7. Solutions

Question 1

The Physical State of Matter Depends on ……….

Answer 1

…..a balance between the kinetic energy of particles, which tends to keep them apart, and the attractive forces between them, which tends to bring them together

Question 2

The Gaseous State

Answer 2

• kinetic energy increases as temperature increases
• at high temperatures, particles possess a high kinetic energy and move so fast that the attractive forces between them are too weak to hold them together - the gaseous state

Question 3

The Liquid State

Answer 3

• at lower temperatures, less kinetic energy means particles move more slowly to the point where the forces of attraction between them have some effect. When the temperature is low enough, a gas will condense - the liquid state

Question 4

The Solid State

Answer 4

• when the temperature is even lower, particles don’t have enough kinetic energy to counteract the attractive forces between them, and so the particles stop moving past each other. Each particle will have a set of nearest neighbours that will not change - the solid state

Question 5

Solution

Answer 5

• an homogeneous mixture whereby a solute is dissolved in a solvent
• distribution of particles in a solution is uniform ie. every part of the solution has the same composition and properties
• components of a solution do not separate upon standing
• solution cannot be separated into its components by filtration
• for any given solute and solvent, it is possible to make solutions of many different compositions
• solutions are almost always transparent
• solutions can be separated into pure components by methods such as distillation and chromotography
• separation is a physical change, not chemical.

Question 6

Solvent

Answer 6

• generally the substance in a solution that is present in largest quantity
• if one substance is a liquid, it is normally considered to be the solvent
• when water is present, it is assumed to be the solvent

Question 7

Solute

Answer 7

• a substance dissolved in a solvent

Question 8

Solubility

Answer 8

• the maximum amount of solute that can be dissolved in a solvent at a specific temperature and pressure
• a physical property

Question 9

Saturated Solution

Answer 9

• a solution that contains as much dissolved solute as it can hold while in equilibrium with undissolved solute

Question 10

Supersaturated Solution

Answer 10

• a solution in which there is more solute in the solvent than it can normally hold at a given temperature under equilibrium conditions
• a supersaturated solution is unstable and when disturbed the excess solute will precipitate + the solution will return to equilibrium ie. a saturated solution

Question 11

Miscible

Answer 11

• two liquids are said to be miscible if they are soluble in each other in all proportions

Question 12

Factors Affecting Solubility

Answer 12

• ‘like dissolves like’
• temperature
• pressure

Question 13

1. ‘Like Dissolves Like’

Answer 13

• polar compounds dissolve in polar solvents because the positive dipole of one molecule will be attracted to the negative dipole of another molecule
• non-polar compounds dissolve in non-polar solvents

Question 14

2. Temperature

Answer 14

• in general, the solubility of most solid + liquid substances in a liquid solvent increases with increasing temperature
• for gases in liquids, increasing the temperature decreases the solubility

Question 15

3. Pressure

Answer 15

• in general, pressure has very little effect on the solubility of liquids or soluds in liquid solvents
• the solubility of gases always increases with an increase in pressure

Question 16

Henry’s Law

Answer 16

• the solubility of a gas in a liquid is directly proportional to the pressure

Question 17

Gas Tension

Answer 17

• the amount of gas held in solution
• it is proportional to the pressure of the gas in the liquid

Question 18

Concentration

Answer 18

• the amount of a solute dissolved in a given quantity of solvent

Question 19

Percent Concentration

Answer 19

For solid in liquid:

weight/volume (w/v)% = (mass of solute / volume of solution) x 100

weight/weight (w/w)% = (weight of solute / weight of solution) x 100

For liquid in liquid:

volume/volume (v/v)% = (volume of solute / volume of solution) x 100

Question 20

Molarity

Answer 20

• the number of moles of solute dissolved in one litre of solution - moles/litre

Molarity (M) = moles of solute (n) / volume of solution (L)

Question 21

Dilution

Answer 21

Solutions are frequently prepared by diluting concentrated solutions rather than measuring out pure solvent. Because only solvent is added during dilution, the number of moles of solute remains unchanged.

before dilution M₁V₁ = moles

after dilution M₂V₂ = moles

and therefore M₁V₁ = M₂V₂

Also %₁V₁ = %₂V₂

Question 22

Parts per Million

Answer 22

ppm = (grams of solute / grams of solution) x 10⁶

Question 23

Parts per Billion

Answer 23

ppb = (grams of solute / grams of solution) x 10⁹

Question 24

Water as a Solvent

Answer 24

• most of the important chemical reactions in living tissue occur in aqueous solution
• water is used as a solvent transporting reactants and products from one place in the body to another
• water is also a reactant or product in many biochemical reactions
• water is a good solvent because of its polarity and its hydrogen bonding capacity

Question 25

Hydration

Answer 25

• when a solid ionic compound is added to water, water molecules surround the ions at the surface of the crystal. The anions attract the positive pole of the water molecule, and the cations attract the negative pole of the water molecule. Each ion attracts multiple water molecules. When the combined force of attraction to water molecules is greater than the force of attraction of the ionic bonds that keep the ions in the crystal, the ions will be completely dislodged. Water molecules now surround the ions removed from the crystal - the ion is hydrated (or solvated for other solvents). The surrounding layer of solvent molecules - the solvation layer - acts as a cushion preventing the hydrated cations and anions from colliding and thereby keeping them in solution

Question 26

Hydrates

Answer 26

• substances that contain water in their crystals
• caused by the attraction between ions and water molecules being so strong that the water molecules become part of the crystal structure
• water molecules in a crystal are called ‘water of hydration’
• often, to remove the water, the crystals must be heated for some time at high temperature
• the crystal without its water of hydration is called ‘anhydrous’
• some crystals rehydrate by absorbing water vapour from the air - these crystals are ‘hydroscopic’

Question 27

Electrolyte

Answer 27

• a substance that conducts an electric current when dissolved in water or when in the molten state
• ions in solution migrate from one place to another independently of one another, and it is this migration that constitutes an electric current
• cations migrate to the cathode (negative electrode)
• anions migrate to the anode (positive electrode)
• a non-electrolyte is a substance that does not conduct electricity
• electric conductance depends on the concentration of ions: the higher the ion concentration, the greater the electric conductance of the solution
• strong electrolytes are compounds that dissociate completely
• weak electrolytes are compounds whereby only partial dissociation occurs
• electrolytes help to maintain the acid-base balance and the water balance in the body

Question 28

Covalent Compounds in Water

Answer 28

• some covalent compounds dissolve in water to produce ions and therefore they behave as electrolytes
• most covalent compounds that dissolve in water have their entire molecule solvated
• provided the solute molecule is fairly small, the solute will form hydrogen bonds with water
• hydrogen bonding is possible between two molecules if one of them contains an O, N or F atom (a hydrogen bond acceptor), and the other contains an O-H, N-H or F-H bond (a hydrogen bond donor). Water has both an O atom and an O-H bond and therefore can behave as a donor or acceptor
• in general, for a molecule to be small enough to be soluble in water, it should have no more than three C atoms for each O or N atom
• usually, covalent molecules that do not contain O or N atoms are insoluble in water, the exception being where the covalent molecules react with water

Question 29

Water in the Body is Important:

Answer 29

• dissolves ionic substances and some covalent compounds
• hydrates all polar molecules
• a vehicle to transport most of the organic compounds, nutrients and fuels used by the body
• a vehicle to transport waste material in the body
• hydration of macromolecules such as proteins, nucleic acids and polysaccharides allows the proper motions within these molecules necessary for functions such as enzyme activity

Question 30

Colloid

Answer 30

• mixture in which particles of one of the components are of a size intermediate between those in a true solution + those in a suspension ie. 2 - 500nm
• colloidal particles usually have a very large surface area which means they scatter light, making them appear turbid, cloudy or milky
• can exist in each of the three phases, forming stable dispersions so they do not separate and settle out
• most colloidal particles carry a large solvation layer which moves with the particle and cushions it from other colloidal particles. It is only the solvation layers that touch and therefore the particles do not stick together - they stay in solution
• the large surface area of colloidal particles acquire charges from the solution. All colloids in a particular solution acquire the same charge ie. either positive or negative. This leaves a net positive or negative charge in the solvent. When a colloidal particle collides with another, the two repel each other
• the combined effects of the solvation layer and the surface charge keep colloidal particles in a stable dispersion
• to remove a colloidal dispersion, we can remove the solvation layer by adding a solvent and/or we can remove the surface charge by adding an electrolyte. In each case, the colloidal particles become unprotected and therefore when they collide they will stick together and precipitate from the solution

Question 31

Suspension

Answer 31

• relatively large particles of one substance are suspended in another
• the particle size is > 500nm
• suspensions settle and separate upon standing as the large particles collide and stick together

Question 32

Emulsion

Answer 32

• a system consisting of a liquid, with or without an emulsifying agent, in an immiscible liquid, usually as droplets of larger than colloidal size

Question 33

Colligative Property

Answer 33

• property of a solution that depends only on the number of solute particles and not on the chemical identity of the solute
  eg. freezing point depression, boiling point elevation, osmotic pressure

Question 34

1. Freezing Point Depression

Answer 34

• decrease in freezing point of a liquid caused by adding a solute
• one mole of any particle, whether it be molecule or ion, dissolved in 1000g of water lowers the freezing point of the water by 1.86°C
• nature of the solute does not matter, only the number of particles
• if a solute is ionic, each mole will dissociate to two or more moles of particles, and the freezing point of the water will be lowered directly proportionally to the number of dissociated particles

Question 35

2. Boiling Point Elevation

Answer 35

• increase in the boiling point of a liquid caused by adding a solute
• one mole of any particle, whether it be molecule or ion, dissolved in 1000g of water raises the boiling point of the water by 0.512°C
• nature of the solute does not matter, only the number of particles
• if a solute is ionic, each mole will dissociate to two or more moles of particles, and the boiling point of the water will be raised directly proportionally to the number of dissociated particles

Question 36

3. Osmotic Pressure

Answer 36

• the amount of external pressure that must be applied to the more concentrated solution to stop the passage of solvent molecules across a semi-permeable membrane

Question 37

Osmosis

Answer 37

• passage of solvent molecules from a less concentrated solution across a semi-permeable membrane into a more concentrated solution
• semi-permeable membrane is a thin slice of some material that contains very tiny holes that allows only solvent molecules to pass through
• solvated solute particles are much larger than solvent particles and cannot pass through

Question 38

Osmolarity

Answer 38

• molarity of the solution multiplied by the number of particles produced by each formula unit of solute
• osmotic pressure generated by a solution is dependent on the osmolarity of the solution
• two solutions with the same osmolarity are ‘isotonic’

Question 39

Hemolysis

Answer 39

• an osmotic process in which water flows into red blood cells throgh the cells’ semi-permeable membrane, causing the cells to burst

Question 40

Crenation

Answer 40

• an osmotic process in which water flows out of red blood cells and into solution through the cells’ semi-permeable membrane causing the cells to shrivel

Question 41

Hypertonic Solution

Answer 41

• a solution that has greater osmotic pressure than that with which it is being compared

Question 42

Hypotonic Solution

Answer 42

• a solution that has lesser osmotic pressure than that with which it is being compared

Question 43

Dialysis

Answer 43

• process in which a solution containing particles of different sizes is placed in a bag made of a semi-permeable membrane. The bag is placed into a solvent or solution containing only small molecules. The solution in the bag reaches equilibrium with that outside, allowing the small molecules to diffuse across the membrane but retaining the large particles

Question 44

Hemodialysis

Answer 44

• dialysis of blood