In this explainer, we will learn how to explain the properties of the polar molecule of water.
Water is critical for the survival of all organisms on our planet. Without adequate and sustainable water supply, agriculture and industry would not function. As individuals, we use water every day for cooking, cleaning, and drinking.
A single molecule of water has one atom of oxygen and two atoms of hydrogen.
There is a covalent bond between each hydrogen atom and the central oxygen atom. So, the molecule has two chemical bonds, and the angle between these chemical bonds is . The molecular formula of water is .
Every element in the periodic table has a property known as electronegativity.
Electronegativity measures how strongly an atom attracts an electron pair (or electron pairs) from a chemical bond.
Let’s consider a molecule of water. Oxygen is more electronegative than hydrogen. So, the electrons of the covalent bonds are closer to the oxygen atom and further from the hydrogen atoms.
We can see this effect illustrated in Figure 2. The orange arrows show how the electrons forming each covalent bond are pulled closer to the oxygen atom due to its higher electronegativity.
The electronegativity of oxygen being greater than hydrogen causes the negatively charged electrons to be positioned closer to the oxygen atom. As the electrons are closer, a partial negative charge forms on the oxygen atom. Partial charges in chemistry are like the full charges you might find on ions. However, a partial charge is much weaker than a full charge.
Scientists call molecules polar when they have permanent partial charges. There are some exceptions to this, but we do not need to investigate this in this explainer; we can say that water is a polar molecule.
Definition: Polar Molecule
A polar molecule is a covalent molecule where the difference in electronegativity between the elements in the molecule is relatively large. In a polar molecule, one end of the molecule is slightly positive, while the other end is slightly negative.
Example 1: Using Electronegativity to Explain Why Water Is a Polar Molecule
A single water molecule consists of two hydrogen atoms and one oxygen atom. Which of the following statements explains why water is a polar molecule?
- Hydrogen atoms are much smaller than oxygen atoms.
- There is a big difference in electronegativity between hydrogen atoms and oxygen atoms.
- Intermolecular forces known as hydrogen bonds exist between water molecules.
- The electronegativity of hydrogen atoms and that of oxygen atoms are very similar.
- There are more hydrogen atoms than oxygen atoms within each water molecule.
A polar molecule is a covalent molecule with a significant difference in electronegativity between its elements. This difference in electronegativity means the molecule has permanent partial charges.
Oxygen has a much higher electronegativity than hydrogen. The pairs of bonding electrons from the two covalent bonds are attracted closer to the oxygen atom.
If we look at the answers, we can see that B is the correct answer; there is a big difference in electronegativity between hydrogen atoms and oxygen atoms.
Example 2: Locating the Partial Charges on a Molecule of Water
Oxygen is more electronegative than hydrogen and attracts the bonding electrons in a molecule of water closer to itself. and are used to show partial charges in chemistry. Which diagram correctly shows the partial charges on a molecule of water?
The strength by which an atom attracts the pair of electrons used to form a chemical bond is measured by electronegativity. Since oxygen has a higher electronegativity value than hydrogen, the electrons that form covalent bonds are drawn closer to the oxygen atom and away from the two hydrogen atoms.
Consequently, the oxygen atom has a partial negative charge , and the two hydrogen atoms have a partial positive charge as a result.
We can use this information to determine that the correct answer is A.
Another common polar molecule is ammonia (). Ammonia is a simple molecule consisting of one nitrogen atom and three hydrogen atoms. Similar to the oxygen in a water molecule, nitrogen is much more electronegative than hydrogen, resulting in partial charges within the molecule.
Returning to our focus on water, the permanent partial charges in each water molecule cause a weak electrostatic attraction to form between the partial charges of neighboring water molecules. These attractions can also be referred to as intermolecular forces.
We call these weak electrostatic attractions between water molecules’ hydrogen bonds. Although we use the word bond, it is not a true chemical bond and is best thought of as an interaction between molecules.
Hydrogen bonds are weaker than covalent bonds but are incredibly important when explaining and describing the physical properties of water.
If you were to try to imagine the billions upon billions of water molecules that exist in a glass of water, you could begin to see how hydrogen bonds form and break continuously as the molecules rotate and move around, colliding with each other and the container. This idea is shown in Figure 5, where the hydrogen bonds are shown as dotted blue lines.
The chemical bonding and intermolecular forces of attraction we have just discussed give water some very unique properties. For example, Figure 6 shows examples of water in all three states: as a liquid in the ocean, as solid ice in the iceberg, and, although we do not see it, as water vapor in the air and clouds.
Water is a polar molecule; as such, other polar compounds can dissolve into water, forming solutions. For this reason, we can describe water as a polar solvent.
Substances such as sugar are polar substances, and they dissolve in water. Also, ionic substances like salt, which have contained charged ions, dissolve in polar substances. We only need to think about salt in oceans or in a cup of tea to realize that we have had first-hand experience of these examples.
A nonpolar substance is a substance that does not contain partial charges because the difference in electronegativity between the elements of the atoms in the molecules is very small.
Definition: Nonpolar Molecule
A nonpolar molecule is a covalent molecule where the difference in electronegativity between the elements in the molecule is very small.
However, nonpolar solvents, such as cooking oil, do not dissolve in water, and when added to water, they separate into two layers.
Water is an excellent polar solvent for most ionic substances, such as salt, and some covalent compounds, such as sugar, that form hydrogen bonds with water molecules.
Example 3: Naming Solvents That Dissolve Ionic Substances
What is the name given to a solvent like water that is able to dissolve ionic substances such as salt?
- A polar solvent
- A nonpolar solvent
- An electrostatic solvent
- An ionized solvent
- A hydrogen solvent
Ionic substances such as sodium chloride and other salts contain ions. Ions have positive and negative charges. When the charge is spread out in this way in chemistry, it is referred to as polarity.
Some other substances without ionic charge still have partial charges and are also referred to as polar. For example, water is a polar molecule.
We can use a beaker of water to dissolve a tablespoon of sodium chloride. A substance that can dissolve polar substances such as ionic salts is known as a polar solvent, so the correct answer is A, a polar solvent.
The hydrogen bonds between water molecules also significantly affect the boiling and melting points of water. Let’s take a moment to compare water with another compound: hydrogen sulfide (). Hydrogen sulfide is very similar to water; it has a sulfur atom instead of an oxygen atom.
Sulfur is in group 16 of the periodic table like oxygen, so we might imagine that the melting and boiling points are very similar. However, as shown in Table 1, the melting and boiling points of water and hydrogen sulfide are very different!
|Compound||Melting Point ()||Boiling Point ()|
We can see from the table above that more energy is required to separate water molecules during melting and boiling. This extra energy is necessary to overcome the hydrogen bonds that exist between the individual water molecules. Hydrogen sulfide molecules cannot form hydrogen bonds as the difference in electronegativity between sulfur and hydrogen is not large enough.
Another remarkable characteristic of water is its density when in the solid form known as ice. Most substances are denser when they are in their solid state; however, water is less dense. As liquid water cools down to less than , the water molecules form large hexagonal crystals due to longer hydrogen bonds between the individual molecules. When the liquid freezes at , these large hexagonal crystals are fully formed, and the longer hydrogen bonds create a relatively large amount of space between molecules.
We know that density is a way to express the amount of mass in a given volume, and when water takes the form of solid ice, the ice has less mass than the same volume of liquid water; therefore, water gets less dense as it freezes and turns into ice. We can see the longer hydrogen bonds and hexagonal arrangement of water molecules in ice in the figure below.
Example 4: Matching the State of Matter of Water to Its Structural Arrangement
The picture shows water molecules in a uniform fixed hexagonal arrangement created by hydrogen bonds.
What state of matter would you expect to find water in when it is in this structural arrangement?
- Ice (solid)
- Water (liquid)
- Steam (gas)
Water molecules form large hexagonal crystals when liquid water freezes at . These hexagonal crystals form due to the existence of hydrogen bonds between the individual molecules.
In their liquid state, individual water molecules are free to move and rotate with hydrogen bonds, continually forming and breaking between different molecules.
In the gaseous state, steam water molecules can be thought of as independent units with little interaction.
If we compare these three descriptions to the diagram above, we can see that the correct answer is A, ice (solid).
This decrease in density as water freezes explains the expansion of ice as water freezes. As the water freezes, the mass of the water of course does not change, but the volume increases, and therefore, the density must decrease. The expansion of ice as water freezes is responsible for physical weathering phenomena such as freeze–thaw action, shown in Figure 9. It might also be witnessed at home when bottles of drinks explode in the freezer!
Example 5: Explaining Why Ice Floats on Water
Which of the following sequences of statements correctly describes why ice floats on water?
So, we can start answering this question by stating that when water freezes, it becomes ice, so we know that answer C is incorrect.
Due to the hydrogen bonds that exist between molecules of water, water has some unique physical properties. When water freezes and turns to ice, longer hydrogen bonds form as the water molecules form hexagonal crystals. These hexagonal crystals have a relatively large amount of space between them due to the longer hydrogen bonds and cause water to expand.
We can now see that answer E is also incorrect.
When water freezes, the amount of water and consequently its mass do not change, but as the water expands when it freezes and becomes ice, it has a greater volume. With this information, we are now able to eliminate answer D.
If any substance transforms to have a greater volume but the same mass, then the density will decrease. In the case of water, this makes ice less dense, and so, the correct answer is A.
Seawater contains salt, and this salt affects its density. In areas of the ocean where the concentration of salt is lower, such as the North and South Poles, it is more difficult to swim as it is harder to generate force against the less dense liquid. In contrast, there are some places, such as the Dead Sea in western Asia, where salt concentrations are so high that people can float with virtually no effort at all!
We will begin to conclude this explainer by looking at some of the chemical properties of water. Water is a neutral compound and does not display any acidic or basic properties. We can test this by using litmus paper. Red or blue litmus does not change color if it is in water.
We can break water apart into hydrogen and oxygen using a process known as electrolysis. During electrolysis, we use electricity to break the bonds in chemical compounds. When we are explicitly performing the electrolysis of water, we use a unique piece of scientific equipment known as the Hofmann voltameter.
The Hofmann voltameter is filled with water we have acidified by adding a small quantity of dilute sulfuric acid. The addition of the acid improves its conductivity. A power pack is attached to the voltameter, and a dc current is passed through the acidified water. The water then separates according to the following chemical equation:
In Figure 14 below, we can see the Hofmann voltameter apparatus. Notice how twice as much hydrogen gas has been produced as oxygen gas.
Let’s look closely at the atoms in this chemical reaction. It is easy for us to see why we get twice as much hydrogen gas as oxygen gas produced. Figure 15 shows how the atoms in two molecules of water rearrange to form two molecules of hydrogen gas and one molecule of oxygen gas during the electrolysis of water.
Water is a fascinating molecule with many unusual characteristics. Many of its properties result from being a polar molecule and the existence of hydrogen bonds between the molecules.
Let’s summarize what we have learned about the properties of water.
- Water is a vital resource.
- One atom of oxygen and two hydrogen atoms make up a single molecule of water.
- Oxygen is more electronegative than hydrogen.
- A polar molecule is a covalent molecule where the difference in electronegativity between the elements in the molecule is relatively large.
- Permanent partial charges exist on a molecule of water, making the molecule polar.
- Electrostatic attractions known as hydrogen bonds exist between water molecules.
- Water can exist as a liquid, as a piece of solid ice, and as gaseous steam.
- Water is a polar solvent, and other polar compounds can dissolve in water and form solutions.
- Water has high melting and boiling points due to the hydrogen bonding between water molecules.
- Water forms hexagonal crystals when it freezes and becomes ice.
- Ice is less dense than water.
- Water has a neutral effect on blue and red litmus paper.
- Electrolysis can be used to decompose water into oxygen and hydrogen.
- A Hofmann voltameter is used for the electrolysis of acidified water.
- During the electrolysis of acidified water, twice as much hydrogen gas is produced as oxygen gas.