Solutions and Mixtures

Solutions and Mixtures

• A solution is a homogeneous mixture of two or more substances
• The substance in smaller amount is the solute, larger amount is the solvent.
• Solutions can be gaseous (air), solid (alloy), or liquid (seawater).
• Electrolytes dissolved in water result in a solution that can conduct electricity.
• Nonelectrolytes do not conduct electricity when dissolved.
• Strong electrolytes are 100% dissociated into ions.
• Dissociation means the breaking of compounds into cations and anions.
• Hydration describes the process where water molecules surround each ion.
• Hydration helps stabilize ions and prevents cations from combining with anions.
• Dissolving a solid in liquid is a two-step process, first is the endothermic breakdown of the crystal lattice and second is the exothermic surrounding of individual particles by solvent.
• The overall process is endothermic or exothermic, depending on the net balance.
• The endothermic heat of NaCl solution, 3.9 kJ/mol, comes from a lattice energy of 769 kJ/mol and an hydration energy of -765 kJ/mol.
• The endothermic heat of KCl solution, 17 kJ/mol, comes from a lattice energy of 700 kJ/mol and an hydration energy of -683 kJ/mol.

Colligative Properties

• Colligative properties depend on the concentration of solute molecules/ions, not their identity which includes: vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.
• Freezing temperature of water lowers with added salt, boiling temperature increases and vapour pressure lowers and alters osmotic pressure.
• Adding alcohol to water lowers the freezing point below that observed in either pure water or alcohol.

Vapour Pressure

• Adding a non-volatile solute to a solvent reduces the surface covered by solvent molecules.
• The vapour pressure of the solution is lower than the pure solvent at the same temperature.
• If P₀ is the vapour pressure of the pure solvent, and Ps is that of the solution, then P₀ - Ps is the lowering in vapour pressure.
• The ratio (P₀ - Ps) / P₀ is the relative lowering of vapour pressure.
• In 1886, Raoult stated the relative lowering in vapour pressure of a dilute solution equals the mole fraction of the solute.
• If n moles of solute are in N moles of solvent, then Po - Ps / Po = n/n + N
• Boiling point is when vapour pressure equals atmospheric pressure.
• Adding a non-volatile liquid to a pure solvent lowers the vapour pressure and the temperature needs to be increased to equalize vapour pressure and atmospheric pressure.
• Elevation in boiling point describes the difference.

Freezing Point Depression

• Freezing point is the temperature at which the vapor pressure of a liquid equals that of its solid.
• Adding a non-volatile solid to a solvent decreases vapour pressure, equaling that of the solid solvent at a lower temperature.
• Depression in freezing point is the difference between the freezing point of the pure solvent and its solution.

Formulas to Note

• ΔT = Kbb, where K is the ebullioscopic constant and the formula for the elevation of boiling point
• ΔT = Kfb, where Kf is the cryoscopic constant and the formula for the depression of freezing point.

Osmosis and Osmothic Pressure

• Osmosis occurs when solvent molecules flow through a semipermeable membrane.
• The semi-permeable membrane allows solvent molecules to pass through but prevents the passage of bigger molecules like solute.
• The flow goes from a pure solvent to a solution or from a dilute to a concentrated solution.
• Osmotic pressure describes the pressure required to stop solvent flow through a semipermeable membrane.
• Osmotic pressure is a colligative property as it depends on the number of solute present.
• Osmotic pressure (π) is directly proportional to molarity (C) and temperature (T).

Solution Types

• Isotonic solutions share the same osmotic pressure at a given temperature, and there is no osmosis.
• Hypotonic solution features a lower osmotic pressure than its surroundings, solute concentration is less, and water moves out if separated by a semipermeable membrane.
• Hypertonic solution features a higher osmotic pressure, the solute concentration is more, and water moves inside if separated by a semipermeable membrane
• Isotonic solutions: 0.9% normal saline and lactated ringers are examples that replenish fluid loss from blood loss, trauma, or dehydration.
• Isotonic solutions have dissolved particle concentration and osmolality (250 to 375 mOsm/L), distribute between intravascular and interstitial spaces and blood is isotonic.
• Tapwater and pure water are hypotonic.

Gas Solubility

• Increased Increasing temperature lowers the solubility of gases in liquids.
• Increased heat reverses forces between gas/solvent molecules, decreasing gas solubility.

Factors Affecting Gas Solubility

• Pressure
• Temperature
• Size
• Chemical reactivity
• Increasing the pressure increases gas solubility in liquids as per Henry's law.

Biology

• We inhale about 500 cm3 of air per breath
• The amount of inhaled air is the result of changes in lung volume from diaphragm depression and chest expansion.
• This decreases pressure by about 100 Pa relative to atmospheric pressure.
• Expiration is marked by a diaphragm rise and chest contraction, raising pressure about 100 Pa above atmospheric.
• Total lung air volume around is 6 dm3.
• Additional air exhaled after normal expiration measure 1.5 dm3.
• Some air remains to prevent alveoli collapse allowing gas exchange alters lung air composition during breathing.
• Alveolar gas is a mixture of new and exhaled air.
• Oxygen concentration in arterial blood equals a partial pressure of 40 Torr (5.3 kPa).
• Freshly inhaled air features around 104 Torr (13.9 kPa).
• Arterial blood remains in the alveolus capillary for about 0.75 s, and saturates with oxygen in about 0.25 s due to pressure gradient.
• Respiratory membrane thickens if lungs collect fluids, slowing diffusion, and causing oxygen starvation.
• Carbon dioxide moves oppositely across the respiratory

Nitrogen

• Elevating partial pressure via a hyperbaric oxygen chamber treats certain diseases and carbon monoxide poisoning consequences of shock.
• Anaerobic bacteria-caused diseases like gas gangrene and tetanus are sensitive to increased oxygen concentrations.
• Scuba diving supplies air at pressure to match surrounding water.
• Water pressure increases by about 1 atm per 10 m descent.
• Nitrogen is more soluble in fatty tissues at high pressures in the central nervous system, bone marrow, and fat reserves causing nitrogen narcosis.
• The bends: Rapid ascent can result in nitrogen bubbles exiting lipid solution producing a painful condition.
• Arterial embolisms: Scuba drowning cases appear to be consequences of air bubbles that cause obstructions and loss of consciousness.

Colloid Overview

• Colloids, solutions, and suspensions are the three primary types of mixtures.
• Colloids feature particulates ranging from 1-1000 nanometers in diameter that remain evenly distributed which are known as colloidal dispersions.
• In colloids, one substance is evenly dispersed in another, in the dispersed phase with is in the continuous phase.
• A colloid's dispersed phase must be larger than a molecule, but smaller than what is visible, quantified as 1-1000 nanometers.
• If dimensions are smaller, it is a solution, larger, a suspension.

Classifying Colloids

• Colloids are classified by the phases of the dispersed substance and medium.
• The different types of colloids include:
  • sol and emulsion
  • foam and aerosol

Colloid Characteristics

• Sol describes colloidal suspensions with solid particulates in a liquid.
• Emusion describes between two liquids
• Foam describes gas particles in a liquid or solid.
• Aerosol describes liquid or solid particles dispersed in a gas.

Colloidal Solutions

• Lyophilic and lyophobic colloidal solutions (sols) are readibly prepared by different methods.
• Lyophilic colloids feature strong affinity between particles of dispersed phase and dispersion medium.
• Simply mixing the dispersed phase and dispersion medium readily forms colloidal solutions.
• Gelatin, gum, starch, egg, albumin create colloidal solutions by readily dissolution with water.
• These are reversible and can be precipitated, then directly converted into a colloidal state.

Lyophobic Preparation

• Lyophobic colloids can be prepared by two main methods:
• Condensation: Smaller particles of dispersed phase are condensed to get it to colloidal size.

By Oxidation

A colloidal solution of sulphur is obtained by bubbling oxygen(oxidising agent) through hydrogen sulphide solution. 2H₂S + O (or any suitable oxidising agent) → 2H₂O + 25

By Reduction

• Metals: Silver, gold and platinum
• Aqueous solutions of metal salts + Suitable reducing agent
• Formaldehyde
• Phenyl hydrazine
• Hydrogen peroxide
• Stannous chloride
• Gold sol has purple colour, called purple of cassius.

Salt Solutions

• Salt solutions can hydrolyse with boiling dilute solutions of their salts.
• Ferric hydroxide
• Aluminium hydroxide
• Prepared by boiling with corresponding chlorides.

Double Decomposition

Arsenic sulphide sol is formed by double decomposition Arseious oxide + Hydrogen sulphide = Arsenic sulphide in cold water

Cooling and Solvent Exchange

Collodial solution by excessive cooling : ice in organic solvent (ether/chloroform) prepared by frezzing water solution in solvent Molecules of water which combine separately when no longer held = particle of colloidal size Collodial particles by exchanger of solvents can be prepared with: Substance soluble in alcohol / insoluble in water can be prepared by poring alcohol solution in excess of water. eg Colodial sulphur formed = Milky collodial solution formed by alcohol sulphur solution poring it into water. Sols of mercury and sulphur + pasing vapour to a cold water containgin stabilizer, ammonium salt or citrate

Colloidal Solutions and Purification

Colloidal solutions by any method = impurities like electrolytes.

• The presence affect the sol stabilizing when in low and destablilizes when in ahigh concentation. To purify = removing the impurities.
• The separation are possible by:
• Dialysis
• Ultrafiltration

Dialysis

• Animal membrane allows passage of crystalloids but retains larger colloids.
• Animal membranes are used for purification of sols- called dialysis.
• Dialysis is the separation of impurity particles (crystalloids) from an impure sol by diffusion via parchment paper or cellophane.
• Dialyser is the apparatus used, which consists of a bag made of parchment/cellophane.
• Bag contains impure sol suspended in pure water tank and electrolyte impurities diffuse, for a pure sol.

Electrodialysis

• Dialysis in the use of electric field as the process is slow
• Field induces impurity ion movement faster to charged electrodes.

Ultrafiltration

Ordinary paper cant to remove salts
Pore is reduced = paper can be used for salts Use Formaldehyde on ordniary paper with collodation and gelatin. Process names: Ultrafiltration and Ultrafilters are the papers

Ultrafiltration Overview

• Ultrafiltration uses a wire mesh to support filters.
• Impure sol pours over the filter leading to an extraction of the electrolytes and retains colloidal particles.
• The process is sped-up with pressure and suction. Series of graded ultra filters = remove impurities even by size different colloids
• By centrifugal force= separate colloidal and impurities, known as ultracentrifugation
• Impure sol in tube with ultracentrifuge
  • Rotation causes colloidal particles settle, removed and mixed.

Questions

• Question 1 Answer: Dialysis
• Question 2 Answer: Cellophane
• Question 3 Answer: Electrodialysis
• Question 4 Answer: Centrifugal force