop the food coloring directly into the water?
  • Try varying the size or shape of the water glass, the amount of the oil, or the amount of the food coloring. Just remember, when you are manipulating variables to only change one thing at a time!

Links

For a more detailed explanation of the miscibility of fluids, along with a more expansive version of this demonstration, head over to the Scientific American website.

Water That Coin

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There is an adage that says, “Like attracts like.” It basically means that similar things tend to stay together. It is possible to witness this in action simply by dropping water onto a penny.

The Experiment

Supplies: A paper towel, a penny, a small glass, an eye dropper or pipette, tap water.

What to do: Wash the penny well with soap and water. Make sure to dry it thoroughly. Set the penny flat on the paper towel. Fill the glass with water. Use the pipette or eye dropper to remove some water from the glass. Carefully place drops of water onto the penny, one at a time. How many drops of water can you place onto the penny before the bulge of water runs off the coin?

What is happening: As you place the drops of water onto the coin, you should see the water forming a small, dome-shaped bulge or bubble. Eventually, the bubble will “burst”, causing the water to run off over the edge of the penny.

The water bubble is a result of two things – cohesion and surface tension. Cohesion is when particles of the same substance stick together – “like attracting like”. Water molecules behave sort of like a bar magnet. They have a positive end and a negative end. The positive end of one water molecule is drawn to the negative end of the next water molecule. This cohesion draws the molecules tightly together, forming the dome-shaped bubble.

The outside of the bubble has surface tension. The attractive force exerted upon the surface molecules of the water by the molecules beneath tends to draw the surface molecules into the bulk of the water and makes the water assume the shape having the least surface area. In other words, the cohesion of the molecules is trying really hard to make the surface of the water (that area of the bubble where the water molecule has no neighboring molecule to cling to) as small as possible. The molecules on the surface are more attracted to the other water molecules than they are the air molecules around them, so they form a bubble. As long as the surface tension is greater than the force of gravity, the bubble will remain intact on top of the penny.

Take It Further

If you repeat this test several times, you will discover that the number of drops you can put on the penny each time changes. This is because the water you are using is changing each time. When an element of an experiment can change, that element is called a variable, because it can vary from test to test.

Try repeating this experiment using different variables. Some options include changing the type of coin you are using. Try it on a nickle, a dime, and a quarter. What do your results look like when comparing water on coins with smooth edges vs. water on coins with ridged edges? Another variable could be the liquid. What happens if you use bottled water instead of tap water? What about using salt water? You could also try using other liquids, like soda, fruit juice, or vegetable oil. You can also try rubbing alcohol or hydrogen peroxide. Just remember, if you are going to change a variable, make sure you only change one thing at a time. That will allow you to identify how the change in variable alters your results.

Links

To see this experiment in action, watch this video from Sick Science!
To explore more about cohesion and adhesion of water, visit Khan Academy.

Exploring Density

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Density describes the relationship between a substance’s mass, or weight, and its volume, or how much space it takes up. Things that are more dense take up less space than things with less density. To visualize density, let’s compare baseballs and marshmallows. A regular baseball weighs about one pound. It is small and compact and fits in the palm of your hand. A 16 oz. bag of marshmallows also weighs one pound, but because there is more air incorporated into the marshmallows, they are less dense. One pound of marshmallows takes up a lot more space, or volume, than a one-pound baseball.

When you combine substances that have different densities, the substances with the greatest density tend to move toward the bottom, while those with lesser densities tend to rise to the top. Gases tend to be lighter and less dense than liquids. Liquids tend to be lighter and less dense than solids. Even within these groups, there are a variety of densities.

The Experiments

Simple Experiment

Supplies: A clear jar with a lid, vegetable oil, water, food coloring (optional).

What to do: Fill the jar about half-full with water. Add food coloring, if desired. Pour in vegetable oil until the jar is almost full. Put the lid on the jar and MAKE SURE IT IS TIGHT. Give the jar a good shake so that the water and oil are thoroughly mixed. Set the jar where it won’t be disturbed and observe the liquid.

What is happening: The oil is less dense than the water, so it rises up to “float” on the surface of the water. The water and oil do not mix because of the molecular properties of each compound. Water molecules tend to want to “stick” to other water molecules, while oil molecules tend to want to stick to other oil molecules. This is because of something called molecular polarity, where the structures of the two molecules are not compatible. They push each other away, similar to a pair of magnets that won’t stick together. The longer the jar sits, the more the water and oil will sort themselves out, until they are completely separate again.

Complex Experiment

Supplies: A large jar or clear glass cylinder, liquid measuring cup with pour spout, a turkey baster, different colors of food coloring, honey or molasses, light corn syrup (Kayro), blue liquid dish soap, rubbing alcohol, yellow corn oil, water.

What to do: Pour 1 cup of honey or molasses into the bottom of your jar. Measure out 1 cup of light corn syrup and add some red food coloring. Stir until well combined. Carefully pour the corn syrup into the jar, making sure to avoid hitting the sides of the jar. Measure out 1 cup of dish soap. Slowly add the dish soap, again avoiding the sides of the jar. Measure out 1 cup of water and add to the jar, but this time use the turkey baster to slowly drizzle the water down the side of the jar. Measure out 1 cup of corn oil and add to the jar, again using the turkey baster. Finally, measure out 1 cup of rubbing alcohol. Add green food coloring and stir well. Add the rubbing alcohol into the jar using the turkey baster.

What is happening: Different liquids have different densities. Liquids like honey and dish soap are more dense than water, while other liquids, like rubbing alcohol and vegetable oil are less dense and will float above the water. Adding the food coloring helps distinguish the different layers. The various densities of the different liquids is what keeps the layers separate.

Links

You could take the Density Column experiment one step further by dropping in solid objects to see where they land, OR you could just watch this great video from the Bearded Science Guy.