Chemistry (5070)
Topic 9 of 13Cambridge O Levels

Experimental Techniques & Chemical Analysis

Mastering lab methods for separating mixtures and identifying unknown chemical substances.

In chemistry, practical skills are as vital as theoretical knowledge. This section covers the fundamental experimental techniques used to measure, separate, purify, and identify chemical substances, forming the bedrock of laboratory work.


### Criteria of Purity and Measurement


The first step in any chemical work is accurate measurement. We use a balance for mass (g), a measuring cylinder, pipette, or burette for volume (cm³), a thermometer for temperature (°C), and a stopwatch for time (s). Pipettes and burettes provide more precise volume measurements than a measuring cylinder.


To determine if a substance is pure, we check its melting point and boiling point. A pure substance has a sharp, fixed melting point and boiling point at a specific pressure. For example, pure water boils at exactly 100°C and melts at 0°C (at standard pressure). The presence of impurities disrupts the regular arrangement of particles, causing the substance to melt over a range of temperatures (and at a lower temperature) and boil over a range (at a higher temperature).


### Methods of Separation and Purification


Most substances in nature are found as mixtures. We use various techniques to separate them based on their physical properties.


  • Filtration: This technique is used to separate an insoluble solid from a liquid. For example, separating sand from water. The mixture is poured through filter paper in a funnel. The liquid that passes through is called the filtrate, and the solid left behind on the filter paper is the residue.

  • Crystallisation: Used to obtain a pure soluble solid from its solution. The solution is heated to evaporate some of the solvent until a saturated solution is formed. This is checked by dipping a glass rod into the solution; crystals will form on it upon cooling. The solution is then allowed to cool slowly, causing the dissolved solid to form pure crystals. These crystals are then separated by filtration and dried.

  • Simple Distillation: This method separates a pure liquid solvent from a solution containing a dissolved solid (e.g., obtaining pure water from salt water). The solution is heated in a distillation flask. The solvent, having a lower boiling point than the solute, turns into vapour. The vapour passes into a condenser, where it is cooled by circulating water and turns back into a pure liquid (distillate), which is collected.

  • Fractional Distillation: This is used to separate a mixture of two or more miscible liquids with different boiling points, such as ethanol and water. The apparatus includes a fractionating column packed with glass beads or rings. When the mixture is heated, the liquid with the lower boiling point evaporates first. The vapours rise up the column, cool, condense, and re-vaporise multiple times. This process enriches the vapour with the more volatile component, which eventually reaches the top of the column and is distilled and collected. This technique is crucial for separating crude oil into fractions like petrol and diesel.

  • Chromatography: A powerful technique to separate a mixture of soluble substances. In paper chromatography, a spot of the mixture is placed on a baseline drawn on chromatography paper (the stationary phase). The paper is then placed in a beaker with a solvent (the mobile phase), ensuring the solvent level is below the baseline. As the solvent moves up the paper, it carries the components of the mixture at different rates depending on their solubility in the solvent and their attraction to the paper. The separated spots can be identified by calculating their Retardation Factor (Rf value).

    • Rf = (distance travelled by substance) / (distance travelled by solvent front)


    The Rf value is a constant for a particular substance under specific conditions (same solvent and paper).


    ### Qualitative Analysis: Identifying Ions and Gases


    Qualitative analysis is the process of identifying unknown substances.


    Tests for Gases:

  • Ammonia (NH₃): Turns damp red litmus paper blue; has a characteristic pungent smell.
  • Carbon dioxide (CO₂): Bubbled through limewater (aqueous calcium hydroxide), it turns the solution milky or cloudy due to the formation of a white precipitate of calcium carbonate.
  • Chlorine (Cl₂): Bleaches damp litmus paper.
  • Hydrogen (H₂): A lighted splint placed at the mouth of the test tube produces a 'pop' sound.
  • Oxygen (O₂): A glowing splint relights when placed in the gas.

    • Tests for Cations (Positive Ions):

    A few drops of aqueous sodium hydroxide (NaOH) or aqueous ammonia (NH₃) are added to the unknown solution.

  • Ammonium (NH₄⁺): Add NaOH(aq) and warm; ammonia gas is produced.
  • Copper(II) (Cu²⁺): Forms a light blue precipitate with both reagents. Precipitate dissolves in excess aqueous ammonia to form a dark blue solution.
  • Iron(II) (Fe²⁺): Forms a green precipitate with both reagents.
  • Iron(III) (Fe³⁺): Forms a red-brown precipitate with both reagents.
  • Zinc (Zn²⁺): Forms a white precipitate with both reagents, which dissolves in excess of either reagent to form a colourless solution (amphoteric).
  • Calcium (Ca²⁺): Forms a white precipitate with NaOH(aq); no (or very slight) precipitate with NH₃(aq).

    • Tests for Anions (Negative Ions):

  • Carbonate (CO₃²⁻): Add a dilute acid (e.g., HCl). Effervescence (fizzing) occurs. The gas produced is CO₂.
  • Chloride (Cl⁻): First, acidify with dilute nitric acid, then add aqueous silver nitrate. A white precipitate (AgCl) forms.
  • Sulfate (SO₄²⁻): First, acidify with dilute nitric acid, then add aqueous barium nitrate. A white precipitate (BaSO₄) forms.
  • Nitrate (NO₃⁻): Add aqueous sodium hydroxide and aluminium foil, then warm. Ammonia gas is produced.

    • Key Points to Remember

    • 1Purity is assessed by fixed, sharp melting and boiling points; impurities lower melting points and raise boiling points over a range.
    • 2Filtration separates an insoluble solid from a liquid, while crystallisation obtains a pure soluble solid from a solution.
    • 3Simple distillation separates a liquid solvent from a solution; fractional distillation separates miscible liquids with different boiling points.
    • 4Chromatography separates mixtures based on differential movement between a stationary and a mobile phase, identified by Rf values.
    • 5Characteristic tests identify gases: O₂ relights a glowing splint, H₂ 'pops', CO₂ turns limewater milky, and NH₃ turns damp red litmus blue.
    • 6Aqueous cations (Cu²⁺, Fe²⁺, Fe³⁺, Zn²⁺) are identified by the colour and solubility of their precipitates with NaOH(aq) and NH₃(aq).
    • 7Key anion tests include adding acid for carbonates, silver nitrate for chlorides, and barium nitrate for sulfates.
    • 8Accurate measurement using balances, pipettes, burettes, and thermometers is fundamental to reliable experimental results.

    Pakistan Example

    Crystallisation in Pakistan's Sugar Industry

    The production of sugar from sugarcane is a major industry in Pakistan, particularly in Punjab and Sindh. The process relies heavily on the principle of **crystallisation**. After extracting juice from the sugarcane, it is heated and concentrated to form a thick, supersaturated syrup. This syrup is then carefully cooled, allowing pure sucrose to form large crystals. These raw sugar crystals are then separated from the remaining liquid (molasses) by centrifugation, which is a large-scale application of filtration. This process is a perfect real-world example of how crystallisation is used to purify a soluble solid on an industrial scale.

    Quick Revision Infographic

    Chemistry — Quick Revision

    Experimental Techniques & Chemical Analysis

    Key Concepts

    1Purity is assessed by fixed, sharp melting and boiling points; impurities lower melting points and raise boiling points over a range.
    2Filtration separates an insoluble solid from a liquid, while crystallisation obtains a pure soluble solid from a solution.
    3Simple distillation separates a liquid solvent from a solution; fractional distillation separates miscible liquids with different boiling points.
    4Chromatography separates mixtures based on differential movement between a stationary and a mobile phase, identified by Rf values.
    5Characteristic tests identify gases: O₂ relights a glowing splint, H₂ 'pops', CO₂ turns limewater milky, and NH₃ turns damp red litmus blue.
    6Aqueous cations (Cu²⁺, Fe²⁺, Fe³⁺, Zn²⁺) are identified by the colour and solubility of their precipitates with NaOH(aq) and NH₃(aq).
    Pakistan Example

    Crystallisation in Pakistan's Sugar Industry

    The production of sugar from sugarcane is a major industry in Pakistan, particularly in Punjab and Sindh. The process relies heavily on the principle of **crystallisation**. After extracting juice from the sugarcane, it is heated and concentrated to form a thick, supersaturated syrup. This syrup is then carefully cooled, allowing pure sucrose to form large crystals. These raw sugar crystals are then separated from the remaining liquid (molasses) by centrifugation, which is a large-scale application of filtration. This process is a perfect real-world example of how crystallisation is used to purify a soluble solid on an industrial scale.

    SeekhoAsaan.com — Free RevisionExperimental Techniques & Chemical Analysis Infographic

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