Chapter 1 - Solutions

Think of your morning tea. You add one spoon sugar to hot water, stir, sugar disappears but taste is sweet everywhere. That is a solution. Solute is small amount that dissolves - sugar, salt. Solvent is large amount that dissolves it - water, tea. Solution is homogeneous means uniform everywhere same taste, same colour, cannot see particles.

Air we breathe is also solution - gas in gas - oxygen nitrogen mixed uniformly. Brass is solid in solid - copper and zinc mixed solid solution alloy.

So solution = solute + solvent, homogeneous, particle size less than 1 nm cannot see even with microscope.

We can mix any state with any state to make solution total 9 types.

Common daily examples: Solid in liquid - sugar in water, salt in water you do daily. Liquid in liquid - alcohol in water sanitizer mixes fully. Gas in liquid - soda bottle CO2 in water, oxygen in water fish breathes. Solid in solid - brass copper+zinc, steel iron+carbon jewellery alloys. Liquid in solid - mercury in gold amalgam dentist filling. Gas in solid - hydrogen in palladium metal absorbs. Gas in gas - air oxygen nitrogen. Liquid in gas - humidity water vapour in air, cloud. Solid in gas - smoke dust particles in air though smoke is more colloid but example.

Most important for us are solid in liquid, liquid in liquid, gas in liquid.

How to say how much sugar in tea? Strong tea means more solute per amount. Different ways to measure strength called concentration.

Mass percent: mass of solute divided by total mass times 100. Example 10% sugar means 10 g sugar in 100 g solution. Used in daily syrups.

Volume percent: volume solute over total volume times 100 used for alcohol in sanitizer 70% alcohol.

Parts per million ppm: mg per kg used for very low concentration like pollutants in water, fluoride in toothpaste.

Mole fraction: moles of one component over total moles no unit, sum of all mole fractions =1. Like share of each person in team.

Molarity M = moles of solute per litre of solution. Most common in lab. Example 1 M NaCl means 1 mole 58.5 g in 1 litre solution. Changes with temperature because volume expands.

Molality m = moles of solute per kg of solvent. Does not change with temperature because mass constant. Used for colligative properties. 1 m means 1 mole per 1000 g water.

Remember: molarity uses litre of solution, molality uses kg of solvent. For dilute water solutions both almost same because 1 litre water ~1 kg.

Solubility means maximum amount of solute that can dissolve in given amount of solvent at given temperature. Like how many spoons sugar can dissolve in one glass water before some stays at bottom.

Saturated solution means no more can dissolve at that temperature extra sits at bottom. Unsaturated can still dissolve more. Supersaturated has more than limit unstable.

Factors: Like dissolves like - polar water dissolves polar sugar salt, non-polar oil does not dissolve in water oil floats. So polar dissolves polar non-polar dissolves non-polar.

Temperature effect: For solid in liquid like sugar, more dissolves in hot water than cold. Hot tea dissolves 3 spoons easily cold water only 1 spoon sits bottom. So solubility increases with temperature for most solids.

For gas in liquid opposite: gas dissolves less in hot water. Cold soda fizz stays longer hot soda loses fizz quickly. Oxygen less in warm river fish struggle in summer.

Open a cold soda bottle you see bubbles fizz escaping. Why? Soda factory fills CO2 gas at high pressure into water so more gas dissolves. When you open pressure drops gas escapes as bubbles.

Henry's law says solubility of gas in liquid proportional to pressure of gas above liquid: P = KH x where P partial pressure gas, x mole fraction gas dissolved, KH Henry constant depends on gas and temperature. Higher pressure more gas dissolves. Higher KH means less soluble.

Daily examples: Soda bottle high pressure more CO2 dissolves cold. Scuba diver deep sea high pressure more nitrogen dissolves in blood if diver comes up fast pressure drops nitrogen bubbles form in blood causes bends pain dangerous so slow ascent. Oxygen less soluble in warm water fish die in hot water, more soluble in cold water good for fish in winter. At high altitude low pressure less oxygen dissolved in blood breathlessness.

KH increases with temperature gas less soluble hot.

Vapour pressure is pressure of vapours above liquid. Pure water has some vapour pressure at given temperature. When you add non-volatile solute like sugar, less water molecules at surface escape so vapour pressure decreases. Raoult says partial vapour pressure of component = mole fraction times vapour pressure of pure component pA = xA pA0 for volatile component.

Ideal solution follows Raoult at all concentrations like benzene toluene similar interactions solute-solvent same as pure. Non-ideal shows deviation positive when solute-solvent weak interaction more volatile vapour pressure higher than ideal like ethanol hexane. Negative deviation when strong interaction H-bond less volatile lower than ideal like chloroform acetone H-bond strong.

Daily: perfume is volatile high vapour pressure smells. Adding salt to water lowers vapour pressure water evaporates slower. Azeotropes are mixtures that boil at constant temperature like ethanol water 95% forms azeotrope cannot separate by distillation fully.

Colligative means depends on number of particles not type. Like number of people in room matters not who. Four properties.

1. Relative lowering vapour pressure: (p0-p)/p0 = x solute = n solute / (n solute + n solvent). More solute more lowering.

2. Elevation of boiling point: water boils at 100 C but salt water boils higher because vapour pressure lowered need higher temperature to reach atmospheric pressure. So Delta Tb = Kb x m where Kb ebullioscopic constant, m molality. Daily adding salt to water for cooking boils at higher temp cooks faster? Actually higher temp but small effect. Antifreeze not this.

3. Depression of freezing point: salt water freezes lower than 0 C. Delta Tf = Kf x m Kf cryoscopic constant. Daily salt spread on icy roads in winter ice melts because freezing point lowered to -5 C, antifreeze ethylene glycol in car radiator prevents freezing in winter and boiling in summer.

4. Osmotic pressure pi = nRT/V = CRT = M R T where C molarity. Osmosis is movement of solvent from low concentration to high concentration through semipermeable membrane like cell wall that allows water not solute. Osmotic pressure is pressure needed to stop osmosis. Daily raisins in salt water shrinks water goes out, potato in salt water shrinks, de-shelled egg in salt water shrinks, fresh water flower stays turgid, salty water flower wilts. Reverse osmosis applies pressure greater than osmotic to make water go opposite used in desalination sea water to drinking water RO purifier at home, also kidney dialysis.

Normally colligative properties give correct molar mass. But some solutes associate or dissociate so particle number changes abnormal.

Association: molecules join together like acetic acid in benzene forms dimer 2 molecules join 1 particle via H-bond. So fewer particles than expected. So colligative effect less, calculated molar mass higher than normal double. van't Hoff factor i = observed colligative / expected = less than 1 for association.

Dissociation: molecules break into ions like NaCl in water breaks into Na+ and Cl- 2 particles from 1 formula unit, KCl, MgCl2 gives 3. So more particles than expected colligative more observed molar mass smaller than normal. i greater than 1.

van't Hoff factor i = (number of particles after association/dissociation) / (number before) = observed value / expected. For association i less than 1 for dissociation i greater than 1 for no change i=1.

Modified equations: Delta Tb = i Kb m, Delta Tf = i Kf m, pi = i CRT, relative lowering = i x solute.

Daily: salt water NaCl i~2 so colligative double, ice melting salt works better because i=2.