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CHE2810 - Food Chemistry > Science of Water > Flashcards

Flashcards in Science of Water Deck (34)
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1

What are the anomalous properties of water?

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.

2

What is the Mpemba phenomena?

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.

3

What is the Leidenfrost effect?

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.

4

What type of bonds make up a water molecule?

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

5

What does polar mean in reference to molecules?

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.

6

What is the bond angle of water?

104.5 Degrees

7

Why is it important that water has a polar characteristic?

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.

8

What is the strongest inter-molecular attraction?

That is the Hydrogen bond.

9

What is the weakest inter-molecular attraction?

Dispersion force.

10

What is a Hydrogen bond?

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.

11

What is unique about hydrogen bonding in H2O?

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.

12

What sort of structures are formed when creating ice?

Ice forms in highly organised hexagonal arrangements.

13

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

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

14

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

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

15

Structurally why is water a good solvent?

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

16

Why is it hard to prepare pure water?

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

17

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

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.

18

What is a solution?

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).

19

What are ionic solutes commonly known as?

Salts.

20

What is a ionic solute made from?

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

21

Why are salts super dissolvable in water?

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

22

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

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

23

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

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.

24

In water, what is autoprotolysis?

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.

25

What is colligative properties? what are some examples?

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.

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?

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).

27

What is osmosis?

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.

28

What is bound water?

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

29

What is water activity?

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'

30

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

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.

31

What is the molal concentration of H2O?

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

32

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

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.

33

What is the difference between molal scale and molar scale?

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

34

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

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.

Aw = Pw / Pwo

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