Experimental design Revision Notes

Time measurement is essential for experiments involving reaction rates or timed observations. The primary instrument is the stopwatch, which measures elapsed time in seconds and fractions of a second.

Stopwatches can be mechanical or digital; digital stopwatches offer greater precision, often to hundredths of a second.

When using a stopwatch, start and stop it exactly at the beginning and end of the event to minimise human error. Repeated timing improves reliability by allowing calculation of an average time.

• Stopwatches measure time intervals, typically in seconds and fractions of a second.
• Digital stopwatches provide higher precision than mechanical ones.
• Repeat measurements to reduce random errors.

Temperature measurement is vital for monitoring reaction conditions and physical changes. Mercury or alcohol thermometers are commonly used in school laboratories.

Thermometers measure temperature by the expansion of a liquid in a calibrated glass tube, with scales in degrees Celsius (°C).

For accurate readings, the bulb of the thermometer must be fully immersed in the substance without touching the container's sides or bottom to avoid conduction errors.

• Thermometers measure temperature in °C.
• Mercury thermometers are used for temperatures above -39 °C; alcohol thermometers are safer for lower temperatures.
• Ensure the bulb is fully immersed and not touching container surfaces for accurate readings.

Mass measurement is fundamental for quantifying reactants and products. Electronic digital balances are standard in modern laboratories, providing precise mass readings to at least 0.01 g.

Before weighing, the balance should be zeroed (tared) with the container or weighing paper to measure only the sample mass.

Place the sample gently to avoid damage or inaccurate readings. Record the mass once the reading stabilises.

• Balances measure mass, typically in grams (g).
• Electronic balances offer precision to 0.01 g or better.
• Always tare the balance before weighing samples.

Accurate volume measurement is crucial for quantitative analysis and titrations.

Burettes are long, graduated tubes with a tap at the bottom, used to deliver variable but precise volumes of liquid, typically in titrations. They allow measurement to the nearest 0.05 cm³.

Volumetric pipettes are designed to transfer a fixed, precise volume of liquid, such as 25.0 cm³, with high accuracy. They have a single graduation mark and must be filled to this mark for correct volume delivery by gravity or blowing out the last drop if specified (blow-out pipettes). They are calibrated at 20 °C, so temperature affects accuracy slightly if different from this standard condition.

• Burettes measure variable volumes with high precision, used mainly in titrations.
• Volumetric pipettes transfer fixed, exact volumes, e.g., 25.0 cm³.
• Both require careful reading of the meniscus at eye level to avoid parallax errors.
• Fill pipettes and burettes with the liquid to the correct mark before use.

Measuring cylinders are cylindrical containers with graduations for measuring approximate volumes of liquids, typically from 10 cm³ to 1000 cm³.

They are less precise than burettes or pipettes but useful for measuring larger volumes where high precision is not critical.

Read the bottom of the meniscus at eye level to avoid parallax error.

• Measuring cylinders measure approximate volumes of liquids.
• They are less precise than burettes and pipettes.
• Used for larger volumes where exact precision is not required.

Gas syringes are used to measure the volume of gases produced or consumed in experiments, such as gas collection during reactions.

They consist of a graduated barrel and a plunger that moves to measure gas volume accurately, typically in cm³.

Gas syringes must be airtight to prevent gas leakage, and the plunger should move smoothly without resistance for accurate volume measurement.

• Gas syringes measure gas volumes accurately, usually in cm³.
• Used in experiments involving gas collection or measurement.
• Ensure airtight connections and smooth plunger movement for accuracy.

Experimental techniques and equipment are fundamental to chemistry investigations. Each technique has specific applications, advantages, and limitations that affect the quality and reliability of results. Choosing the correct method depends on the nature of the experiment, required accuracy, available resources, and safety considerations.

Common experimental techniques include titration, filtration, chromatography, distillation, and use of apparatus such as burettes, pipettes, balances, and thermometers. Understanding their pros and cons helps in designing effective experiments.

• Techniques vary in precision and complexity.
• Equipment choice affects measurement accuracy and ease of use.
• Safety and cost are important factors in method selection.

Titration is a quantitative analytical technique used to determine the concentration of an unknown solution by reacting it with a solution of known concentration (the titrant). It is highly precise when performed correctly but requires careful technique.

Advantages include high accuracy, relatively simple apparatus (burette, pipette, conical flask), and clear end-point detection using indicators.

Disadvantages include the need for skilled handling to avoid systematic errors, time consumption for multiple repeats, and potential human error in reading volumes or detecting colour changes.

• Requires accurate measurement of volume using burettes and pipettes.
• End-point detection depends on suitable indicators and observer judgement.
• Repeated titrations improve reliability but increase time and resource use.

Chromatography separates components of mixtures based on their different affinities for stationary and mobile phases. Techniques include paper chromatography and thin-layer chromatography (TLC).

Advantages include the ability to separate complex mixtures, small sample size, and visual identification of components by spot colour and position (Rf values).

Disadvantages include qualitative rather than quantitative results in many cases, potential difficulty in interpreting overlapping spots, and sensitivity to experimental conditions such as solvent choice and temperature.

• Useful for identifying substances and purity checks.
• Rf values depend on solvent and stationary phase, requiring careful control.
• Results can be subjective without instrumentation for detection.

Filtration separates solids from liquids using filter paper; it is simple and effective for insoluble solids.

Distillation separates liquids based on boiling points; simple distillation is suitable for separating a solvent from a solution, while fractional distillation separates liquids with closer boiling points.

Advantages of filtration include ease of use and minimal equipment; disadvantages include incomplete separation if particles are very fine or if the solid dissolves slightly in the liquid. Distillation advantages include obtaining pure solvents or liquids; disadvantages include equipment complexity, energy consumption, and potential loss of volatile substances.

• Filtration is quick but limited to insoluble solids.
• Distillation requires heating and apparatus like condensers.
• Both methods require careful technique to avoid contamination or loss.

Measuring instruments such as balances, thermometers, measuring cylinders, volumetric pipettes, and gas syringes vary in accuracy and ease of use.

Balances provide mass measurements with high precision but require calibration and careful handling to avoid errors.

Volumetric pipettes and burettes offer precise volume measurements essential for titrations, whereas measuring cylinders are less precise but quicker for approximate volumes. Gas syringes measure gas volumes accurately but must be handled to avoid leaks and pressure changes.

• Choose equipment based on required precision.
• Calibrate and handle apparatus carefully to reduce errors.
• Consider ease of use and cost when selecting equipment.

When selecting experimental methods, consider: accuracy (closeness to true value), precision (repeatability), cost, availability, ease of use, time required, and safety.

For example, titration is preferred for precise concentration determination, while chromatography is chosen for mixture analysis.

Safety considerations include handling corrosive chemicals, heating apparatus, and glassware to prevent accidents.

• Balance accuracy and practicality for the experiment's purpose.
• Repeat measurements to improve reliability.
• Follow safety protocols to minimise risks.

A solvent is the substance in which another substance dissolves. It is usually present in the largest amount in a solution. For example, in saltwater, water is the solvent.

A solute is the substance that dissolves in the solvent. It is usually present in a smaller amount. For example, in saltwater, salt (sodium chloride) is the solute.

A solution is a homogeneous mixture formed when one or more solutes dissolve in a solvent. The particles of solute are dispersed evenly at the molecular or ionic level throughout the solvent, resulting in a single-phase system. For example, sugar dissolved in water forms a sugar solution.

• Solvent: dissolves the solute; usually liquid (e.g., water).
• Solute: substance dissolved in solvent (e.g., salt, sugar).
• Solution: homogeneous mixture of solute(s) in solvent.

A saturated solution contains the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature. Beyond this point, any additional solute will remain undissolved as a solid.

Solubility is the maximum mass of solute that can dissolve in 100 g of solvent at a given temperature. Solubility generally increases with temperature for most solids in liquids.

For example, if 36 g of sodium chloride dissolves in 100 g of water at 20 °C, the solution is saturated at that temperature. Adding more salt will result in undissolved salt at the bottom of the container.

• Saturated solution: maximum solute dissolved at given temperature.
• Excess solute remains undissolved as solid residue.
• Solubility varies with temperature; usually increases as temperature rises.

Residue is the solid material left behind after a separation process such as filtration, evaporation, or distillation. For example, when filtering a mixture of sand and water, the sand remains on the filter paper as residue.

Filtrate is the liquid or solution that passes through the filter during filtration. It contains dissolved substances but not the solid residue.

In evaporation, the residue is the solid left after the solvent evaporates. For example, evaporating saltwater leaves salt crystals as residue.

• Residue: solid left after separation (e.g., sand after filtration).
• Filtrate: liquid that passes through filter, free of solid residue.
• Residue and filtrate are complementary parts of separation outcomes.

Suppose you add salt to 100 g of water at 25 °C until no more dissolves and some solid remains undissolved. The solution is saturated at 25 °C.

If you filter this mixture, the undissolved salt remains on the filter paper as residue.

The liquid collected after filtration is the filtrate, a saturated salt solution containing dissolved sodium and chloride ions.

• Add salt to water until no more dissolves → saturated solution formed.
• Filter mixture → residue is undissolved salt on filter paper.
• Filtrate is saturated salt solution passing through filter.

When preparing solutions, stirring and temperature control help dissolve solutes faster and increase solubility.

Observing undissolved solids indicates saturation has been reached.

Filtration requires appropriate filter paper and apparatus (e.g., funnel, conical flask). The filtrate should be clear and free of solid particles if filtration is successful.

• Stirring and heating increase solubility and speed dissolution.
• Undissolved solids indicate saturated solution or excess solute.
• Filtration separates solids (residue) from liquids (filtrate) effectively.