LEARNING OBJECTIVES
What you will be able to do
- select suitable measuring apparatus
- plan fair, safe and reliable investigations
- carry out acid–base titrations
- use chromatography and calculate Rf
- choose separation and purification methods
- identify common ions and gases
- evaluate data, errors and improvements
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Choose apparatus, separate substances, analyse ions and gases, and evaluate experimental evidence.
Practical chemistry depends on choosing measurements and techniques that answer a clear question. Good conclusions are supported by observations, processed data and controlled variables.
Chemical analysis identifies substances through characteristic reactions, colours, precipitates and gas tests.
SECTION 01
Apparatus, variables and measurement
Use a stopwatch for time, thermometer for temperature, balance for mass, gas syringe for gas volume, and measuring cylinder, volumetric pipette or burette for liquid volume.
A pipette transfers one accurate fixed volume; a burette delivers accurate variable volumes; a measuring cylinder is quicker but less precise.
Identify the independent variable, dependent variable and controlled variables. Use a suitable range, repeat readings, calculate means and identify anomalies.
DETAILED EXPLANATION
- Record measurements to the precision of the apparatus with units.
- Risk assessment links hazard, possible harm and a practical precaution.
SECTION 02
Solutions and titration
A solvent dissolves a solute to form a solution. A saturated solution contains the maximum dissolved solute at that temperature. Residue remains; filtrate passes through filter paper.
Rinse the pipette with the solution it will deliver and the burette with the solution it will contain. Use a pipette filler, record initial and final burette readings and swirl the flask.
Add indicator to the pipetted solution. Approach the endpoint dropwise, then repeat until concordant titres are obtained.
SECTION 03
Chromatography
Paper chromatography separates soluble substances because they have different attractions to the paper and different solubilities in the solvent.
Draw the baseline in pencil, keep it above the solvent and mark the solvent front before it evaporates. A pure substance gives one spot; a mixture may give several.
Colourless substances require a locating agent. Match spots and Rf values only under the same conditions.
Calculating an Rf value
- A spot travels 4.2 cm.
- The solvent front travels 7.0 cm.
- Rf = 4.2 ÷ 7.0.
Answer: Rf = 0.60.
SECTION 04
Separation and purification
Filtration separates an insoluble solid from a liquid. Crystallisation obtains a soluble solid by concentrating a solution and cooling it.
Simple distillation obtains a solvent or separates liquids with very different boiling points. Fractional distillation separates miscible liquids with closer boiling points.
A suitable solvent can selectively dissolve one substance. Melting and boiling points assess purity: impurities usually broaden and change the observed range.
DETAILED EXPLANATION
- The condenser cools vapour into liquid; water enters at the lower end.
SECTION 05
Tests for anions and gases
Carbonate plus dilute acid releases carbon dioxide. Acidified silver nitrate gives chloride white, bromide cream and iodide yellow precipitates.
For sulfate, acidify with nitric acid then add barium nitrate for a white precipitate. Nitrate warmed with aluminium foil and sodium hydroxide releases ammonia. Sulfite decolourises acidified potassium manganate(VII).
Ammonia turns damp red litmus blue; carbon dioxide turns limewater milky; chlorine bleaches damp litmus; hydrogen gives a squeaky pop; oxygen relights a glowing splint; sulfur dioxide decolourises acidified potassium manganate(VII).
SECTION 06
Tests for cations
With aqueous sodium hydroxide: Cu²⁺ gives light blue, Fe²⁺ green and Fe³⁺ red-brown precipitates that are insoluble in excess. Cr³⁺ gives a green precipitate soluble in excess. Al³⁺ and Zn²⁺ give white precipitates soluble in excess; Ca²⁺ gives a white precipitate insoluble in excess.
With aqueous ammonia: Cu²⁺ gives a light-blue precipitate dissolving in excess to a dark-blue solution; Zn²⁺ gives white dissolving in excess. Al³⁺ gives white and Cr³⁺ green precipitates insoluble in excess; Ca²⁺ gives no precipitate or only a very slight white precipitate. Iron precipitates remain green or red-brown.
Ammonium ions release ammonia when warmed with sodium hydroxide. An iron(II) precipitate turns brown near its surface on standing.
Flame colours: Li⁺ red, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Ba²⁺ light green and Cu²⁺ blue-green.
SECTION 07
Processing and evaluating evidence
Present results in a table with units in headings. Plot the independent variable on the x-axis and use a sensible scale with a best-fit line or curve.
Use patterns to form a conclusion, quote data and explain chemically. An anomaly should be identified and repeated rather than silently removed.
Random error causes scatter and is reduced by repeats. Systematic error shifts results in one direction and needs method or apparatus correction.
DETAILED EXPLANATION
- A control experiment isolates the effect of the independent variable.
- Precision, accuracy and reliability describe different qualities.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Match apparatus precision to the measurement.
- Control variables and repeat readings.
- Titration endpoints require concordant titres.
- Chromatography separates through different affinities.
- Separation methods depend on physical properties.
- Qualitative tests require reagent, conditions and observation.
- Conclusions must be supported by processed evidence.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- select suitable measuring apparatus
- plan fair, safe and reliable investigations
- carry out acid–base titrations
- use chromatography and calculate Rf
- choose separation and purification methods
- identify common ions and gases
- evaluate data, errors and improvements