3.2 Water (2.1.2)

Question 1

Describe why water molecules are polar

Answer 1

The oxygen atom having a higher affinity (pull) for the shared electrons makes it slightly negative as it has the greater share of negative electrons. This makes hydrogen slightly positive. The unequal distribution of charge due to the difference in electronegativity between hydrogen and oxygen makes water molecules polar.

Question 2

Describe hydrogen bonding

Answer 2

• A hydrogen bond is an interaction which happens wherever molecules contain a slightly negatively charged atom bonded to a slightly positively charged hydrogen atom.
• Hydrogen bonds are relatively weak interactions, which break and reform between the constantly moving water molecules.
• The hydrogen bonding between water molecules make it more difficult for them to escape to become a gas.

Question 3

What makes water harder to change state and become a gas?

Answer 3

The hydrogen bonds between the water molecules make it more difficult for them to escape to become a gas.

Question 4

How can we prove that hydrogen bonds make water harder to change state (liquid at room temperature)?

Answer 4

The hydrogen bonds between the water molecules make it more difficult for them to escape to become a gas. By contrast, other less polar, but similarly-sized molecules (e.g. H2S) are gases at room temperature. We know that the hydrogen bonds play a role in this as stronger hydrogen bonds form with more polar molecules.

Question 5

Water has a low viscosity. What does this mean?

Answer 5

It flows easily.

Question 6

Because water is a liquid at room temperature, it can: (4)

Answer 6

• Provide habitats for living things in rivers, lakes, and seas.
• Form a major component of the tissues in living organisms.
• Provide a reaction medium for chemical reactions.
• Provide an effective transport medium, e.g. in blood and vascular tissue.

Question 7

Describe water’s density property

Answer 7

When most liquids get colder, they become more dense. If this were the case with water, then water at the top of a pond would freeze and sink. The water replacing it at the top would do the same, until the pond would be full of ice. But water behaves differently. It becomes more dense as it gets colder until about 4 degrees. As it goes from 4 degrees to freezing point, because of its polar nature, the hydrogen bonds fix the positions of the polar molecules slightly further apart than the average distance in the liquid state. This produces a giant, rigid but open structure, with every oxygen atom at the centre of a tetrahedral arrangement.

Question 8

What would happen if solid ice was more dense than liquid water?

Answer 8

If this were the case, then water at the top of a pond would freeze and sink. The water replacing it at the top would do the same, until the pond would be full of ice. This would mean that aquatic organisms don’t have a stable environment to live in and the pond isn’t insulated against extreme cold.

Question 9

Because ice is less dense than water: (2)

Answer 9

• Aquatic organisms have a stable environment in which to live through the winter.
• Ponds and other bodies of water are insulated against extreme cold. The layer of ice reduces the rate of heat loss from the rest of the pond.

Question 10

Water is a good solvent for many substances found in living things. What does this include?

Answer 10

• Ionic solutes such as sodium chloride
• Covalent solutes such as glucose

Question 11

Describe how water acts as a good solvent

Answer 11

Because water is polar, the positive and negative parts of the water molecules are attracted to the negative and positive parts of the solute. The water molecules cluster around these charged parts of the solute molecules or ions and will help to separate them and keep them apart. At this point, they dissolve and a solution is formed.

Question 12

Because water is a good solvent: (2)

Answer 12

• Molecules and ions can move around and react together in water. Many such reactions happen in the cytoplasm of cells which is over 70% water.
• Molecules and ions can be transported around living things whilst dissolved in water.

Question 13

What happens to the shared negative hydrogen electrons in a water molecule?

Answer 13

They are pulled towards the oxygen atom.

Question 14

What charge do the hydrogen atoms have in a water molecule?

Answer 14

A slightly positive charge (δ+)

Question 15

What charge does the unshared negative electrons give to the oxygen atom in a water molecule?

Answer 15

A slightly negative charge (δ-)

Question 16

What is the effect of the oxygen atom having a greater number of positive protons in a water molecule?

Answer 16

It exerts a stronger attraction for the shared electrons, meaning it is left with a slightly negative charge.

Question 17

What is cohesion in the context of water molecules?

Answer 17

Cohesion is the attraction between water molecules due to hydrogen bonds. Hydrogen bonding between the molecules pulls them together.

Question 18

Describe surface tension in water

Answer 18

The water molecules at the surface are all hydrogen-bonded to the molecules beneath them, and hence more attracted to the water molecules beneath them than to the air molecules above. This means the surface of the water contracts (because the molecules are being pulled inwards) and it gives the surface of the water an ability to resist force applied to it.

Question 19

What is adhesion in relation to water molecules?

Answer 19

Adhesion in the attraction of water molecules to other surfaces, such as narrow tubes.

Question 20

What is water temperature a measure of?

Answer 20

The kinetic energy of the water molecules.

Question 21

Why does water have a high specific heat capacity?

Answer 21

The water molecules are held together quite tightly by hydrogen bonds. Therefore, you have to put in a lot of heat energy to increase their kinetic energy and temperature.

Question 22

What is the specific heat capacity of water?

Answer 22

4.2kJ/℃