Chapter 1 Experimental Chemistry – Comprehensive Study Notes

Physical Quantities, Units & Laboratory Measurement

Key Reasons for Quantification

Exact, reproducible data are mandatory for comparing results, scaling-up reactions or resolving disputes (e.g. the 1960 Olympic swimming controversy that led to electronic touch pads).
Chemistry relies on the International System of Units (SI) so results from any laboratory are intelligible worldwide.

Time

SI base unit = second ( $\text{s}$ ).
Common laboratory units: minute ( $\text{min}$ ) and hour ( $\text{h}$ ).
- $1\,\text{min}=60\,\text{s}$
- $1\,\text{h}=60\,\text{min}=3600\,\text{s}$
Apparatus
- Digital stopwatch — accuracy $\pm0.01\,\text{s}$ .
- Analogue/mechanical stopwatch — accuracy $\pm0.1\,\text{s}$ .
Choice criterion: pick the device whose least count is small enough for the phenomenon studied.

Temperature

SI base unit = kelvin ( $\text{K}$ ). Conversion:
$T{\text{(K)}} = T{(^{\circ}\text{C})}+273$
Celsius remains common in school labs.
Apparatus & ranges
- Alcohol thermometer: $\approx-10^{\circ}\text{C}\,\text{to}\,110^{\circ}\text{C}$ .
- Mercury thermometer (less common now for safety).
- Digital laboratory thermometer: wider range, fast response.
- Temperature probe + data-logger: enables automated real-time recording.
Essential conventions
- No degree symbol with kelvin.
- $\text{K}$ values can never be negative.

Length

SI base unit = metre ( $\text{m}$ ).
Decimal prefixes
- $1\,\text{m}=10\,\text{dm}=100\,\text{cm}=1000\,\text{mm}$
- $1\,\text{km}=10^{3}\,\text{m}$ , $1\,\mu\text{m}=10^{-6}\,\text{m}$ etc.
Apparatus
- Metre rule: accuracy $\pm0.1\,\text{cm}$ .
- Measuring tape: accuracy $\pm0.1\text{–}0.5\,\text{cm}$ , flexible, long lengths.
- Microscopic/atomic scales (Å, nm) implied for molecular dimensions.

Mass

SI base unit = kilogram ( $\text{kg}$ ).
- $1\,\text{kg}=1000\,\text{g}=1\,000\,000\,\text{mg}$
- Industrial scales: $1\,\text{t}=1000\,\text{kg},\;1\,\text{kt}=1000\,\text{t}$ .
Apparatus
- Electronic balance (digital): accuracy $\pm0.01\,\text{g}$ .
- Beam balance (traditional): accuracy set by calibration marks & counterweights.

Volume

SI derived unit = cubic metre ( $\text{m}^3$ ).
- $1\,\text{m}^3=1000\,\text{dm}^3=1\,000\,000\,\text{cm}^3$ .
- Note: conversion is cubic, not linear (a common misconception: $1\,\text{m}\neq10\,\text{dm}\;\Rightarrow\;1\,\text{m}^3\neq10\,\text{dm}^3$ ). A $1\,\text{m}$ cube actually holds $1000\,\text{dm}^3$ .
Liquid-measuring glassware (choose by required volume & precision)
- Pipette: fixed, high precision; e.g. $10.0\,\text{cm}^3,25.0\,\text{cm}^3$ .
- Volumetric flask: fixed but larger (e.g. $100\,\text{cm}^3,250\,\text{cm}^3$ ).
- Measuring cylinder: variable reading, to $\pm0.5\,\text{cm}^3$ .
- Burette: variable, to $\pm0.05\,\text{cm}^3$ — best for titration.
- Meniscus reading: eye level with curve; concave (water) read bottom, convex (Hg) read top; avoids parallax.
Gas measurement
- Gas syringe: typically $0\text{–}100\,\text{cm}^3$ .

Collection & Drying of Gases

Collection Methods (choose by solubility & density)

Method	Solubility	Density requirement	Typical gases
Water displacement	Insoluble / slightly soluble	Any	$\text{H}<em>2,\,\text{O}</em>2,\,\text{CO}_2$
Downward delivery	Soluble or insoluble	Denser than air	$\text{Cl}<em>2,\,\text{HCl},\,\text{SO}</em>2$
Upward delivery	Soluble or insoluble	Less dense than air	$\text{NH}_3$
Gas syringe	Usually small volumes, highest accuracy	Irrelevant	Universal

Take $M_r(\text{air})\approx30$ as density yardstick.

Drying Agents

Agent	Suitable gases	Notes
Concentrated $\text{H}<em>2\text{SO}</em>4$	Most, incl. $\text{Cl}_2,\,\text{HCl}$	Not for reactive bases like $\text{NH}_3$
Quicklime (CaO)	$\text{NH}_3$	Must be freshly heated; absorbs $\text{CO}_2$ too
Fused $\text{CaCl}_2$	$\text{H}<em>2,\,\text{N}</em>2,\,\text{CO}_2$	Reacts with $\text{NH}_3$ ; keep freshly fused

Gas is bubbled or passed over the drying agent before use.

Separation & Purification of Mixtures

Summary of Techniques

Mixture Type	Appropriate Methods
Solid–solid	Magnetic attraction, sieving, suitable solvent, sublimation
Solid–liquid	Filtration, evaporation to dryness, crystallisation, simple distillation
Liquid–liquid	Separating funnel (immiscible), chromatography (miscible), fractional distillation (miscible with $bp$ difference)

Solid–Solid in Detail

Magnetic attraction — isolates Fe, Ni, Co etc. (recycling plants, iron filings + sulfur).
Sieving — relies on particle size (baker’s flour, archaeology). Use mesh with suitable pore size.
Suitable solvent — exploit differential solubility (salt dissolves in water, sand remains).
Sublimation — one solid sublimes (dry ice, iodine, naphthalene) leaving non-volatile residue; collect solid on cool surface.

Solid–Liquid Mixtures

Filtration — separates insoluble solid (residue) from filtrate; coffee sock analogy.
Evaporation to dryness — removes solvent completely, recovers solute (sea-salt pans).
- Limitation: heat-sensitive solute decomposes (e.g. sugar → char).
Crystallisation — gentle; produce pure crystals from saturated solution; steps:
1. Heat to concentrate until saturation.
2. Cool slowly → crystals.
3. Filter, wash with cold distilled water, dry between filter papers.
Simple distillation — collects pure solvent AND retains solute; key parts: flask, condenser (counter-current cooling), thermometer at side-arm.

Liquid–Liquid Mixtures

Immiscible (heterogeneous) — separate by density using separating funnel; open tap sequentially.
Miscible but differing $bp$ — fractional distillation; fractionating column (glass beads) provides repeated condensation/evaporation, enriching lower-bp component at top.
- Example: ethanol ( $78^{\circ}\text{C}$ ) vs water ( $100^{\circ}\text{C}$ ).
- Column length ↔ ability to separate close $bp$ .
- Industrial uses: crude-oil refining, liquefied-air separation, spirit distillation.

Chromatography

Principle: components move at different rates with mobile phase (solvent) over stationary phase (paper/column) due to differential solubility & adsorption.
Procedure (paper/TLC version)
1. Draw graphite start line.
2. Spot sample + references.
3. Suspend with spot above solvent.
4. Allow solvent front to travel, remove, mark.
Retention factor ( $Rf$ ): $Rf = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}$
- Constant for same solvent & temperature irrespective of run time.
Identifying colourless compounds
- Use locating agents (e.g. ninhydrin for amino acids) producing coloured spots.
- Observe under UV light where compounds fluoresce.
Applications: food safety (pesticides), anti-doping (urine/blood), forensic DNA fragment separation.

Purity Analysis via Melting & Boiling Data

Pure substance → sharp, fixed $mp$ / $bp$ .
Mixture → range; impurity amount widens range & shifts value.
- Non-volatile impurities raise $bp$ , lower $mp$ (e.g. road salt lowers ice freezing point).
Experimental test
1. Carefully heat/ cool sample in melting-point apparatus.
2. Record temperature where last crystal disappears (melting) or first bubble forms (boiling).
3. Compare to literature value.
Example analysis
- Solid X melts at $0^{\circ}\text{C}$ → pure water ice.
- Solid Y melts from $-5^{\circ}\text{C}$ to $5^{\circ}\text{C}$ → mixture (e.g. tridecane + tetradecane).

Real-World & Ethical Connections

Municipal water: Singapore blends rainwater, imported water, NEWater (reverse osmosis + UV) and desalinated seawater (distillation); each method balances purity, energy cost, independence and environmental impact.
Electronics: semiconductor-grade silicon must reach parts-per-billion impurity levels; otherwise chip failure occurs.
Medicines: trace impurities can introduce toxicity or reduce efficacy; regulatory bodies enforce pharmacopeial purity.
Environmental/food safety: chromatography prevents harmful pesticide or dye adulteration.

Helpful Formulae & Quick-Reference Conversions

$1\,\text{kg}=10^3\,\text{g}=10^6\,\text{mg}$
$1\,\text{t}=10^3\,\text{kg}$ ; $1\,\text{kt}=10^6\,\text{kg}$
$1\,\text{m}^3=10^6\,\text{cm}^3=10^3\,\text{dm}^3$
$1\,\text{dm}^3=1000\,\text{cm}^3=1000\,\text{mL}=1\,\text{L}$
$\Delta T{\text{K}} = \Delta T{^{\circ}\text{C}}$ (size of degree is identical).

Experimental Accuracy & Good Practice Reminders

Always read liquid meniscus at eye level to avoid parallax.
Use boiling chips to prevent bumping in distillation.
Mount thermometer bulb level with sidearm exit for true vapour temperature.
In fractional distillation, change receiver immediately once $bp$ rises — prevents cross-contamination.
Prepare drying agents freshly (CaO, CaCl $2$ ) because they absorb atmospheric moisture/CO $2$ .
Label chromatogram solvent front as soon as paper is removed; solvent evaporates swiftly.

Frequently Asked Conceptual Questions & Model Responses

Why choose a burette over a measuring cylinder?
- $\pm0.05\,\text{cm}^3$ precision vs $\pm0.5\,\text{cm}^3$ ; allows titration drop-wise control.
Disadvantage of pipette?
- Fixed volume only; inflexible when multiple volumes needed.
Collection method for a gas lighter than air & insoluble in water (e.g. $\text{H}_2$ )?
- Upward displacement (upward delivery) or over water via displacement.
Drying a gas that reacts with acids?
- Avoid $\text{H}2\text{SO}4$ ; use CaO or fused CaCl $_2$ depending on reactivity.
Two separation techniques involving change of state?
- Distillation (liquid→vapour→liquid) and sublimation (solid→gas→solid).

Macro Concept Map (Textual)

Physical Quantities → Accurate Data → Proper Apparatus → Enables → Experiment Design & Separation Techniques → Produce Pure Substances → Verified by $mp/bp$ & Chromatography → Applied to Healthcare, Tech, Environment.

Each link emphasises that precise measurement is foundational to all downstream chemical analysis, industrial processing and societal applications.