Name
Bio A – Basic Chemistry
Energy and Water: use chapter 2 to answer the following questions.
Chemical Reactions
1. What is energy?
2. Name and contrast the three states of matter (tell how they are different).
3. List the states of matter in increasing order of the energy present.
4. When chemicals react, bonds between atoms are broken and formed in new combinations. When this
happens, how is energy involved?
5. In a chemical reaction, the substances that are available to react are called the __________________,
and they are shown to the left of an arrow and the resulting substances are the ___________________,
shown to the right of the arrow. Although most chemical reactions in living things are reversible, we
will generally only worry about one direction, where the arrow is pointing to the right. A specific and
very common chemical reaction is described in Figure 2-6; write the reaction below. Label the
reactant(s) and product(s).
6. Most biological reactions require a certain amount of energy to be added before the reactants will
react. This necessary energy is called the ________________________ energy of the reaction and a
substance called a ___________________ can lower the amount needed to start the reaction. In
biology, enzymes often play this role. We will learn more about enzymes in the next chapter. We
will also worry about oxidation and reduction later.
Water
1. Write the chemical formula for one molecule of water.
2. Remember that water is a polar molecule. What does that mean?
3. Draw a molecule of water and show the partial charges; just use + and – signs for the positive and
negative poles.
4. The attractions between H atoms and other atoms in polar molecules are called __________________
____________. These are not real chemical bonds, but they are strong attractions that hold molecules
together.
5. Because of hydrogen bonds, water molecules show cohesion. Explain what this is.
6. Water molecules will also stick to other substances that have actual or partial charges. This property
is called ______________________.
7. Water’s cohesion means that it must absorb a great deal of energy before enough hydrogen bonds
break to allow the molecules to move so freely that it changes state from a solid to a liquid, or from a
liquid to a gas. Because of this, water is said to have a high heat capacity. It is also necessary to
remove a great deal of energy from water before hydrogen bonds have a chance to form in going from
a gas to a liquid, or to lock the molecules into position when forming ice. Explain what all this means
in terms of keeping temperatures stable in the environment. Think: when it is hot out, would you
rather spend time near water or in a desert???
Now think back to the water cycle. What process would require energy to be added to water
molecules to cause a change in state?
In what process would energy being released from water molecules cause a state change?
8. Hydrogen bonds between water molecules also mean it is unique in that its molecules are less densely
packed together in the solid form than the liquid form. Ice floats on liquid water. Draw molecules of
water as hydrogen bonds connect them in liquid form and in solid form. Use dotted lines to represent
the hydrogen bonds.
9. Liquid mixtures. The polarity of water also allows it to surround and dissolve many substances.
When a substance dissolves in water, it becomes the solute in a solution, while water is the solvent. In
a true solution, the liquid mixture is clear because the water molecules completely surround the solute
particles. This happens with small polar molecules and ionic compounds. Many life processes
depend on solutions.
If the particles of solute are too large to dissolve, or are nonpolar, the resulting liquid mixture is called
a _____________________, and it will remain cloudy. In a suspension, the particles of solute may
temporarily be evenly distributed (suspended) in the solvent, but they will eventually separate and
either sink or float.
In forming both solutions and suspensions, there is not a chemical reaction that changes the solute.
Removing the water leaves the solute unchanged.
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If you stir a little salt into water, will it form a solution or a suspension?
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How about if you stir a little sand into water?
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How about adding oil?
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Problem: Is blood a solution or a suspension? Explain!
A third kind of liquid mixture exists as well. This is a colloid and it can be recognized because it acts
like a suspension of undissolved solute, but the solute particles never separate. Think of something
like jello or mayonnaise. The cytoplasm of a cell is best described as a colloid with organelles
suspended in it.
10. A true solution can be described in terms of its concentration. The concentration tells us how much
solute is present in a certain volume of solvent. In Biology, we will usually describe solutions in
terms of percent. Think in terms of money: how many pennies, or cents, are there in one dollar? A
specific percentage solution means telling how many grams of solute you will find in 100 ml of
solvent, and in Biology, that solvent will always be water. This acts like a recipe that tells you how to
make the solution, no matter how much of the solution you need. Actually, because the solute will
displace (take up the space) of some of the solvent, you would need to add enough of the solvent to
make the total volume 100 ml, but we will simplify things by just saying 100 ml of water.

For example, a 5% solution of sucrose means that for every 100 ml of the water, you will add
5 grams of sucrose. How would you make 200 ml of a 5% sucrose solution?

How about a liter (1000 ml) of the solution?

How about 25 ml of the solution?
11. A more concentrated solution will have more solute in a given amount of solute. Which is more
concentrated – a 10% solution or a 1% solution?
12. Draw lines on the two beakers below to show the same amount of solution in each. In the beakers,
draw dots to represent solute. Show a solution that is more concentrated in the left beaker than in the
right.
13. If you need to increase the concentration of a solution, you can add more solute. If you have a
solution that is too concentrated, how can you decrease the concentration? You can’t remove solute
very easily. Hint: what do you need to do to a concentrate before you use it?
14. If you have a solution that is of lower concentration than you need, you might say it is too dilute. If
you can’t add solute, how could you make the solution more concentrated? Hint: When a cook wants
to increase the flavor of a sauce, or thicken it without adding more ingredients, what might he or she
do?
The concentrations of solutions inside of cells is a critical factor in maintaining homeostasis.
Materials always “want” to move from where they are more concentrated to where they are less
concentrated and they will try to do this until they are evenly distributed. This is not always a good
thing for life. It is often necessary to keep certain materials concentrated inside cells or to remove
other materials that might be poisonous. Sometimes adding or removing water is the fastest and
easiest way to adjust concentrations of materials in cells. We will see that the membranes of cells
have a number of ways to control what can pass through them.