Experimental Techniques

Lab Equipment

Gas Collection Methods

Gas Collection Methods

To measure the volume of gases, a graduated gas syringe can be used or a measuring cylinder or burette can be used in place of the gas jar in the water displacement method.

Methods for drying of gases

Separation Techniques

Mixtures consist of more than one compound or element, mixed but not chemically combined. The components can be mixed in any proportion. Examples of mixture include air, seawater, crude oil and alloys.

solid solid mixture

Magnetic Attraction

Description of procedure

Magnetic separation is used to separate magnetic substances (Ferromagnetic and paramagnetic) from non-magnetic ones in a mixture.

ferromagnetic materials are strongly attracted and can become permanent magnets (like iron) paramagnetic materials are weakly attracted (like aluminium) diamagnetic materials are weakly repelled (like copper, gold)

Sublimation

Sublimation separates a mixture of solids, one of which sublimes, where the solid will undergo a change in state from solid to vapour upon heating, the other does not.

Examples of substances which Sublimate

• Iodine
• Dry ice ($\ce{CO2}$)

Solid Liquid Mixture

Filtration

The solid collected in the filter paper is called the residue. The liquid/solution passing through the filter paper is called the filtrate.

Filtration has some disadvantages. Solid residue will be contaminated with some fluid and thus they must be washed thoroughly (often with deionised water). The filtrate, depending on the pore size of the filter paper, may contain fine particles that are smaller in diameter than the pore size.

Filtration under pressure (Vacuum Filtration)

Is similar to normal filtration but is much faster as filtrate is "sucked" through the filter paper, leading to faster collection of residue.

Decanting

Decanting separates an insoluble solid from liquid by pouring off the liquid or solution.

Centrifuging

Decanting can be made more effective by using a centrifuge.

Centrifuging can be used to separate very fine solid from a solid-liquid mixture. It involves revolving the mixture at high speed which causes the fine insoluble solid to settle at the bottom quickly. Due to this speed, a liquid-filled casing must be placed opposite the casing with the sample to prevent damage to the centrifuge. After this, the solid is stuck to the bottom of the pellet, overturning it will not cause the solid to come loose

Solid Liquid Solution

Crystallisation

• Crystallisation is used to separate a dissolved solid from from its solvent.
  • It can only be used for thermally stable salts. e.g. $\ce{NaCl}$
• When a solution is heated, most of the solvent is evaporated off, creating a saturated solution.
• As the solution cools, the solubility of the solute decreases
• The difference in mass of solute that can dissolve in solution between the two temperatures will result in the dissolved solute appearing as crystals
• Impurities remain dissolved in the solution.
  

Testing For saturation

A clean glass rod is dipped into the solution and removed. There will be a small quantity of solution on the rod. If small crystals form on the rod as the solution cools, the solution is saturated.

Evaporation to dryness

• Evaporation to dryness involves heating the solution until all the solvent has evaporated, leaving behind the solid.
• e.g. $\ce{NaCl}$ from seawater
• Is not suitable in the following cases
  • The solute decomposes on heating The solid obtained contains water of crystallisation. Evaporation to dryness will result in loss of water of crystallisation, forming an anhydrous solid.

Water of crystallisation refers to water molecules that are chemically bound within the structure of a crystalline solid, and are essential for maintaining the crystal structure of the compound

Not suitable for hydrated Salts e.g.

• Hydrated Copper Sulphate (Blue) ($\ce{CuSO4 \cdot 5H2}$) $\xrightarrow{\text{Heated}}$ Anhydrous Salt (White) ($\ce{CuSO4 + 5H2}$)

Miscible Liquid Liquid Mixtures

Distillation

simple distillation is to obtain a pure solvent from a solution while fractional distillation is to separate a mixture of 2 or more liquids or solutions that are miscible.

Simple distillation can technically be used on miscible liquids, but only if their boiling points are at least $25^\circ \text{C}$ (Rough guideline). Reason being in a homogeneous solution, the vapour will contain both liquids. The more volatile liquid (lower boiling point) will exist in the majority, but the less volatile liquid vapour will still be mixed in.

Simple distillation

Note

• Bulb must be at the mouth of the opening where the vapour condenses
• Boiling stones/chips ensure smooth & even boiling chips
• Water Enters from the bottom and exits from the top in the Liebig Condenser
  • Water entering from the top would introduce air bubbles and reduce effectiveness of heat exchange
  • This direction maximises the difference of temperature between cooling water and the condensing vapour
• Distillation is considered a separation and purification technique because the resulting distillate is relatively pure

Fractional Distillation

A electric heating mantle is used instead of a Bunsen burner as ethanol is a flammable liquid. A naked flame should not be used as a safety precaution.

• Like simple distillation but with an additional glassware - the fractionating column
• A fractionating column contains many glass beads and is used to separate the liquids in order of relative boiling points.
• The glass beads in the column allows repeated cycle of condensation and vaporisation to occur. They provide a large surface area for condensation to occur easily
• Liquid with the lowest boiling point boils first. It is thus distilled first and collected first.

Fractional distillation of petroleum

• petroleum is a mixture of hydrocarbon molecules with a range of boiling points.
• petroleum needs to be refined to make useful products and fuels.
• Fractional distillation is used for separating petroleum into fractions.
  • Properties of fractions
    • Insoluble in water
    • Burn in air
    • As boiling points of fractions increase,
      • Molecules become larger
      • Liquids flow less easily
      • Liquids burn with a more smoky flame
      • Liquids burn less easily

Fractional Distillation of Air

Mixture of dry air consists gases like $\ce{N2}$ (78%), $\ce{O2}$ (21%) and 1% of other gases like argon (0.97%) and $\ce{CO2}$ (0.03%). Nitrogen is one of the essential reactants to form ammonia (Haber Process). Nitrogen can be obtained from liquified air using fractional distillation as it has a boiling point of -195.8°C while oxygen oxygen -183°C.

Immiscible Liquid Liquid Mixtures

Separating Funnel

The narrow bottom minimizes the surface area where the two liquid layers meet, allowing for precise, clean removal of the bottom layer through the stopcock.

Description of Procedure

• Pour the mixture into a separating funnel
• Allow the two liquids to separate into two layers
• The liquid with a lower density (oil) will form the upper layer
• The liquid with a higher density (water) will form the lower layer
• Open the tap to run out the liquid in the lower layer into a beaker
• Use another beaker to collect the the liquid in the other layer

Chromatography

• Separates mixture components by distribution between a mobile phase (liquid or gas) and a stationary phase (solid, or thin liquid film on inert solid)
• Can separate colourless compounds too: spray with a locating agent to make spots visible
  • e.g.
    • ninhydrin → amino acids and Tollens'
    • Fehling's → reducing sugars
    • iodine → most organic compounds
• Advantage: detects very small amounts (< 10⁻¹² of the substance)

Paper Chromatography

• solvent is the Mobile phase; cellulose is the stationary phase
• Components most soluble in solvent move up the most (weakest attraction to cellulose)
• Solvent front = boundary between wet and dry paper
• Lid on to reduce solvent evaporation
• Separation due to:
  • Different solubilities of solutes in the same solvent
  • Selective adsorption by the paper
• use pencil not pen for start line
  • Pen ink is soluble in the solvent, so it runs up the paper and forms its own spots, interfering with the results
  • Graphite (pencil) is insoluble, so it stays put
• steps
  1. Draw start line in pencil (not pen, ink would run)
  2. Place a drop of sample on start line
  3. Suspend paper in solvent, start line above solvent
  4. Remove paper, let it dry

Types of paper chromatography

Rf Value

• Rf = retardation factor
• $R_f = \frac{x}{y}$
  • x = distance moved by substance
  • y = distance moved by solvent front
• Ratio, so no units, between 0 and 1
• Constant for a given solvent and temperature, under saturated conditions
• One spot is pure; >1 spots is impure
• Substances are the same if they have the same no. of spots, same colours and same Rf value

2D Chromatography

• In normal chromatography, some spots overlap because they have similar Rf in one solvent
• 2D method: Run it twice with different solvents
  1. Run in solvent 1
  2. Turn paper 90°
  3. Run in solvent 2 (different polarity)
• Different solvent results in different $R_{f}$ and different sample distance, so the overlapping spots split apart
  

Other methods to determine purity

• melting and boiling point data
  • Pure substance will have specific melting/boiling point under fixed conditions
  • Mixtures melt/boil over a range of temperatures
  • More impurities result in larger change in melting/boiling point

Effect of Impurities (e.g. salt in water)

• Dissolved particles interfere with forming a solid lattice (lower freezing point)
• Need more heat to overcome intermolecular forces (higher boiling point)
• e.g. salt on roads in winter lowers freezing point, so ice does not form, reducing slippery conditions