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6 Activities for Testing The Properties Of Water

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Testing The Properties Of Water - The Homeschool Scientist
 

Testing the Properties of Water

Before testing the properties of water, it’s important for your student is familiar with the properties of water.

In these activities, we are observing the following properties:

  • Polarity
  • Cohesion
  • Adhesion
  • Evaporative Cooling

But before we get started on the activities, let’s understand these properties.

Polarity

A water molecule is made up of two hydrogen atoms and one oxygen atom.

There are two ends to a water molecule. The two positively charged hydrogen atoms at one end and the negatively charged oxygen atom at the other end give the water molecules two poles. The negative charge is on the oxygen side of the molecule and the positive charge is on the hydrogen side of the molecule.

This combination of atoms makes water a polar molecule.  Polar molecules have a negative side and a positive side.

In the illustration below are two water molecules. Have your student point out the positive and negative sides of each molecule.  (Hint: the 2 hydrogen atoms are the positive side where you see the + below. The negative side of the atom is the oxygen end where you see the – below.)

 

Water is also neutrally charged. This means it has an equal number of protons and electrons.

However, even though a water molecule has an equal number of protons and electrons, there is an uneven distribution of the electrons within each water molecule that makes it polar.

The oxygen atom spends more time with electrons than do the hydrogen atoms. Since electrons spend more time with the oxygen atom, the oxygen atom carries a partial negative charge.

So, polarity describes the distribution of electrical charges around a molecule. Charge is unevenly distributed in a polar molecule and evenly distributed in a nonpolar molecule.

In the diagram below you can see a variety of polar molecules.

Before we move on, let’s talk about nonpolar molecules. Non-polar molecules do not have a positive end and a negative end. The electrical charges of nonpolar molecules are evenly distributed across the molecule. Oxygen, carbon, and carbon dioxide molecules are examples of nonpolar molecules.
 

So remember, polar molecules are attracted to polar molecules. Nonpolar molecules are attracted to nonpolar molecules.

We’ll observe polarity when we complete the activities below that test the properties of water.

Cohesion and Adhesion Properties of Water

The polarity of water gives it some special properties, like cohesion and adhesion, that can be easily demonstrated.

Molecules that are polar will mix with each other. This is called cohesion. This happens in water because the negative charge of the oxygen atom in a water molecule is attracted to the positive charge of the hydrogen atoms in another water molecule.

 Cohesion simply means that water molecules like to stick to each other. This is caused by the slightly negative charge of the oxygen atom of one water molecule being attracted to the slightly positive charge of the hydrogen atoms of another water molecule.

The hydrogen bonds between water molecules is illustrated below. Again, have your student point out the positive and negative end of each water molecule.


If you have two magnets handy, a great way to demonstrate the concept of a negative end of a water molecule being attracted to a positive end of another water molecule is to hold the north pole of a magnet to the south pole of a magnet. They are attracted to each other and “stick” together.

Surface Tension

Water molecules like to stick together. On the surface where the water meets the air, water molecules cling even more tightly to each other. This causes a “skin” to form on the surface of the water. This skin is so strong that it can hold a weight that normally would sink in water. This is called surface tension.

When the surface tension is disrupted, the heavy object that is floating on the skin will sink. A light object on the surface will be pulled by the attraction of the water molecules if the surface tension is disrupted. This easy experiment will demonstrate that phenomenon.

Cohesion

The surface tension of water is cause by cohesion.  Cohesion means that the water molecules like to stick to each other. This is caused by the slightly negative charge of the oxygen atom of one water molecule being attracted to the slightly positive charge of the hydrogen atoms of another water molecule.

Adhesion

Adhesion is the attraction of molecules of one kind to molecules of a different kind. For example, water droplets on a flower is an example of adhesion because the water molecules are attracted to the molecules on the flower.

Evaporative Cooling

Evaporative cooling is the process of removing heat from a surface due to the evaporation of water. When we are outside on a hot day and sweat forms on our skin, that is an example of evaporative cooling. The heat energy from our bodies is transferred to water molecules (sweat), and that evaporating water removes the heat energy from our bodies, helping to keep our bodies cooler.

Let’s get started testing the properties of water. You may want to print the worksheet linked below and have your student fill it out for the liquids used here and for other liquids on hand.

Related post: Properties Of Liquids Worksheet

Testing the Properties of Water – Surface Tension, Cohesion, Polarity

Testing the Properties of Water:  Surface Tension

You can test surface tension by filling a glass with water and gently laying a needle on the surface of the water using. The needle has a higher density than water, it will sink if just dropped in the water. However, if the surface tension is not broken, the needle can be placed on the water in such a way that the surface tension of the water holds the needle on the surface.

Testing the Properties of Water – Cohesion

Test the cohesion properties of water using an eyedropper, water, and a coin.

Testing the Properties of Water

Slowly, drop water onto a coin. Watch as the drops of water stick together to form a larger drop.

Testing the Properties of Water

The water molecules will stick together and form a dome over the coin. Keep adding drops until the drop breaks and spills off of the coin. This is caused by gravity overcoming the force of the cohesion.  How many drops of water can you fit on a coin?

You can also observe cohesion by filling a glass of water to the top. Next, use a dropper to very carefully add more water until the water is forming an arc of water slightly over the top of the glass. The cohesive properties of the water are holding the molecules together so that they will not spill over the top of the glass. There will come a point when the weak hydrogen bonds can no longer hold and the water will spill over.

Testing the Properties of Water – Polarity

In this test we are going to use water, vegetable oil, and wax paper.

As we mentioned before, water is polar. What did we say about polar molecules being attracted to polar molecules?

Vegetable oil is nonpolar. Again, what did we say above about nonpolar molecules being attracted to nonpolar molecules?

What you need to gather for this test:

  • Water
  • Food coloring (Optional, but it does help your student see the testing results.)
  • Vegetable oil
  • Several dinner plates
  • A dropper or you can carefully drop the liquids from a spoon
  • Glass for mixing the water and food coloring
  • Spoon
  • Tablespoon (if you want to be precise. We just poured small amounts on to the dish.)

Start out by mixing some food coloring into about 8 ounces of water. You can always mix more, if needed.

Pour a tablespoon or so of the colored water on a dish. Then pour the same about of water next to it. Move the dish so the water drops are able to run into each other. What happens?

Explanation: They become a bigger drop of water together. Remember polar molecules are attracted to each other.

Clear the plate, dry it well or use a second dish.

Place a tablespoon or so of oil on the dish. Next, place a second amount oil next to it. Move the dish so the oil drops touch. What happens?

Explanation: The two drops of oil jump together because they are both nonpolar.

Grab a new plate or wash and dry the first plate really well.

Place a tablespoon or so of oil on the dish. Next, place a tablespoon or so of the colored water onto the same dish. Move the dish so the drops touch. What happens? Do the drops form a bigger drop together? Notice how the water will move around the perimeter of the oil? Even when the two liquids do seem to come together, the water can move off of the oil, and they don’t completely come together.

Explanation: This is because oil is nonpolar and water is polar.

More Polarity Testing

For this activity you’ll need:

  1. A few tablespoons of colored water or plain water
  2. A few tablespoons of vegetable oil
  3. A dropper or small spoon
  4. A sheet of wax paper

Instructions:

  1. Tear off a square sheet of wax paper and place the waxy side up.
  2. Place it on a dish.
  3. On one end of the wax paper, place a teaspoon of water.
  4. Next, pour a teaspoon of oil on the wax paper a few inches apart from the water.
  5. Tilt the wax paper slightly and let the water and oil run down the paper.

Are the two liquids sliding down at the same rate? If one is faster, which is it? Why do you think that liquid is faster?

Explanation: The wax paper is nonpolar and the oil is nonpolar, so they have some attraction that causes the oil to “hang onto” the paper as it moves down. The water is polar and since polar and nonpolar molecules are not attracted to each other, the water slides down the wax paper at a faster rate than the oil.

Testing the Properties of Water – Evaporative Cooling

As discussed above evaporative cooling is the process of removing heat from a surface due to the evaporation of water. The sweat our body produces when we are hot is one example. Here’s another example…

You’ll need some hand sanitizer. Squeeze some onto your student’s hand and have them rub their hands together.

Do their hands feel cooler now that they are wet from the sanitizer? They should answer yes.

Wat a few minutes until the hand sanitizer evaporates. Ask if their hands are drier. They should answer yes.

Have them wipe their hands dry and wait a few minutes.

Next, squirt hand sanitizer back into their hands and distribute it all over their hands. Have them wave their still-wet hands in the air. Ask if their hands feel cooler.

They should answer yes because moving their hands through the air caused the hand sanitizer to evaporate more quickly. During the evaporation process, the heat energy on the surface of their hands was transferred off.

Testing the Properties of Water – Adhesion

As discussed above, adhesion is the attraction of molecules of one kind to molecules of a different kind.

There are two activities you can do to demonstrated adhesion.

Activity 1

You’ll need:

  • a sheet of paper towel
  • bowl
  • 2 cups of water
  • a few drops of food coloring
  1. Mix the water and food coloring and stir.
  2. Dip one corner of the paper towel into the water in the bowl.
  3. Watch the water seep up into the paper towel. The water molecules are attracted to each other and to the paper molecules.

Activity 2

You’ll need:

  • 2 plastic cups (clear is best)
  • 1 cup of water
  • a few drops of food coloring
  • piece of cotton string
  • tape
  1. Pour the water into one of the cups and mix in some food coloring. Stir.
  2. Tape one end of the string into the other plastic cup.
  3. Hold the cup with the colored water and string above the empty cup and to the side of it until the string is taut. Also, make sure the string in the dry cup is not touching the sides of the cup.
  4. Slowly pour the colored water out of the cup and down the string.
  5. Notice how the water travels down the string. The coloring helps us see how the water molecules are traveling along the molecules in the string.

What is happening:

The water molecules that are moving down the string are attracted to molecules in the string and the other water molecules. The property of adhesion is when the water molecules are sticking to molecules of a different type.

Now not all of the water molecules stick because some of the water did drip into the cup. This is because adhesion is not as strong as the property of cohesion.

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