Experimental Chemistry – Study Notes (Measurements, Gases, Separating Mixtures)

MEASUREMENTS

  • Topic: Experimental Chemistry focusing on measurements, gases, separating mixtures.

  • SI units and typical apparatus are introduced for key physical quantities.

  • Time

    • SI Unit: Second (s)
    • Apparatus: analogue stop watch, clock, digital stopwatch

  • Temperature

    • SI Unit: Kelvin (K)
    • Daily life unit: degree Celsius (°C)
    • Conversion: K=°C+273K = °C + 273
    • Types of thermometers: mercury-in-glass, alcohol-in-glass, thermocouple

  • Mass

    • SI Unit: kilogram (kg)
    • Other units: gram (g), milligram (mg)
    • Large mass/volume: tonne (t) where 1 exttonne=1000 kg1\ ext{tonne} = 1000\ \text{kg}
    • Apparatus: electronic balance, beam balance

  • Volume (liquids and gases)

    • SI Unit: cubic metre (m^3)
    • Large-volume unit: decimetre cubed (dm^3); 1 dm^3 = 1000 cm^3
    • Everyday unit: millilitres (mL) or litres (L); 1 L = 1000 mL
    • Apparatus for measuring liquids depends on volume and required accuracy: measuring cylinder, beaker, burette, pipette, gas syringe

  • Apparatus overview (typical tools used)

    • Stop watches (digital or analogue)
    • Clock, Thermometer (mercury-in-glass or alcohol-in-glass)
    • Temperature sensors: Thermocouple
    • Mass measuring devices: Electronic balance, Beam balance
    • Volume measuring devices: Measuring cylinder, Burette, Pipette, Gas syringe, Beaker

  • Accuracy and reading guidance for liquids

    • Reading liquids with a measuring cylinder: accuracy to 1 cm^3; read near the bottom of the meniscus. Do not record 23.0 cm^3 if you’re reading to the nearest 0.1 cm^3 because the instrument is accurate to 1 cm^3.
    • Burette: long scale from 0–50 cm^3; accuracy to 0.1 cm^3. Read the liquid level before and after the tap is opened; difference gives the volume delivered.
    • Bulb pipette: measures exact volumes such as 20.0, 25.0, or 50.0 cm^3 (not 31.0 cm^3).
    • Meniscus reading technique: for concave (low) meniscus, read at the bottom; for convex (high) meniscus, read at the top; ensure correct eye level.

  • Temperature and mass precision (typical values)

    • Temperature accuracy: ±0.5 °C
    • Mass accuracy (electronic balance): ±0.01 g
    • Liquid volumes in burette: ±0.1 cm^3 or better depending on device; pipette: ±0.05 cm^3

  • Reading hints

    • Lower/upper limits of the liquid levels influence accuracy; align eye with the correct part of the meniscus (bottom for concave, top for convex).
    • Example reading: if a burette reads 50.0 cm^3 at one point and 50.25 cm^3 at another, the difference is 0.25 cm^3, etc.

  • GASES AND GAS COLLECTION

  • Gas collection is influenced by two main factors:

    • Solubility of the gas in water
    • Density of the gas relative to air

  • Gas collection devices and capacity

    • Gas syringe for collecting and measuring gas volumes up to about 100 cm^3; can also be used to study rate of gas evolution (volume per unit time).

  • Solubility/density concepts (relative to air)

    • Gases differ in solubility in water; as temperature increases, gas solubility decreases.
    • Oxygen is more soluble than nitrogen in water (both have low solubility), affecting gas exchange in aquatic environments.
    • Relative molecular mass (Mr) influences density relative to air; density ∝ Mr.
    • Air has an approximate Mr ≈ 29 (mixture: ~20% O2, ~80% N2).
    • Example: CO_2, Mr = 44, is denser than air.

  • Methods of gas collection (general rules)

    • Downward displacement of water (collect over water): suitable for slightly soluble gases or gases with densities similar to air (e.g., O2, N2, CO, CO2, H2).
    • Downward delivery of gas or upward displacement of air (collect over air): for gases denser than air and when collecting a dry gas (e.g., CO2, SO2, Cl_2, HCl).
    • Upward delivery of gas or downward displacement of air (collect over air): for gases less dense than air and when collecting a dry gas (e.g., H2, NH3).
    • Gas syringe: for dry gases with similar density to air; also used to measure gas volumes over time.

  • Practical notes on gas collection (Table-style summary)

    • Hydrogen (Mr = 2): density less dense than air; very insoluble; method: downward displacement of water.
    • Oxygen (Mr = 32): slightly denser than air; soluble a little; method: downward displacement of water.
    • Carbon dioxide (Mr = 44): denser than air; moderately soluble in water; method: downward delivery of gas (over water) or downward displacement of water.
    • Ammonia (Mr = 17): less dense than air; highly soluble in water; collected by upward delivery or downward displacement of air depending on setup; not collected over water.
    • Sulfur dioxide (Mr = 64): denser than air; very soluble; method: downward delivery of gas.

  • PREPARATION AND DRYING OF GASES

  • Preparation of gases usually requires a reaction vessel; three main vessel types are used:
    1) Dropping funnel with a conical flask: liquid flow can be controlled by the tap; use when poisonous gases might be evolved; funnel tube kept above liquid level.
    2) Thistle funnel with a conical flask: tube goes into the liquid to prevent gas loss through tubing; used to prevent gas escape.
    3) Combustion tube: used for solid and gaseous reactants (e.g., steam with a metal to generate hydrogen).

  • Drying of gases

    • After gas generation, pass the gas through a drying agent to remove water vapor.
    • Drying agents types:
    • Liquid drying agent: conc. sulfuric acid (gas bubbled into acid in conical flask).
    • Solid drying agents: quicklime (CaO), anhydrous fused calcium chloride (CaCl2), soda-lime (NaOH + Ca(OH)2 mixture), silica (SiO_2).
    • Do not use acidic drying agents to dry alkaline gases (e.g., ammonia) and do not use basic drying agents to dry acidic gases (e.g., chlorine, SO2, CO2, HCl).

  • COLLECTION OF GASES (methods summarized)

  • The method chosen depends on gas properties:

    • Downward displacement of water: for slightly soluble gases or gases with density close to air (e.g., O2, N2, CO, CO2, H2).
    • Downward delivery of gas or upward displacement of air: for gases denser than air; collect dry gas.
    • Upward delivery of gas or downward displacement of air: for gases less dense than air; collect dry gas.
    • Gas syringe: for dry gases with density near air; can measure volume per unit time.

  • NOTE on drying and solubility

    • If a gas is required dry, it must not be collected over water.
    • A soluble gas must not be collected over water.

  • Normal method of collecting common gases (Table 1, abbreviated)

    • Hydrogen: Mr = 2; density less dense; insoluble; method: downward displacement of water.
    • Oxygen: Mr = 32; slightly denser; soluble modestly; method: downward displacement of water.
    • Carbon dioxide: Mr = 44; denser; soluble moderately; method: downward delivery of gas or downward displacement of water.
    • Ammonia: Mr = 17; less dense; very soluble; method: upward delivery of gas (or downward displacement of air) but typically collected by upward displacement of air due to being lighter than air and highly soluble.
    • Sulfur dioxide: Mr = 64; denser; very soluble; method: downward delivery of gas.

  • PREPARATION OF DRY HYDROGEN CHLORIDE GAS

  • Laboratory preparation from sodium chloride and concentrated sulfuric acid

  • Reactants: NaCl and conc. H2SO4

  • Procedure (summary):

    • Add conc. H2SO4 to NaCl in a suitable vessel via thistle funnel; heating drives gas evolution.
    • HCl gas is evolved with effervescence and dried by passing through conc. H2SO4.

  • Chemical reactions:

    • Below 200°C: ext{NaCl} + ext{H}2 ext{SO}4
      ightarrow ext{NaHSO}_4 + ext{HCl}
    • Above 200°C: 2\text{NaCl} + ext{H}2 ext{SO}4
      ightarrow ext{Na}2 ext{SO}4 + 2\text{HCl}

  • Purification: dry HCl by passage through conc. sulfuric acid.

  • Collection: HCl collected by upward displacement of air (gas heavier than air, Density of HCl ~ 1.28 times that of air).

  • Precautions:

    • Maintain temperature near 200°C.
    • Delivery tube end dipped in drying agent.
    • Lower end of thistle funnel dipped in conc. sulfuric acid.

  • PREPARATION OF AMMONIA GAS

  • From ammonium chloride and calcium hydroxide (classic):

    • Reactants: 2 NH4Cl + Ca(OH)2
    • Reaction: 2NH<em>4Cl+Ca(OH)</em>2→CaCl<em>2+2H</em>2extO+2NH32\text{NH}<em>4\text{Cl} + \text{Ca(OH)}</em>2 \rightarrow \text{CaCl}<em>2 + 2\text{H}</em>2 ext{O} + 2\text{NH}_3
    • Drying: pass gaseous NH3 through drying tower containing quicklime (CaO).
    • Collection: downward displacement of air (NH3 is lighter than air and highly soluble in water, so not collected over water).

  • From metal nitrides (alternative source):

    • Reactants: warm water on metal nitrides (e.g., Mg3N2, AlN)
    • Reaction: AlN+3H<em>2O→Al(OH)</em>3+NH3\text{AlN} + 3\text{H}<em>2\text{O} \rightarrow \text{Al(OH)}</em>3 + \text{NH}_3
    • Drying: pass through quicklime (CaO) in a drying tower.
    • Collection: NH3 collected by holding a jar with its mouth downwards over the delivery tube (downward displacement of air).

  • SEPARATING MIXTURES

  • Mixtures and separation goals

    • Pure substance: single substance with fixed properties (e.g., white sugar, copper sulfate crystals, distilled water).
    • Mixture: contains two or more substances (e.g., seawater, milk) and can be separated by physical methods.

  • Purification vs Separation

    • Purification: separation of mixtures into pure substances using physical methods without chemical reactions.

  • DESERT ISLAND: SEPARATING SALT, SAND, AND WATER

  • Goal: extract clean fresh water from seawater; plan includes equipment list and stepwise procedures with reasoning.

  • Emphasis on understanding why each step is needed (based on science knowledge).

  • FILTRATION

  • Used to separate insoluble solids from liquids (solid-liquid mixtures).

  • Filtration removes solid impurities from water or other liquids.

  • Experimental procedure (summary):

    • Place filter paper in a filter funnel.
    • Pour sand-water mixture into the funnel.
    • Liquid passes through; sand remains as residue on filter paper.

  • Applications

    • Water filtration in water purification plants.
    • Biological filtering roles: Nose and kidneys filtering particles and wastes.

  • CRYSTALLIZATION

  • Definition: separation of a dissolved solid from a solution by formation of crystals.

  • Process:

    • Evaporate solvent to reach crystallization point where solute crystallizes.
    • Filter to collect crystals and dry them.

  • Experimental steps (summary):

    • Heat solution to concentrate; allow to cool; crystals form.
    • Filter to obtain pure crystals; dry by pressing between filter papers.

  • Extra precautions:

    • Check saturation by dipping a clean rod; if crystals form on cooling, solution is saturated.
    • Avoid over-evaporation beyond saturation, which may produce powder instead of crystals.

  • Uses: purifying solids (e.g., pure sugar); obtaining soluble salts.

  • DISTILLATION

  • Distillation: method of obtaining a pure liquid (distillate) from a solution or separating a volatile liquid from a non-volatile solute.

  • Distillation setup components (summary):

    • Distillation flask (solution boils, vapor rises to condenser).
    • Condenser (cooling; condensate collected as distillate).

  • Experimental flow:

    • Heat solution in the flask to boiling; vapor travels to condenser; cooling forms distillate.
    • Collect distillate in a beaker after condenser.

  • Important operational steps:

    • Do not dip the thermometer into the solution; it should measure the boiling point of the distillate.
    • The condenser has two tubes: cold water enters at the bottom and exits at the top.
    • The condenser should slope so that distillate runs into the receiver.
    • If the distillate is volatile, place the receiver in a container of ice.

  • Uses: obtaining pure water from seawater.

  • FRACTIONAL DISTILLATION

  • Purpose: separate a mixture of miscible liquids with different boiling points.

  • Setup components: Thermometer, Fractionating Column, Distillation Flask, Condenser.

  • Concept:

    • Fractionating column increases surface area (glassy beads or wool) to improve separation.
    • Vapors rise; components condense on higher boiling points and return to the flask, allowing the lower-boiling liquid to distill first.
    • The beaker/receiver can be changed after each liquid is distilled.

  • Uses: industrial separation of nitrogen, argon, oxygen from air; refining crude oil into fractions; separating ethanol and water; separating miscible liquids.

  • SEPARATING FUNNEL (immiscible liquids)

  • purpose: separate immiscible liquids that do not mix.

  • Procedure summary:

    • Pour mixture into separating funnel; allow two layers to form (less dense on top).
    • Remove stopper, open tap to drain bottom layer into a separate container, leaving top layer behind.

  • PAPER CHROMATOGRAPHY

  • Definition: a method to separate and identify mixtures; can determine purity.

  • Uses: separation of coloured substances in dyes, analysis of urine, drugs, blood; used in sports drug testing to identify banned substances.

  • Procedure for separating coloured substances:

    • Draw a pencil line on filter paper and place a drop of dye above the line.
    • Place paper in a solvent with the level below the spot; solvent travels up, carrying components at different speeds.
    • More soluble components move further up; less soluble components lag behind; thus, separation occurs.

  • Identifying mixtures of coloured substances:

    • The resulting chromatogram may show multiple colored spots indicating multiple dyes; comparison with known samples helps identification.

  • Identifying colorless substances:

    • Use a locating agent sprayed on the paper; locating agent reacts with substances to produce visible colored products.

  • Distance-based identification (Rf values):

    • Solute retention factor: Rf=distance moved by solutedistance moved by solventR_f = \frac{\text{distance moved by solute}}{\text{distance moved by solvent}}
    • Example calculations (typical values shown in notes):
    • Rf(red) = \frac{10.0\,\text{cm}}{2.5\,\text{cm}} = 0.25
    • Rf(blue) = \frac{10.0\,\text{cm}}{7.5\,\text{cm}} = 1.333… (note: in data, denominator and numerator positions determine actual Rf; use given distances for your calculation)

  • Purity assessment via paper chromatography:

    • Compare heights/positions of spots or calculate Rf values against known substances.

  • CHECKING THE PURITY OF SUBSTANCES

  • Pure substances have fixed melting and boiling points:

    • Example: pure water boils at 100 °C and melts at 0 °C.

  • Impure substances have a range of melting/boiling points (no fixed values).

    • Example: salt in water raises the boiling point to about 102 °C.

  • Impurity effects on melting/boiling points:

    • Impure substances often have lower melting points and higher boiling points compared to the pure substance.

  • WHY IS PURITY IMPORTANT?

  • Food safety: pure substances ensure safe consumption; contaminated foods/drinks can cause adverse effects.

  • Pharmaceuticals: impure drugs can trigger harmful allergic reactions.

  • Electrical wiring: pure copper is needed for efficient conductivity.

  • Electronics manufacturing: silicon chips require very low impurity levels; impurities reduce device efficiency.

  • QUIZ PREPARATION (key prompts to practice)

  • Name appropriate apparatus with the SI unit for the measurement of time, temperature, mass, volume of liquid, and volume of gas.

  • Identify the two factors that determine the method of gas collection.

  • List the four methods used to collect gas.

  • NOTE: Throughout the content, be aware of the interplay between solubility, density, and the appropriate experimental setup when handling gases, liquids, and mixtures.