Introduction: The Chemistry Toolkit

Hello! Welcome to the final chapter in the Chemistry section. While previous chapters focused on *what* chemistry is (atoms, bonds, reactions), this chapter focuses on the crucial question: How do we actually *do* chemistry?

This section, "Experimental techniques and chemical analysis," is your practical guide. It teaches you how to measure things accurately, how to separate complex mixtures (like separating salt from sand), and how to identify mysterious substances in a test tube. These are the fundamental skills you need to be a real chemist!

C12.1 Experimental Design: Measuring Up

Essential Laboratory Apparatus

To perform accurate chemistry experiments, you must use the correct tools. Choosing the right piece of equipment often depends on the level of precision (how detailed the measurement is) required.

Measuring Time, Temperature, Mass, and Volume
  • Time: Measured using stop-watches or digital timers. (Precision matters for rate of reaction experiments!)
  • Temperature: Measured using thermometers. Always read the thermometer scale carefully to avoid parallax error.
  • Mass: Measured using balances. These are digital scales used to find the mass of reactants or products.
  • Volume (Highly Precise Liquids):
    • Burettes: Long, narrow glass tubes used for titration. They measure the volume of liquid dispensed very accurately (usually to \(\pm 0.05 \text{ cm}^3\)).
    • Volumetric Pipettes: Used to measure and transfer one exact, fixed volume of liquid (e.g., exactly \(\text{25 cm}^3\)).
  • Volume (Less Precise Liquids/Gases):
    • Measuring Cylinders: Used for measuring approximate volumes of liquids. They are less precise than burettes or pipettes.
    • Gas Syringes: Used to collect and measure the volume of gas produced during a reaction.

Understanding Solutions and Mixtures

When you make a cup of coffee, you are dealing with several key chemical concepts, all of which are important definitions in the lab!

  • Solvent: The substance that dissolves the solute. It is usually the substance present in the largest amount (e.g., the water in your coffee).
  • Solute: The substance that is dissolved in the solvent (e.g., the sugar or coffee powder).
  • Solution: The resulting mixture formed when the solute is dissolved in the solvent. It is typically transparent (e.g., the whole cup of coffee).
  • Saturated Solution: A solution containing the maximum concentration of solute dissolved in the solvent at a specified temperature. If you add any more solute, it won't dissolve—it will just sink to the bottom.

Did you know? Increasing the temperature usually increases how much solute can dissolve in a solvent. This is why you can dissolve more sugar in hot tea than in iced tea!

Key Definitions from Separation Processes
  • Residue: The solid substance that remains behind after a separation process like filtration or evaporation.
  • Filtrate: The liquid or solution that has passed through the filter paper during filtration.

Quick Review: The solvent does the dissolving; the solute gets dissolved. Filtration leaves the solid residue and produces the liquid filtrate.

C12.3 Separation and Purification Techniques

In the lab, chemicals are often mixed up or impure. We need techniques to separate these mixtures or purify the substances we want.

1. Separation by Solvent and Filtration

This method is used when you have a mixture of two solids, where only one solid is soluble (dissolves) in a specific solvent.

Step-by-step example (separating salt from sand):

  1. Add a suitable solvent: Add water (the suitable solvent) to the mixture of salt (soluble solute) and sand (insoluble solid). Stir to dissolve the salt.
  2. Filtration: Pour the mixture through filter paper in a funnel. The sand (residue) stays behind, and the salt solution (filtrate) passes through.

2. Purification by Crystallisation

Crystallisation is used to obtain a pure solid solute from a solution, often after filtration.

  1. Heating (Evaporation): Gently heat the filtrate (the salt solution from the example above) to evaporate most of the solvent (water). Stop heating when the solution is near saturation (i.e., when small crystals start to form around the edge—this is called the point of crystallisation).
  2. Cooling: Allow the remaining hot, concentrated solution to cool slowly. As it cools, the solubility of the salt decreases, and pure crystals form.
  3. Collection: Filter the crystals, wash them with a little cold distilled water, and dry them.

3. Simple Distillation

Distillation is used to separate a solvent from a solute (e.g., getting pure water from salt water).

  • The process relies on the difference in boiling points. The solvent (lower boiling point) boils first, turns into a gas, and is then cooled and condensed back into a liquid (the distillate). The solute (higher boiling point) is left behind.
  • This technique is used to obtain pure liquid (like pure water) from a solution.

4. Fractional Distillation (For liquids mixing together)

This technique is used to separate two or more liquids that are miscible (they mix completely, like water and ethanol).

  • A tall column (fractionating column) is placed above the distillation flask.
  • The liquid mixture is heated. Vapours rise up the column.
  • Because the column is cooler at the top, the vapour of the liquid with the lower boiling point reaches the top and passes into the condenser, separating it effectively. The liquid with the higher boiling point tends to condense and drip back down.
  • Real-world example: This is the main industrial method used to separate crude oil into useful fractions (like petrol and diesel).

Key Takeaway: Use filtration for solids/liquids. Use crystallisation for obtaining pure solids. Use distillation for separating liquids (simple for solvent/solute, fractional for liquid/liquid mixtures).

C12.2 Chromatography

Chromatography is a powerful technique used to separate mixtures of soluble coloured substances (like inks or dyes) using a suitable solvent.

How Paper Chromatography Works (Core Content)

  1. A spot of the mixture (e.g., black ink) is placed on a line drawn near the bottom of a piece of chromatography paper (the stationary phase).
  2. The bottom edge of the paper is dipped into a solvent (the mobile phase), ensuring the solvent level is below the spot line.
  3. The solvent moves up the paper, carrying the components of the mixture with it.
  4. Different substances travel at different speeds, depending on two factors:
    • How soluble they are in the solvent.
    • How strongly they are attracted to the paper.
  5. Components separate out into different spots, forming a chromatogram.
Interpreting the Chromatogram (Core Content)
  • Pure Substances: A pure substance shows only one spot on the chromatogram.
  • Impure Substances/Mixtures: An impure substance or mixture will separate into two or more spots.
  • Identifying Unknowns: If an unknown substance produces a spot that travels the exact same distance as a known substance (when run under identical conditions), the unknown substance likely contains the known substance.

The Rf Value (Retention Factor) (Supplement Content)

For Extended candidates, the R_f value provides a numerical way to identify substances.

The R\(_f\) value is the ratio of the distance travelled by the substance to the distance travelled by the solvent front.

$$ R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}} $$

Since the substance can never travel further than the solvent, the R\(_f\) value is always between 0 and 1.
It is a constant for a specific compound, solvent, and temperature, making it a reliable way to confirm identity.

Memory Aid: R\(_f\) is distance of the *Spot* over distance of the *Solvent Front*.

C12.4 Chemical Analysis: Identification Tests

Qualitative analysis is about identifying what chemicals are present. You need to know the specific reagents (the chemicals you add) and the specific, observable results (colour change, precipitate formation, gas produced).

1. Tests for Anions (Negative Ions)

These tests usually involve observing a precipitate or the production of a specific gas.

Test for Carbonate Ion, \(\mathbf{CO_3^{2-}}\)
  1. Add: Dilute acid (e.g., dilute hydrochloric acid).
  2. Observe: Immediate effervescence (fizzing/bubbling) of a colourless gas.
  3. Test the gas: Bubble the gas produced through limewater (aqueous calcium hydroxide).
  4. Result: Limewater turns milky (confirming the gas is carbon dioxide, \(\text{CO}_2\)).
Tests for Halide Ions (Chloride, Bromide, Iodide)

This test uses aqueous silver nitrate (\(\text{AgNO}_3\)) after acidifying with dilute nitric acid (\(\text{HNO}_3\)) to remove any interfering carbonate ions.

  1. Acidify: Add dilute nitric acid to the solution.
  2. Add: Aqueous silver nitrate.
  3. Results:
    • Chloride, \(\mathbf{Cl^-}\): White precipitate (\(\text{ppt.}\)) formed.
    • Bromide, \(\mathbf{Br^-}\): Cream precipitate formed.
    • Iodide, \(\mathbf{I^-}\): Yellow precipitate formed.
Test for Sulfate Ion, \(\mathbf{SO_4^{2-}}\)

This test uses aqueous barium nitrate (\(\text{Ba(NO}_3\text{)}_2\)) to check for the presence of sulfates.

  1. Acidify: Add dilute nitric acid.
  2. Add: Aqueous barium nitrate.
  3. Result: A white precipitate is formed (confirming sulfate ions are present).

2. Tests for Cations (Positive Ions) using Aqueous \(\mathbf{NaOH}\) and Aqueous \(\mathbf{NH_3}\)

Many metal ions (cations) form coloured precipitates when reacted with sodium hydroxide (\(\text{NaOH}\)) or aqueous ammonia (\(\text{NH}_3\)). Observing the colour and whether the precipitate dissolves in excess reagent helps identify the ion.

Cation Effect of Aqueous Sodium Hydroxide (\(\text{NaOH}\)) Effect of Aqueous Ammonia (\(\text{NH}_3\))
Ammonium (\(\mathbf{NH_4^+}\)) Ammonia gas produced on warming. — (No reaction)
Calcium (\(\mathbf{Ca^{2+}}\)) White ppt., insoluble in excess. No ppt. or very slight white ppt.
Copper(II) (\(\mathbf{Cu^{2+}}\)) Light blue ppt., insoluble in excess. Light blue ppt., soluble in excess, giving a dark blue solution.
Iron(II) (\(\mathbf{Fe^{2+}}\)) Green ppt., insoluble in excess. (Turns brown near surface on standing). Green ppt., insoluble in excess. (Turns brown near surface on standing).
Iron(III) (\(\mathbf{Fe^{3+}}\)) Red-brown ppt., insoluble in excess. Red-brown ppt., insoluble in excess.
Zinc (\(\mathbf{Zn^{2+}}\)) White ppt., soluble in excess (giving a colourless solution). White ppt., soluble in excess (giving a colourless solution).

Important note on Ammonium ions (\(\text{NH}_4^+\)): The test for ammonium is unique. You must heat the sample with aqueous sodium hydroxide. The gas produced is ammonia (\(\text{NH}_3\)), which turns damp red litmus paper blue.

3. Tests for Gases

You must know the test and the result for five key gases:

  • Ammonia (\(\mathbf{NH_3}\)): Turns damp red litmus paper blue. (It's an alkali gas).
  • Carbon Dioxide (\(\mathbf{CO_2}\)): Turns limewater milky.
  • Chlorine (\(\mathbf{Cl_2}\)): Bleaches damp litmus paper (i.e., turns it white).
  • Hydrogen (\(\mathbf{H_2}\)): Produces a distinctive 'pop' sound when tested with a lighted splint.
  • Oxygen (\(\mathbf{O_2}\)): Relights a glowing splint (makes it burst into flame).

4. Flame Tests for Metal Ions

Some metal ions emit specific colours when heated strongly in a Bunsen burner flame. This is used to confirm their presence.

Procedure: Dip a clean wire (usually platinum or nichrome) into the solid or solution and hold it in the hot, blue part of the flame.

Metal Ion Flame Colour Mnemonic/Aid
Lithium (\(\mathbf{Li^+}\)) Red (Christmas) Li-ghts are Red
Sodium (\(\mathbf{Na^+}\)) Yellow Sodium streetlights are Yellow
Potassium (\(\mathbf{K^+}\)) Lilac (Pink/Purple) K is for Lilac (like flowers)
Copper(II) (\(\mathbf{Cu^{2+}}\)) Blue-green Copper statues turn Greenish-blue

Accessibility Tip: Don't worry if 'lilac' and 'blue-green' seem difficult. Focus on the distinctive colour of each one. Sodium (Yellow) is the most common test and must be known.

KEY TAKEAWAY: Mastery of this section requires memorizing the required apparatus, definitions, separation methods (and when to use them), and all the specific chemical tests and their results found in the qualitative analysis notes.