Click the following link for a student study guide to the Chemistry and Properties of Water
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1. Introduction: Water Makes Life Possible
Liquid water is the environment in which life occurs. You can think of this on two levels.
1.1. Living things are mostly water
Step on a scale. If you took all of the water out of your body, your weight would only be about 40% of what it is now (source: US Geological Survey). And, of course, you wouldn’t be alive. Most humans can survive for a few weeks without food. But stop drinking, and, in a few days, life ends. And this isn’t only true of humans: it’s true of almost all living things.
1.2. Life Depends on Water.
Astronomically speaking, life only exists where there is water in a liquid form. It’s no accident that of the planets in our solar system, Earth is the one with life. That’s because Earth is in a narrow zone where average surface temperatures let water exist in its liquid form, as opposed to gaseous steam or solid ice. Life needs liquid water. So let’s see how water works.
Interactive Reading 1: The Structure of Water
[qwiz qrecord_id=”sciencemusicvideosMeister1961-Properties of Water Interactive Reading (M4)”]
[h]The Structure of Water: Interactive Reading 1
[i]
[q]
2.1 In Basic Chemistry for Biology Students, we learned how to use the Octet Rule to draw diagrams of molecules. Just for review, draw two diagrams of hydrogen gas, H2. Remember that a hydrogen atom has one proton and one electron. Your first diagram should be an electron distribution model (showing the atomic nuclei and the electron energy levels). The second should be a structural formula.
[c*]Show the answer
[f]ANSWER
| Hydrogen, showing energy levels and electron sharing | Hydrogen, structural formula |
[q]Now let’s look at water. Water’s chemical formula is H2O. Hydrogen has one proton and one electron. Oxygen has 8 protons, 8 neutrons, and 8 electrons. Use that information to draw a diagram of a water molecule that shows electron sharing between the single oxygen and the two hydrogens. Then, draw the structural formula. When you’re done, click below to see the solution.
[c*] show the answer
[f] ANSWER
| Water, showing energy levels and electron sharing | Water, structural formula |
[q]If you orient water so that the hydrogens are on top, you should be reminded of a very famous Disney cartoon character. Which one?
[c] [c] [c*]
[f]Tweety bird? I was hoping you would see the resemblance to Mickey mouse!
[f]Homer Simpson? I was hoping you would see the resemblance to Mickey mouse!
[f]Way to go! I was hoping you would see the resemblance to Mickey mouse!
[q labels = “top”]There’s one more important representation of water that you’ll need to understand. This one is called a space-filling model, and it shows water in a 3-D (three dimensional) form. Knowing what you know about water, identify the hydrogen and oxygen atoms in the image below.
[l]hydrogen
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]oxygen
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[x][restart][/qwiz]
Interactive Reading 2: Water and Polarity
[qwiz qrecord_id=”sciencemusicvideosMeister1961-Water, Polarity, Hydrogen Bonding Interactive Reading (M4)”]
[h]Interactive Reading 2: Water, Polarity, and Hydrogen Bonding
[i]Now that we know what water looks like on a molecular level, let’s see how water’s structure determines its chemical properties.
[q labels = “top”]We’ll begin with the concept of polarity.
The Earth is polar. What that means is that it has two _______. A north and a south pole.
Magnets are also polar. Like the Earth, a bar magnet has a north and a south pole. The north pole of one ________ is attracted to the _______ pole of another magnet. Two south poles, by contrast, push each other ________ (as do two north poles).
[l]apart
[fx] No. Please try again.
[f*] Good!
[l]magnet
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]poles
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]south
[fx] No. Please try again.
[f*] Great!
[q labels = “top”]The rules for magnets are
- Like poles __________________
- Opposite poles ____________________
[l]attract (pull together)
[fx] No. Please try again.
[f*] Good!
[l]repel (push away)
[fx] No. Please try again.
[f*] Correct!
[q labels = “top”]Like a magnet, water is also polar. We’ll get into the details in a moment, but for now, just take a good look at this electron distribution model of water, and identify where you think there’ll be a more negatively charged side, and where there’ll be a more positively charged side.
[l]more negatively charged
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[l]more positively charged
[fx] No, that’s not correct. Please try again.
[f*] Great!
[q labels = “top”]
Let’s dig a little bit deeper so we can understand why water is a polar molecule.
- The nucleus of the oxygen atom has eight protons, or eight _________ charges.
- The two hydrogen nuclei each have only one __________, or one positive charge.
- Those eight protons in the oxygen nucleus are going to pull on the 10 __________ in the molecule with a lot more force than the single proton in each hydrogen nucleus.
- As a result, the oxygen side of the molecule will tend to have more electrons, and have a slight _________ charge. By contrast, the region around the two hydrogen nuclei will tend to have ______ electrons, and have slight positive charge.
- In sum, water isn’t just a covalently bonded molecule. Because electron sharing is unequal, and results in polarity, the bonds have special name: they’re called ______ _________ bonds.
[l]electrons
[fx] No. Please try again.
[f*] Correct!
[l]less
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]negative
[fx] No. Please try again.
[f*] Correct!
[l]polar covalent
[fx] No. Please try again.
[f*] Great!
[l]positive
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]proton
[fx] No. Please try again.
[f*] Great!
[q labels = “top”] The charges on water are not full charges, like the ones you’d find on a positive or negative ion. They’re much weaker (though, as we’ll see, they have important consequences). These partial charges are represented with the symbol”?” (Greek for “delta”). So, a partial positive charge is represented by ?+ and a partial negative is represented by ?–. Use these symbols to label all three of the representations of water below.
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[l]?+
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]?–
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[/qwiz]
Interactive Reading 3: Water’s Polarity Leads to Hydrogen Bonding
[qwiz qrecord_id=”sciencemusicvideosMeister1961-Hydrogen Bonding, Interactive Reading (M4)”]
[h]Hydrogen Bonding; Interactive Reading 3
[i]The polarity of water has significant consequences. Read on.
[!]Card 1 [/!]
[q]Here are two water molecules. Because water molecules are polar, weak bonds will form between any two water molecules that get close to one another. The dotted line below represents one of these weak bonds. Which of the images below makes sense?
[c]
[c*]
[f]
No. Remember that opposites attract. The correct representation of this attraction is what’s shown here. Note how the positive side of one water molecule (which is where the hydrogens are) is attracted to the negative side of another water molecule (where the oxygen is).
[f] Excellent. You correctly chose a representation where the positive side of one water molecule (which is where the hydrogens are) is attracted to the negative side of another water molecule (where the oxygen is).
[!]Card 2 [/!]
[q labels = “top”]Water molecules, as you can see here, are “sticky.” Remember that this stickiness is on a molecular level. Here are two key points to keep in mind.
- The oxygen end of one water molecule, with its partially ___________ charge, will temporarily bond with the __________ end of another water molecule, with its partially positive charge.
- The bond that forms between one water molecule and the next is called a hydrogen bond.
[l]hydrogen
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]negative
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[q]Hydrogen bonds are weak and temporary. But, in any body of water–anything from the tiniest droplet to an ocean – they have enormous consequences. We’ll look at these consequences in the next tutorial. Before that, though, let’s compare water to another molecule that we’ve met before: methane: CH4.
Take a good look at the electron sharing between carbon and the four hydrogens in methane. Based on the way it looks, do you think that methane is polar or non-polar?
[c]polar [c*]non-polar
[f]No. Water is polar because all of the electrons are bunched up on the oxygen side of a water molecule, while fewer electrons are on the hydrogen side of the molecule. In methane, all of the electrons are equally distributed. There are no charged poles, making methane non-polar.
[f]That’s right. In methane, all of the electrons are equally distributed. There are no charged poles, making methane non-polar.
[q]Will two methane molecules form hydrogen bonds with one another?
[c]Yes
[c*]No
[f]That’s not right. The only molecules that can form hydrogen bonds are molecules that are polar, like water. Methane, as a non-polar molecule, can’t form hydrogen bonds.
[f]Excellent. The only molecules that can form hydrogen bonds are molecules that are polar, like water. Methane, as a non-polar molecule, can’t form hydrogen bonds.
[q]Methane, with its non-polar electron sharing, is the opposite of a magnet. There’s no positive or negative pole. Methane molecules aren’t attracted to one another (or to anything else).
Keeping that in mind, make a prediction (and feel free to use prior knowledge in your answer). Methane and water have just about the same molecular weight (they both have about the same number of neutrons and protons). At room temperature, methane will be a
[c]liquid
[c*]gas
[f]No. Because methane molecules don’t form hydrogen bonds, they won’t stick together in an open container at room temperature, as water does. As a result, methane is a gas at room temperature. To make methane into a liquid, you have to get it to be really cold…as in -161 degrees, Celsius.
[f]Correct. Because methane molecules don’t form hydrogen bonds, they won’t stick together in an open container at room temperature, as water does. As a result, methane is a gas at room temperature. To make methane into a liquid, you have to get it to be really cold…as in -161 degrees, Celsius.
[q]And just to make sure you’ve got this, label the diagram below (and the ones that follow).
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[l]sticky
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]not sticky
[fx] No. Please try again.
[f*] Excellent!
[q]
|
|
|
[l]non-polar
[fx] No. Please try again.
[f*] Good!
[l]polar
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[q]
|
|
|
[l]hydrogen bonding
[fx] No. Please try again.
[f*] Great!
[l]no hydrogen bonding
[fx] No. Please try again.
[f*] Excellent!
[q]Here’s a different version of the molecules
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|
|
[l]non-polar
[fx] No. Please try again.
[f*] Excellent!
[l]polar
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]sticky
[fx] No. Please try again.
[f*] Good!
[l]non-sticky
[fx] No. Please try again.
[f*] Correct!
[x][restart]
[/qwiz]
Key Concepts of Water: A Quick Review
Before we go on to learning about the properties of water caused by its polarity, let’s review what we’ve learned so far. Read over these key ideas, then work on the flashcards below.
- Water is the most important molecule, by weight, in living things.
- When atoms in a molecule share electrons unequally, the molecule becomes polar.
- Water molecules are polar.
- Because water molecules are polar, they form hydrogen bonds with one another.
[qwiz qrecord_id=”sciencemusicvideosMeister1961-Chemistry of Water Quiz (M4)”]
[h]Quiz: The Chemistry of Water
[i]This quiz includes both multiple choice and fill-in-the blanks questions.
[q]Water is the most important compound by ___________ in any living thing.
[hangman]
[c]weight
[f]Nice! Water is the most important compound by weight in any living thingweight
[q] In a water molecule, the region with a partially negative charge is near the _______ atom(s)
[hangman]
[c]oxygen
[f]Excellent! In a water molecule, the region with a partially negative charge is near the oxygen atom
[q] In a polar molecule (like water), electrons are shared ____________ among the atoms in that molecule.
[hangman]
[c]unequally
[f]That’s correct. In a polar molecule, electrons are shared unequally among the atoms in that molecule.
[q] In a water molecule, the region with a partially positive charge is near the _______ atom(s)
[hangman]
[c]hydrogen
[f]Yes. In a water molecule, the region with a partially positive charge is near the hydrogen atom
[q] Because water molecules are polar, they form ___________ bonds with one another.
[hangman]
[c]hydrogen
[f]Way to go! Because water molecules are polar, they form hydrogen bonds with one another.
[q] If you compared hydrogen, covalent, and ionic bonds, the weakest one would be the _________ bond.
[c*]hydrogen
[c]covalent
[c]ionic
[f]Correct. Hydrogen bonds are much weaker than either covalent or ionic bonds.
[f]No.The weakest of the three bonds is that one that occurs between polar molecules. Which bond is between molecules (as opposed to within a molecule)
[f]No. The weakest of the three bonds is that one that occurs between polar molecules. Which bond is between molecules (as opposed to within a molecule)
[q]Two bonds that are relevant to water are the covalent and the hydrogen bond. Which of these bonds is INSIDE a water molecule?
[c]hydrogen
[c*]covalent
[f]No. Hydrogen bonds occur between water molecules
[f]Excellent. Covalent bonds occur between the hydrogen and oxygen atoms inside of a water molecule.
[!] Card 2+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++[/!]
[q]In this diagram, the dotted line between the two water molecules indicates a ___________ bond.[hangman]
[c]hydrogen
[f]That’s right. The dotted line betweent he two water molecules indicates a hydrogen bond.
[!]3+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++[/!]
[q] In a molecule like methane, all of the electrons are shared equally among the atoms in the molecule. As a result, methane is a ___________.
[c]polar molecule
[c*]non-polar molecule
[c]ionic compound
[f]No. In a molecule like methane, all of the electrons are shared equally among the atoms in the molecule. As a result, methane is a non-polar molecule.
[f]Yes. In a molecule like methane, all of the electrons are shared equally among the atoms in the molecule. As a result, methane is a non-polar molecule.
[f]No. Ionic compounds are compounds where ionic bonds hold together charged ions.
[!]Card 3+++++++++++++++++++++++++++++++++++[/!]
[q] True or false: Non-polar molecules won’t form hydrogen bonds with one another
[c*]True
[c]False
[f]Nice job. The statement above is true: Non-polar molecules won’t form hydrogen bonds with one another
[f]That’s not right. Non-polar molecules won’t form hydrogen bonds with one another
[q]The best description for the bonds that hold the oxygen and hydrogen atoms together inside a water molecule would be
[c]ionic bonds
[c]covalent bonds
[c*]polar covalent bonds
[c]hydrogen bonds
[f]No. Ionic bonds hold together charged ions. A different kind of bond is at work within a water molecule.
[f]No, but you’re close. The bonds that are at work within a water molecule are indeed covalent, but there’s a more specific way to describe them.
[f]Excellent. The bonds that hold the oxygen and hydrogen atoms together inside a water molecule are called polar covalent bond: “covalent” because of the shared electrons, “polar” because of the unequal sharing that results in water’s overall polarity.
[f]No. Hydrogen bonds occur between water molecules (or between water and other polar molecules, or between other polar molecules).
[x]
If you want more practice, please press the restart button below. Otherwise, continue below.
[restart]
[/qwiz]
Next steps
Links
- The properties of water (a virtual lab, and the next tutorial in this series about water)