Chapter 1: Experimental Chemistry & Separation Techniques

Physical Quantities and Measurement Apparatus

• Physical quantities form the quantitative foundation of experimental chemistry. Each physical quantity possesses a standard SI unit as well as supplementary units used in experimental procedures.

• Time:

  • Definition: The measurement of how long it takes for a reaction or event to occur.

  • SI Unit: Second ($s$).

  • Other Units: Minute ($min$), Hour ($h$).

  • Unit Conversions:

  • $1\,min = 60\,s$

  • $1\,h = 60\,min = 3600\,s$

  • Apparatus:

  • Digital Stopwatch: Provides digital readings with an accuracy down to $\pm 0.01\,s$ (e.g., $0.87\,s$).

  • Analogue Stopwatch: Provides manual timing with an accuracy down to $\pm 0.1\,s$ (e.g., $15\,s$).

• Temperature:

  • Definition: The measure of how hot or cold a substance is.

  • SI Unit: Kelvin ($K$).

  • Other Units: Degree Celsius ($^\circ C$).

  • Unit Conversions:

  • $K = ^\circ C + 273$

  • Apparatus:

  • Alcohol Thermometer: Uses expanding alcohol to measure temperature across common experimental ranges (e.g., $-10\,^\circ C$ to $110\,^\circ C$).

  • Mercury Thermometer: Uses liquid mercury expansion across standard lab temperature ranges.

  • Digital Thermometer: Provides direct digital readout of temperature.

  • Data Loggers: Sensors linked to electronic recording devices that offer a significantly wider measurement range and continuous digital monitoring.

• Length:

  • Definition: The accurate distance measured between two distinct points.

  • SI Unit: Metre ($m$).

  • Other Units: Millimetre ($mm$), Centimetre ($cm$), Decimetre ($dm$).

  • Unit Conversions:

  • $1\,m = 10\,dm = 100\,cm = 1000\,mm$

  • Apparatus:

  • Metre Ruler: Used for straight line, rigid length measurements.

  • Measuring Tape: Used for longer or flexible distance measurements.

• Mass:

  • Definition: The quantitative measure of how much matter is present in a given substance.

  • SI Unit: Kilogram ($kg$).

  • Other Units: Gram ($g$), Milligram ($mg$), Tonne ($t$), Kilotonne ($kt$).

  • Unit Conversions:

  • $1\,kg = 1000\,g = 1\,000\,000\,mg$

  • $1\,t = 1000\,kg = 1\,000\,000\,g$

  • $1\,kt = 1000\,t = 1\,000\,000\,kg$

  • Apparatus:

  • Beam Balance: Mechanical balance comparing unknown mass against known weights.

  • Electronic Balance: Digital high-precision mass balance.

• Volume:

  • Definition: The measure of the amount of three-dimensional space a substance occupies.

  • SI Unit: Cubic metre ($m^3$).

  • Other Units: Cubic decimetre ($dm^3$), Cubic centimetre ($cm^3$).

  • Unit Conversions:

  • $1\,m^3 = 1000\,dm^3 = 1\,000\,000\,cm^3$

  • Apparatus Selection and Precision:

  • Measuring Cylinder: Used to deliver variable ranges of volumes; accurate to the nearest $0.5\,cm^3$

  • Burette: Used to deliver variable exact liquid volumes; accurate to the nearest $0.05\,cm^3$

  • Pipette: Used to accurately measure and transfer fixed volumes of liquid (e.g., exactly $10.0\,cm^3$ or $25.0\,cm^3$).

  • Volumetric Flask: Used to measure highly accurate fixed liquid volumes, typically for larger specific standard solution preparation.

Methods of Gas Collection and Gas Drying

• Selection of gas collection and drying apparatus depends entirely on the chemical reactivity, solubility in water, and relative density compared to air of the target gas.

• Gas Collection Methods:

  • Water Displacement:

  • Principle: The gas is bubbled into an inverted water-filled vessel.

  • Requirements: Applicable exclusively for gases that are insoluble or only slightly soluble in water.

  • Gas Density: Relative gas density to air does not affect collection via this method.

  • Upward Delivery:

  • Principle: Gas enters upward into an inverted vessel, forcing air downward out of the container.

  • Requirements: Gas must be less dense than air.

  • Water Solubility: Water solubility does not affect upward delivery collection.

  • Downward Delivery:

  • Principle: Gas enters downward into an upright vessel, forcing air upward out of the container.

  • Requirements: Gas must be denser than air.

  • Water Solubility: Water solubility does not affect downward delivery collection.

• Gas Drying Methods:

  • Concentrated Sulfuric Acid ($\text{H}_2\text{SO}_4$):

  • Nature: Acidic drying agent.

  • Application: Used to dry acidic and neutral gases by passing moist gas through a delivery tube into concentrated acid.

  • Exception/Restriction: Cannot be used to dry basic gases like ammonia ($\text{NH}_3$) because ammonia reacts chemically with concentrated sulfuric acid.

  • Calcium Oxide (Quicklime, $\text{CaO}$):

  • Nature: Basic drying agent.

  • Application: Used to dry neutral or basic/alkaline gases such as ammonia ($\text{NH}_3$).

  • Exception/Restriction: Cannot dry gases that react chemically with calcium oxide.

  • Fused Calcium Chloride ($\text{CaCl}_2$):

  • Nature: Neutral drying agent.

  • Application: Used in a drying tube to absorb moisture from neutral and non-reactive gases.

Chemical Neutralization Reactions

• A classical aqueous reaction involved in chemical measurements is the neutralization of an acid by an alkali to produce a salt and water:

• Word Equation:   $\text{acid} + \text{alkali} \rightarrow \text{salt} + \text{water}$

• Specific Example:   $\text{Hydrochloric acid} + \text{Potassium hydroxide} \rightarrow \text{Potassium chloride} + \text{water}$

• Balanced Chemical Equation with State Symbols:   $\text{HCl}(aq) + \text{KOH}(aq) \rightarrow \text{KCl}(aq) + \text{H}_2\text{O}(l)$

Purity and Separation Techniques

• Purity of a Substance:

  • A pure substance consists of only one single compound or element and exhibits exact fixed physical properties, such as a sharp, specific melting point or boiling point under fixed atmospheric conditions.

• Separation of Solid-Solid Mixtures:

  • Magnetic Attraction: Employs a magnet to extract magnetic solids (e.g., iron filings) away from non-magnetic solid mixtures.

  • Sieving: Uses a mesh sieve to separate solid mixtures containing particles of distinctly different sizes.

  • Suitable Solvents: Applied to solid-solid mixtures where only one component dissolves in a specific solvent. The soluble solid dissolves, while the insoluble solid can be filtered out.

  • Sublimation: Used when one solid component transitions directly from a solid state into a gaseous state upon heating without passing through a liquid phase.

• Separation of Liquid-Liquid Mixtures:

  • Separating Funnel:

  • Application: Used to separate immiscible liquids (liquids that do not mix and form distinct layers, such as oil and water).

  • Principle: The denser liquid settles at the bottom and is drained out via the stopcock first, leaving the less dense liquid inside the funnel.

  • Fractional Distillation:

  • Application: Used to separate miscible liquids (liquids that completely mix) possessing different boiling points.

  • Mechanism: Heating the mixture vaporizes the liquid with the lower boiling point first, which ascends the fractionating column to reach the condenser.

  • Liebig Condenser Configuration:

    • Cold water enters the condenser at the lowest point ("water in") and exits from the top ("water out").

    • Purpose of Counter-Current Flow: Maximizes heat exchange efficiency and maintains the highest continuous temperature gradient across the condenser tube.

    • The bottom entry ensures the lowest outlet region remains the coolest point, guaranteeing that all vapor fully condenses back into a liquid distillate prior to collection.

• Separation of Solid-Liquid Mixtures:

  • Simple Distillation: Used to recover and separate a pure solvent (liquid) from a solution containing dissolved solids.

  • Evaporation to Dryness: Used to separate a heat-stable dissolved solid from its liquid solvent by heating the mixture until all liquid solvent has completely vaporized.

  • Crystallisation: Used to obtain a pure solid crystal sample (e.g., Copper(II) sulfate crystals, $\text{CuSO}_4$) from its saturated solution without thermal decomposition.

• Paper Chromatography:

  • Purpose: Used to separate and distinguish mixtures of substances (such as dye components) based on their differing solubilities in a given mobile solvent.

  • Mechanism:

  • As the solvent moves up the stationary chromatography paper, components separate along the strip.

  • The most soluble components travel the furthest up the paper, while denser or less soluble components travel shorter distances.

  • Locating Agents: Chemical reagents sprayed or applied onto chromatography paper to react with colorless spots, turning them into visible colored spots for analysis.

  • Retention Factor ($R_f$ Value):

  • Function: Provides a standardized ratio used to identify individual constituent components in a dye or mixture.

  • Formula:       $R_f = \frac{\text{distance travelled by spot}}{\text{distance travelled by solvent}}$