Science of Water

Question 1

What are the anomalous properties of water?

Answer 1

1) Water has unusually high melting and boiling points. 2)Unusually high surface tension and viscosity. 3)Water is denser than Ice. 4)High heat capacity (it requires more energy to change the temperature of water). 5)Hot water may freeze faster than cold water. 6)Colder water surfaces may vaporise faster than hot water surfaces. 7)Protons and Hydroxide ion mobilities are fast.

Question 2

What is the Mpemba phenomena?

Answer 2

This phenomena states that hot water freezes faster than cold water, there are multiple potential explanations that have been put forward to explain this phenomena. As an example a sample at 35 degrees will freeze faster than a sample at 100 degrees.

Question 3

What is the Leidenfrost effect?

Answer 3

This states that there is a temperature point on a surface that once surpassed allows cold water to evaporate faster than hot water on contact. This is due to hot water creating an insulating vapour barrier between the water and surface slowing evaporation.

Question 4

What type of bonds make up a water molecule?

Answer 4

A water molecule consists of an Oxygen atom covalently (sharing) bonding with 2 electrons from 2 Hydrogen atoms.

Question 5

What does polar mean in reference to molecules?

Answer 5

Polar refers to a molecule containing a dipole moment, this comes about when a molecule with covalent bonds do not share the electrons equally. In water the Oxygen pulls more charge from a covalent bond with Hydrogen creating a field that is more electronegative around the oxygen (partial negative charge). In this instance the hydrogens are less electronegative (partial positive charge). This unequal sharing creates a dipole relationship and that is what is meant by a polar molecule.

Question 6

What is the bond angle of water?

Answer 6

104.5 Degrees

Question 7

Why is it important that water has a polar characteristic?

Answer 7

The dipole moment causes H2O molecules to exert more attractive forces to other polar molecules, these forces allow for water to have exceptional solubility for polar molecules (including H20). The dipole-dipole attractive forces are considered to be of intermediate strength.

Question 8

What is the strongest inter-molecular attraction?

Answer 8

That is the Hydrogen bond.

Question 9

What is the weakest inter-molecular attraction?

Answer 9

Dispersion force.

Question 10

What is a Hydrogen bond?

Answer 10

A hydrogen bond is where a Hydrogen is bound to either Oxygen, Nitrogen or Flourine. Creating a Partial Positive Charge on the Hydrogen. This positive charge can then interact with a small, highly electronegative atom with an electron pair.

Question 11

What is unique about hydrogen bonding in H2O?

Answer 11

This is the only molecule that can form 4 hydrogen bonds. Two bonds are formed with its own Hydrogen bonds, and two more are formed when the partial positive charge of other water molecules are attracted to the lone pairs of the current water molecule. 2 donors and 2 acceptors.

Question 12

What sort of structures are formed when creating ice?

Answer 12

Ice forms in highly organised hexagonal arrangements.

Question 13

What happens to dissolved substances(eg. salt) when water converts to ice? What does this action require?

Answer 13

The formation of ice expels dissolved substances out of its hexagonal arrangements, this requires more energy to freeze aqueous solutions.

Question 14

When water freezes there is a volume change, what is it? and why is this so important?

Answer 14

The is an increase of approximately 9% in volume, this increase allows ice to float due to is density being less than water.

Question 15

Structurally why is water a good solvent?

Answer 15

Water has a ‘open structure’ which means there is plenty of space between the water molecules to accommodate solutes.

Question 16

Why is it hard to prepare pure water?

Answer 16

Waters ability to form solutions with solutes is very high, due to its open structure and polarity.

Question 17

What is a mixture? What can they be presented as?

Answer 17

A mixture is made from two or different substances that are mixed together but not chemically combined. They can be presented as colloids, suspensions and solutions.

Question 18

What is a solution?

Answer 18

A solution is a homogeneous mixture of 2 or more substances. They are stable, cannot be filtered and are of one phase (liquid, gas or solid).

Question 19

What are ionic solutes commonly known as?

Answer 19

Salts.

Question 20

What is a ionic solute made from?

Answer 20

An ionic solute is made from both cations (positive) and anions (negative) ion which form a very stable structure.

Question 21

Why are salts super dissolvable in water?

Answer 21

The polarity of water is able to disrupt the weaker salts ionic bonds by providing an influx of positive and negative attractive forces.

Question 22

What is a limitation to hydrophillic solutes when dissolving in water?

Answer 22

If a molecule has a polar functional group available it will only interact if the molecule itself is stabilized wholly.

Question 23

Non-polar substances(hydrophobic) do not interact with water, however water can accommodate them, how is this possible?

Answer 23

As water is an open structure it can create a small pocket/cavity called a clathrate. This clathrate can accept small non-polar molecules which prevents unfavourable interactions between water and the hydrophobes.

Question 24

In water, what is autoprotolysis?

Answer 24

This is the ability of a molecule to transfer a hydrogen ion from one molecule to another. This is especially prevalent into H20 as it can react with itself to create OH- (Hydroxide ion) and H3O+ (hydronium ion). Pure water is has a balanced ratio between the two ions.

Question 25

What is colligative properties? what are some examples?

Answer 25

This is a property that is not determined by the characteristic of a molecule but the actual concentration itself. Examples include: the elevation of the boiling point, decreasing of the freezing point, lowering of vapour pressure (atmospheric) and osmotic pressure.

Question 26

If you make a solution of water and a polar solute what could you expect to happen to the boiling point and the freezing point?

Answer 26

If water creates a strong inter-molecular bond with the solute then the boiling point raises, this is due to the increase of energy required to separate water from this bond. As for the freezing temperature, a higher energy is required to expel the solute from the crystalline structure water forms (ice).

Question 27

What is osmosis?

Answer 27

This is the movement of a solvent through a semi-permeable membrane from a lower gradient to a higher gradient of solute (concentration). This movement aims to balance both sides of the membrane.

Question 28

What is bound water?

Answer 28

Bound water is water that has interacted (inter-molecular bonds) with other molecules. This bond restricts waters mobility, making it inaccessible for other reactions.

Question 29

What is water activity?

Answer 29

Water activity is the representation of free water in a system, this water is not impeded by any bonds and therefor has a high mobility. Pure water has a water activity of ‘1’

Question 30

How does the concepts of bound water and water activity affect food perish-ability?

Answer 30

Bound water is inaccessible to micro-organisms typically, meaning that they are not able to utilise it to grow or produce toxins. Water activity is the measure of ‘free’ water which micro-organisms are able to use. Therefor the greater the water activity the greater the degradative effects in a food material such as microbial growth.

Question 31

What is the molal concentration of H2O?

Answer 31

it is 55.5 moles per kg 1 mole roughly equals 18grams

Question 32

What are some limitations of the relationship between water activity and bound water?

Answer 32

Real solutions deviate considerably from an ideal solution, this is due to factors such as ‘attractiveness’ not being supported in the relationship. It is hard to determine where exactly water transitions from being bound to free.

Question 33

What is the difference between molal scale and molar scale?

Answer 33

Molal takes into account mass, where molar accounts for volume. In terms of accuracy molal if preferred as it is not affected by temperature.

Question 34

How can we measure water activity? Is there any limitations to this?

Answer 34

We can compare the vapour pressure of a system to that of pure water. This creates a rough estimate of the percentage of free water.

A_w = P_w / P_w^o

Unfortunately this is only an approximate, so it should be reffered to as Relative vapour pressure (RVP)