Experimental Techniques and Chemical Analysis Notes

Measurement of Physical Quantities

• Measuring Time

  • The S.I. unit for time is the second ($s$).

  • Other units for longer intervals include the minute ($min$) and the hour ($h$).

  • Conversion factors:

    • $1\,h = 60\,min$

    • $1\,min = 60\,s$

• Measuring Temperature

  • The S.I. unit for temperature is the Kelvin ($K$).

  • The degree Celsius ($^{\circ}C$) is a commonly used alternative unit.

  • Conversion formula: $\text{Temperature in K} = \text{Temperature in }^{\circ}\text{C} + 273$.

  • In scientific notation, there is no degree sign ($^{\circ}$) placed before the symbol for Kelvin ($K$).

• Measuring Mass

  • The S.I. unit for mass is the kilogram ($kg$).

  • Conversion factors:

    • $1\,kg = 1000\,g$

    • $1\,\text{tonne (t)} = 1000\,kg$

  • Mass is measured using an electronic balance.

• Measuring Volume of Liquids

  • The S.I. unit for volume is the cubic metre ($m^3$).

  • Other units include the cubic centimetre ($cm^3$) and the cubic decimetre ($dm^3$).

  • Conversion factors:

    • $1\,m^3 = 1000\,dm^3$

    • $1\,dm^3 = 1000\,cm^3$

  • Apparatus categorization:

    • 'To contain' (TC)

    • ‘To deliver’ (TD)

  • Reading the meniscus:

    • Liquids in containers form a curved surface called a meniscus.

    • To obtain an accurate reading, align the eyes to the liquid level.

    • For water or solutions, read the scale at the bottom of the meniscus.

    • Example measurement in a burette: $25.60\,cm^3$.

• Measuring Volume of Gases

  • A gas syringe is used to measure gas volume.

  • A standard gas syringe measures reaching a maximum volume of $100\,cm^3$.

Methods for Collecting Gases

The method of gas collection depends on two physical properties of the gas: its solubility in water and its density compared to air.

• Displacement of Water

  • Used for gases that are insoluble or only slightly soluble in water, such as hydrogen or oxygen.

• Downward Delivery

  • Used for gases that are soluble in water and denser than air, such as chlorine or hydrogen chloride.

• Upward Delivery

  • Used for gases that are soluble in water and less dense than air, such as ammonia ($NH_3$).

Experimental Set-ups for Drying Gases

• Concentrated Sulfuric Acid ($H_2SO_4$)

  • Used to dry most gases.

  • Limitation: It cannot be used to dry ammonia ($NH_3$) because it reacts with it.

  • The gas-introducing tube must be immersed in the acid; the exiting tube must not be.

• Fused Calcium Chloride ($CaCl_2$)

  • Used for drying most gases.

• Quicklime (Calcium Oxide, $CaO$)

  • Specifically used for drying ammonia ($NH_3$).

Common Experimental Terms and Solutions

• Definitions

  • Solute: The substance that dissolves in a solvent to form a solution.

  • Solvent: The liquid in which a solute dissolves.

  • Solution: A mixture formed when a solute dissolves in a solvent.

  • Saturated Solution: A solution containing the maximum concentration of a solute dissolved in a solvent at a specified temperature. Adding more solute to this solution results in no further dissolution.

  • Filtrate: The liquid that passes through filter paper during filtration.

  • Residue: The solid trapped on filter paper during filtration.

• Examples of Solutions

  • Soft drinks (solutes: sugar, carbon dioxide; solvent: water).

  • Vinegar (solute: ethanoic acid; solvent: water).

  • Tincture of iodine (solute: iodine; solvent: ethanol).

  • Sugar syrup (solute: sugar; solvent: water).

Acid-Base Titrations

• Purpose

  • To determine the exact volumes of acid and alkali that must be mixed to create a solution containing only salt and water (neutralization).

• Apparatus Required

  • Burette

  • Volumetric pipette

  • Pipette filler (used to draw liquid into the pipette)

  • Conical flask

  • Retort stand

  • Filter funnel

  • Indicator (to identify the end-point)

Chromatography

Chromatography is used to separate two or more components that dissolve in the same solvent.

• Procedure

  1. Draw a pencil line approximately $1\,cm$ from the bottom of the paper.

  2. Place a drop of the mixture (e.g., green food colouring) on the line and let it dry.

  3. Dip the paper into a solvent (e.g., ethanol), ensuring the solvent level is below the pencil line.

  4. The solvent travels up the paper, carrying components at different speeds based on their solubility. More soluble substances move further.

• Interpreting Chromatograms

  • Purity: A pure substance produces a single spot. A mixture produces multiple spots.

  • Identification: Identical substances produce spots at the same height and colour when using the same solvent.

  • Locating Agents: Used to identify colourless substances (e.g., amino acids). Ninhydrin is a locating agent that reacts with amino acids to form deep blue or purple spots.

• Retardation Factor ($R_f$)

  • The ratio between the distance traveled by the substance and the distance traveled by the solvent.

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

  • Rf values are constant for a substance under fixed conditions (same solvent and temperature).

  • Example calculation: If the substance travels $3\,cm$ and the solvent front is at $4.5\,cm$, $R_f = \frac{3}{4.5} = 0.67$.

Separation and Purification Techniques

• Filtration: Separates insoluble solid particles from a liquid (e.g., sand from water).

• Evaporation to Dryness: Recovers a soluble solid from a solution by heating until all water boils off (e.g., salt from seawater).

• Crystallisation: Recovers pure crystals by heating a solution until hot and saturated, then allowing it to cool to room temperature (e.g., sugar from sugarcane).

• Simple Distillation: Separates a pure solvent from a solution (e.g., pure water from salt solution). It involves boiling the liquid and condensing the vapour.

  • Condenser setup: Cold water enters at the bottom and leaves through the top.

  • Thermometer: Placed beside the side arm of the flask to measure the boiling point of the distillate.

• Fractional Distillation: Separates a mixture of miscible liquids with close boiling points. The liquid with the lowest boiling point distils over first.

Purity Assessment via Melting and Boiling Points

• Melting Point

  • A solid is pure if it has an exact and fixed melting point (e.g., benzoic acid at $122\,^{\circ}C$).

  • Impurities lower the melting point and cause melting to occur over a range of temperatures (e.g., $118\text{--}121\,^{\circ}C$).

• Boiling Point

  • A liquid is pure if it has a fixed boiling point (e.g., methanoic acid at $101\,^{\circ}C$).

  • Impurities increase the boiling point and cause boiling to occur over a temperature range.

Identification of Ions and Gases

• Anion Tests

  • Carbonate ($CO_3^{2-}$): Add dilute acid; check for $CO_2$ (limewater turns milky).

  • Chloride ($Cl^-$): Acidify with dilute nitric acid, add aqueous silver nitrate; results in a white precipitate.

  • Bromide ($Br^-$): Acidify with dilute nitric acid, add aqueous silver nitrate; results in a cream precipitate.

  • Iodide ($I^-$): Acidify with dilute nitric acid, add aqueous silver nitrate; results in a yellow precipitate.

  • Nitrate ($NO_3^-$): Add aqueous sodium hydroxide and aluminium foil; warm carefully. Ammonia gas is given off (turns red litmus blue).

  • Sulfate ($SO_4^{2-}$): Add dilute nitric acid, then aqueous barium nitrate; results in a white precipitate of barium sulfate.

  • Sulfite ($SO_3^{2-}$): Add acidified aqueous potassium manganate(VII); solution changes from purple to colourless.

• Cation Tests (Aqueous)

  • Aluminium ($Al^{3+}$): White precipitate with $NaOH$ (soluble in excess) and ammonia (insoluble in excess).

  • Ammonium ($NH_4^+$): $NaOH$ plus heat produces ammonia gas; no precipitate with ammonia.

  • Calcium ($Ca^{2+}$): White precipitate with $NaOH$ (insoluble in excess); no precipitate with ammonia.

  • Chromium(III) ($Cr^{3+}$): Green precipitate with $NaOH$ (soluble in excess) and ammonia (insoluble in excess).

  • Copper(II) ($Cu^{2+}$): Light blue precipitate with $NaOH$ (insoluble) and ammonia (dissolves in excess to form deep blue solution).

  • Iron(II) ($Fe^{2+}$): Green precipitate with both reagents (insoluble in excess).

  • Iron(III) ($Fe^{3+}$): Red-brown precipitate with both reagents (insoluble in excess).

  • Zinc ($Zn^{2+}$): White precipitate (soluble in excess for both reagents).

• Flame Tests for Cations

  • Lithium ($Li^+$): Red

  • Sodium ($Na^+$): Yellow

  • Potassium ($K^+$): Lilac

  • Calcium ($Ca^{2+}$): Orange-red

  • Barium ($Ba^{2+}$): Light green

  • Copper(II) ($Cu^{2+}$): Blue-green

• Gas Identification

  • Ammonia ($NH_3$): Pungent smell; turns moist red litmus paper blue.

  • Carbon Dioxide ($CO_2$): Odourless; produces a white precipitate in limewater.

  • Chlorine ($Cl_2$): Pungent smell; bleaches moist litmus paper.

  • Hydrogen ($H_2$): Odourless; pops with a lighted splint.

  • Oxygen ($O_2$): Odourless; relights a glowing splint.

  • Sulfur Dioxide ($SO_2$): Pungent smell; turns acidified aqueous potassium manganate(VII) from purple to colourless.

Questions & Discussion

• Question: Name the pieces of apparatus shown in the picture on the previous slide. What are their uses?

• Answer: Apparatus pictured includes burettes (for delivering precise volumes) and pipettes (for accurate fixed volume transfer).

• Question: What are some quantities that are commonly measured in experiments?

• Answer: Time, temperature, mass, and volume of liquids or gases.

• Question: What are some safety precautions taken by the chemist?

• Answer: Wearing protective gear, handling concentrated acids (like sulfuric acid) carefully, and ensuring proper ventilation during the production of pungent gases.

• Question: Suggest the type of compounds that form the coloured mixtures.

• Answer: Dyes used in food coloring or inks typically form these mixtures.