Water
What Are the Properties of Water?
The Water Molecule
Teacher Note: Connections
In this lesson, students will investigate the properties of water and be able to infer the behavior of water based on its molecular structure. To help students better understand the properties of water, encourage students to use the “Journals” strategy throughout the lesson. Each time students watch a video or read a passage in the lesson, have them record summaries of what they have learned in their journals. Remind them of the molecular structure of a water molecule as they learn about its properties so they can relate its properties back to the structure of the molecule. For instance, when students are learning about cohesion, remind them the molecules of water are attracted to one another because they are unbalanced, with regions that are slightly positive and negative, and because opposite charges attract, the molecules will be attracted to one another. The “Journals” strategy is found on the Professional Learning tab. Click on Strategies & Resources, then click on Spotlight on Strategies (SOS). “Journals” is found under “Summarizing.”
As students read and comprehend complex texts, view the videos, and complete the interactives, labs, and other Hands-On Activities, have them summarize and obtain scientific and technical information. Students will use this evidence to support their initial ideas about how to answer the Explain Question or their own question they generated during Engage. Have students record their evidence using “My Notebook.”
A waterglossary term (opens in a new window) molecule consists of one oxygen atom and two hydrogen atoms. In this molecule, H2O, the hydrogen-oxygen bond is a polar covalent bondglossary term (opens in a new window). In a polar covalent bondglossary term (opens in a new window), the electrons shared between the atoms tend to spend more time closer to the nucleus of the more electronegative atom. Therefore, in water, the electrons tend to stay near the oxygen atom. This polarity means that, in the water molecule, the region of the oxygen atom is slightly negative and the regions of the hydrogen atoms are slightly positive.
Due to the polarity of water molecules, they bond with one another through hydrogen bonds. A hydrogen bondglossary term (opens in a new window) between water molecules is an attractive force between the hydrogen atom of one water molecule and the oxygen atom of another water molecule. Hydrogen bonds between water molecules are weaker bonds than are the polar covalent bonds within water molecules. Hydrogen bonding between water molecules is a dynamic process. Hydrogen bonds that connect water molecules are constantly forming, breaking, and reforming.
Two water molecules can dissociate to form one hydronium ion and one hydroxide ion. A hydronium ion (H3O+) contains three hydrogen atoms and one oxygen atom and is positively charged. A hydroxide ion (OH-) contains one hydrogen atom and one oxygen atom and holds a negative charge. The chemical reaction of water dissociation to form these ions is as follows:
This reaction proceeds back and forth in both directions. In its liquidglossary term (opens in a new window) state, water molecules are constantly switching back and forth between these forms.
Teacher Note: Misconception
Students may think that the water molecule is stable and that liquid water molecules remain in their H-O-H configuration. In fact, water molecules constantly dissociate and reform into hydroxide (OH-) and hydronium (H3O+) ions, while the reaction remains in equilibrium.
Properties of Water: Stickiness
Water is different from other substances in many ways. Notably, it exists in all three states of matter within a narrow range of temperature— 0 to 100 degrees Celsius—on Earth’s surface. Most water exists as a liquid or ice on Earth’s surface. A small amount exists as water vapor in the air.
Its polar covalent bonding, together with its hydrogen bonding, gives water unique properties, including cohesion, the ability to moderate temperature, the ability to dissolve many other materials, and the ability to expand upon freezing.
Cohesion refers to the attraction of two like materials. In the case of water, cohesion refers to the binding of water molecules via hydrogen bonds. In other words, water molecules cohere to one another. Cohesion is very important to life on Earth. One useful outcome of cohesion is that it helps water travel from the roots upward in plants. Starting at the roots of the plant, water molecules cohere and travel upward in columns in the plant’s vascular tissue. When one molecule of water evaporates from the leaf, it pulls the water molecule lying below it to the surface of the leaf, and in this way water travels upward from roots to leaves, driven by evaporation.
Water molecules can also stick to other materials and surfaces; the attraction of two different materials is called adhesion. For example, adhesion is also critical to plants because it allows water to stick to cell walls. This helps prevent water from falling downward with gravity. It is also important deep inside of your lungs, where a wet surface is essential for gas exchange.
Another important property of water is its high surface tensionglossary term (opens in a new window). Due to hydrogen bonding, it is relatively difficult to “break” the surface of liquid water compared to other liquids. Water’s high surface tension can be demonstrated by filling a container with water. Water can actually be filled slightly above the height of the container without spilling because of the hydrogen bonds between water molecules that hold the molecules together. High surface tension allows some insects to move on the surface of water. The insects’ legs do not produce enough pressure to break water’s surface tension.
Teacher Note: Connections
In this item, students will use a model to simulate the behavior of water molecules within a larger system. To extend this item, have students perform a simple experiment using water and pennies. Divide students into small groups and have each group predict how many drops will fit on a penny. Then have students use eyedroppers or pipets to add drops to the penny until it overflows. Students can report their data, then graph the class results. Have students write a short descriptive paragraph explaining the properties of water that led to their results.
Properties of Water: The “Universal Solvent”
Another important property of water is its ability to dissolve more substances than any other liquid, making it known as the universal solventglossary term (opens in a new window). A solventglossary term (opens in a new window) is any fluid in which one or more substances can be dissolved. When water dissolves a substance, water molecules surround a molecule of a substance and form a shell of hydration. This shell effectively dissolves the substance.
Water’s ability to act as a solvent for many substances is crucial to life for many reasons. For example, the water inside cells must be able to dissolve the proteins, carbohydrates, ions, and other molecules necessary for cellular function. Water readily dissolves ions and polar molecules. It does not dissolve certain other molecules such as lipids. This is why oil and water do not mix well. Also, because water dissolves many substances, water provides an important medium for molecules to combine and form new molecules.
Properties of Water: Expansion Upon Freezing
Water has the unique property of expanding upon freezing. It is one of the few substances on Earth that is denser in its liquid form than its solid form. Again, this expansion property is due to the polarity and hydrogen bonding of water molecules. Normally, liquid water molecules constantly form and reform their hydrogen bonds with one another. The molecules constantly move closer and farther from one another. When temperatures reach freezing, however, all water molecules form a hydrogen-bonded crystal lattice. The configuration of this lattice holds water molecules stretched out from one another, creating a less dense structure than liquid water. Thus, ice floats in water because frozen water is less dense than liquid water.
The ability of ice to float in water may not seem very important to life on Earth. However, imagine what would happen to large bodies of water in cold climates if this property of water did not exist. Instead of floating, ice would sink. There would be no suitable liquid water underneath the ice to house aquatic life. Eventually, these bodies of water would most likely freeze over completely, making them an uninhabitable environment for most organisms.
Properties of Water: Specific Heat and the Ability to Moderate Temperature
Water has the extraordinary ability to moderate temperatures on Earth. It does this by absorbing thermal energy from warmer air and releasing thermal energy into cooler air. Large bodies of water temper the climate around them. They hold local temperatures within a relatively small range by heating or cooling the air through energy absorption and release.
The ability of water to moderate temperatures is made possible by its very high heat capacity. Heat capacity of a material is a property of the material that is often referred to as the material’s “specific heatglossary term (opens in a new window).” Specific heat is a measure of how much energy is required to change the temperature of a given mass of a substance by a set amount. The specific heat of water is 1 calorie/gram °C. This means that one calorie of energy is required to change the temperature of one gram of water by one degree Celsius. (When specific heat is described in SI units, specific heat is described as a change in joules [J] per Kelvin [K].) The specific heat of water is very high relative to other substances. Water requires a great deal of energy to change its temperature.
The outcome of a high specific heat is that water maintains its temperature within a relatively narrow range, even when absorbing large amounts of energy. As a result, large bodies of water can absorb and release a lot of energy from the surrounding air, moderating air temperatures with relatively small changes to their own temperatures. Similarly, water inside the cells of an organism can absorb and release heat while maintaining fairly stable temperatures within the organism.
Teacher Note: Practices
In this item, students make a qualitative claim between independent variables (the properties of water) and dependent variables (the benefits to living things). To expand this item, have students rank the relative importance of the properties of water as they relate to living things. Rankings will likely vary widely from student to student, so have students also compose an essay in which they provide supporting evidence for their ranking, including specific details and relevant data in their argument.
