Experimental Chemistry

Experimental methods

An experimental method earns marks when every apparatus choice has a reason. Name the quantity, choose an instrument with a suitable range and precision, and state how the result will be read or collected.

1. Definition

An experimental method is an ordered, safe procedure for making observations or measurements that answer a question. A measurement is a numerical value recorded with its unit.

2. Key Ideas

• Measure time with a stopwatch or data logger, temperature with a thermometer, mass with a balance, and volume with suitable volumetric apparatus.
• A measuring cylinder is suitable for approximate liquid volumes; a pipette or burette is used when greater precision is required.
• A gas syringe measures gas volume directly.
• Collect a gas over water only if it is not very soluble in water.
• Use displacement of air when water is unsuitable, choosing the jar orientation from the gas density.
• Read a liquid scale at eye level and record the instrument’s precision.
• In an acid–alkali titration, use a pipette for a fixed volume, a burette for a measured variable volume and the given indicator to show the end-point.

3. Detailed Explanations

Selecting an instrument

First identify the quantity and expected range. Then choose the smallest useful scale divisions without exceeding the instrument range. For a change found from two readings:

For reaction-rate measurements, keep time intervals consistent and measure a quantity such as gas volume or mass loss.

Collecting gases

Collection over water is convenient when the gas has low solubility and does not react with water. If water is unsuitable, use density to choose the jar orientation:

• upward delivery collects a gas less dense than air in an inverted gas jar; the gas rises and pushes air downwards;
• downward delivery collects a gas denser than air in an upright gas jar; the gas sinks and pushes air upwards.

A gas syringe avoids relying on density and gives a volume reading, but connections must be gas-tight.

Carrying out an acid–alkali titration

Rinse the pipette with the solution it will transfer, then use it to place a fixed volume in a conical flask. Add the suitable given indicator. Rinse and fill the burette with the other solution, remove the funnel, record the initial reading and add solution while swirling the flask. Near the end-point, add it drop by drop until the indicator changes colour permanently. Record the final burette reading and calculate the volume delivered. Repeat until close results are obtained. Focus here on the apparatus, safe method, readings and end-point; use the Chemical Calculations lesson for any calculation based on titration data.

4. Common Mistakes

5. Exam Tips

Use the pattern quantity → apparatus → reading or collection method → unit.

6. Worked Examples

7. Mind Stretchers

8. Quiz

Before attempting the assessment, check that you can:

• select apparatus for time, temperature, mass and volume;
• explain a choice using range and precision;
• read a scale correctly with units;
• select gas collection from solubility and density;
• carry out an acid–alkali titration using a pipette, burette and given indicator;
• identify and prevent gas loss.

Try this next: choose an unfamiliar gas from supplied solubility and density data, then write a one-sentence justification for its collection method.

Separation and purity

Choose a separation method from a difference in physical properties. State what passes through, what remains, and how the required substance is recovered.

1. Definition

Separation divides a mixture without forming new substances. Purification removes unwanted substances. A pure substance contains one substance and has characteristic physical properties.

2. Key Ideas

• Filtration separates an insoluble solid from a liquid.
• Crystallisation obtains a dissolved solid without heating the solution to dryness.
• Simple distillation obtains a solvent from a solution.
• Fractional distillation separates miscible liquids with different boiling points.
• Paper chromatography separates soluble substances because they travel differently with a solvent.
• A pure substance melts or boils sharply at its characteristic temperature.

3. Detailed Explanations

Match method to evidence

Use filtration when particle size and insolubility differ. Use crystallisation when a solid is soluble and may decompose on strong heating. Use distillation when volatility or boiling point differs.

For a solid–solid mixture, choose a solvent that dissolves only one solid. Add the solvent, stir, filter, and wash and dry the insoluble residue. Recover the dissolved solid from the filtrate by crystallisation or evaporation. For a solid–liquid mixture, filter if the solid is insoluble; if it is dissolved, use crystallisation, evaporation or distillation according to the product required. For miscible liquids, use fractional distillation when their boiling points differ.

Choose evaporation when the solvent is not required and the dissolved solid is stable on heating. Choose crystallisation when crystals are wanted or heating to dryness could decompose the solid. Choose simple distillation when the solvent itself must be collected.

Carry out simple and fractional distillation

For simple distillation, heat the solution so that the more volatile liquid boils. Its vapour enters the condenser, cools and changes back to liquid. Collect this liquid as the distillate; the less volatile dissolved substance remains in the flask. Keep the thermometer bulb level with the side arm so it measures the vapour entering the condenser. Cooling water enters the lower condenser port and leaves from the upper port, keeping the water jacket full.

For fractional distillation, place a fractionating column between the heated flask and condenser. Repeated condensation and vaporisation in the column allow vapour richer in the lower-boiling liquid to reach the condenser first. Use this method for miscible liquids, especially when their boiling points are close enough that simple distillation would separate them poorly.

Do not heat a closed apparatus. The receiver remains open to the air so pressure cannot build up.

Set up and read paper chromatography

In chromatography, place the sample on a pencil baseline above the solvent. The solvent moves through the paper and carries components different distances. Here, interpret the number of spots and their positions compared with known samples. Where your syllabus also requires calculated R_f values, the methods of separation and purification lesson teaches that calculation.

Judge purity

An impurity usually lowers and broadens a solid’s melting range. A pure liquid boils at a sharp characteristic temperature at a stated pressure. One chromatogram spot supports purity only for the solvent and locating conditions used.

4. Common Mistakes

5. Exam Tips

Use the pattern property difference → method → named apparatus action → product obtained.

6. Worked Examples

7. Mind Stretchers

8. Quiz

Before attempting the assessment, check that you can:

• select a method from solubility, boiling point or particle-size evidence;
• use a suitable solvent to separate two solids and recover both products;
• distinguish residue, filtrate, distillate and crystals;
• explain a paper chromatography setup and chromatogram;
• use melting and boiling behaviour as purity evidence;
• state the limits of one purity test.

Practise this: for each new mixture, write four parts before naming a method: the mixture type, the physical-property difference, the required product and the apparatus action.