Polarity of Water
As you may already have realized, water is a very special molecule. Without it, life on Earth would not exist, or at least not any life we know about! What about your life? Have you ever thought about how water helps you constantly in your everyday life...even if you don’t like the taste?
Previously, we learned that water is polar, with a partially negative oxygen end and a partially positive hydrogen end. This polarity allows hydrogen bonds, weak attractions between polar molecules, to form between individual water molecules. Although individual hydrogen bonds can be formed and broken fairly easily, collectively the hydrogen bonds in water create a set of very important emergent properties, including capillary action, surface tension, and solvency.
In liquid water, hydrogen bonds form and break quickly, but the collective action of these hydrogen bonds between water molecules results in cohesion, or water molecules sticking together instead of spreading apart. Have you ever seen diabetics test their blood sugar, or had a small blood sample taken from your finger? If so, you may have noticed how the blood can be collected into small tubes. Even without suction, the blood moves upward, against gravity, and into the collection tube! Because blood is mostly water, cohesion and a related property called adhesion (where water molecules form hydrogen bonds with other polar molecules as well) are responsible for this movement. These cohesive, or sticky, water molecules pull each other upward, against gravity, and into the glass tube. The phenomenon is called capillary action.
Trees are able to grow as tall as they do because of this same property. Inside the “veins” of a tree trunk, water moves upward against gravity from the ground roots to the highest leaves. Without hydrogen bonding, trees could never transport groundwater up to their leaves.
- Food coloring (optional)
- Paper towels
- Mix a drop of food coloring with a small amount of water in an open bowl. You may use tap water alone.
- Hold a paper towel vertically as shown in the image, and gently touch the corner to the colored water.
- Hold in place for several seconds and watch as the colored water climbs up against gravity onto your paper towel. Cohesion of the water molecules creates this capillary action.
Surface tension is another property that arises from cohesion. On the surface of any liquid body of water, the hydrogen bonds between water molecules are collectively strong enough that they effectively form a thin film over the surface of the water that resists breaking. Because of surface tension, insects like a water strider can land on and walk across the surface of a pond without falling into the water. You may have experienced surface tension yourself if you’ve ever skipped a rock across the surface of a pond. If you throw it just right, the downward pressure is slight enough to allow the surface tension to hold and the rock bounces instead of sinking!
Try This at Home #2
What you need:
- Small paperclip
- Glass cup or bowl
- Fill a glass cup or bowl all the way to the top.
- Gently place a small paperclip onto the top of the water horizontally as shown.
- Without water’s hydrogen bonds creating surface tension, the paperclip, which has a higher density than water, would sink to the bottom of the glass immediately.
- What happens when you place the paperclip into the water vertically?
Finally, hydrogen bonding makes water an excellent solvent. Substances that dissolve into a solvent are called solutes, and the resulting mixture is a solution. Water is an excellent solvent because its polar water molecules form hydrogen bonds with ions and polar molecules, allowing ionic and polar covalent compounds to disperse easily in water.
When an ionic compound like table salt is placed in water, the partially negative oxygen atoms of the water molecules face and surround the sodium cations. At the same time, the partially positive hydrogen atoms of other water molecules face and surround chlorine anions on the surface of each salt crystal. These spheres of hydration around each individual ion separate the anions from the cations, breaking the ionic bonds. As the ions on the surface of the compound are dissolved into the water, layers underneath are exposed and dissolved, until eventually all of the ions are separated and dissolved into the water. Salt dissolves readily into a cup of water because of hydration.
Similarly, polar covalent compounds such as table sugar (sucrose) are dissolved by the same process. Although the covalent bonds are not broken, each individual sugar molecule is separated from the others by spheres of hydration, resulting in total dissolution of the solid sugar.
How do we identify whether a substance will dissolve readily into water? Polarity. Substances that have charge or polarity are hydrophilic, and are likely to dissolve because they have either full or partial charge areas to form hydrogen bonds with water molecules. These substances are soluble in water. Substances that are mainly nonpolar are hydrophobic, and are unlikely to dissolve because they have no full or partial charge areas to form hydrogen bonds with water molecules. Because water molecules cannot interact to form hydrogen bonds with nonpolar substances, they cannot separate the individual molecules of nonpolar substances with hydration spheres, and the substance remains intact.
Fats such as cooking oils are examples of insoluble substances due to their lack of polarity. Even with vigorous shaking, you cannot get the oil in an oil and vinegar salad dressing to stay mixed with the vinegar (water-based substance) for very long. The insoluble oil is pushed out of the polar water as hydrogen bonds are formed between water molecules, resulting in the formation of oil “clumps” at the surface of the vinegar.
Try This at Home #3
What you need:
- Cooking oil
- Glass cup or bowl
- Fill a glass cup or bowl a little more than halfway with water.
- Add about 3 tablespoons of cooking oil and observe as the oil drops into the water.
- Using a spoon or fork, rapidly mix the oil into the water until no large droplets remain.
Watch as large oil droplets re-form at the surface as hydrogen bonds (too small to be seen) are formed between individual water molecules, excluding the insoluble oil from the water.
Why do biologists care about water as a solvent? Because our entire bodies consist of substances dissolved in water! Each of the human body’s millions of cells is around 70-90% water, the liquid in which everything is dissolved. Cell membranes are made of nonpolar substances; therefore, they do not dissolve in water. This insolubility creates a safe barrier to protect cell interiors. Cells are filled with proteins that provide us with energy, produce what we need to grow and reproduce, and respond to our environments. Without water, these proteins would not remain separated inside the cells where they need to move easily to perform their cellular jobs. As we continue through this semester, remember that all cellular activities are taking place in water. It is truly the molecule of life.
Will It Dissolve?
Ionic compounds and polar covalent compounds will dissolve in water. Nonpolar covalent compounds will not. Can you differentiate which molecules will dissolve in water?