Experimental design Revision Notes

Time measurement is crucial in experiments involving rates or durations, such as reaction times or oscillations. Stopwatches are handheld devices designed to measure elapsed time accurately, often to the nearest 0.01 seconds.

A digital stopwatch is preferred for precision and ease of reading. For example, timing how long a pendulum takes to complete 20 swings allows calculation of the period by dividing the total time by 20.

When using a stopwatch, start and stop it exactly at the event's beginning and end to reduce human reaction error. Repeating the timing multiple times and calculating an average improves reliability.

• Stopwatches measure elapsed time, typically in seconds and fractions of seconds.
• Digital stopwatches offer higher precision than analogue ones.
• Multiple trials reduce random errors in timing measurements.

Temperature affects reaction rates, solubility, and physical states, so accurate measurement is essential. Thermometers measure temperature by the expansion of liquids (like mercury or alcohol) or electronic sensors.

Mercury thermometers provide precise readings but are less common due to toxicity. Alcohol thermometers are safer and visible due to coloured liquid. Digital thermometers with thermistors or thermocouples give quick and accurate readings.

For example, measuring the temperature of a water bath during an enzyme activity experiment ensures conditions remain constant. The thermometer bulb must be fully immersed without touching container sides to avoid false readings.

• Thermometers measure temperature in degrees Celsius (°C).
• Liquid-in-glass thermometers rely on thermal expansion; digital thermometers use electronic sensors.
• Proper immersion and placement are vital for accurate temperature readings.

Mass measurement is fundamental for quantifying substances. Electronic balances provide digital mass readings, often to 0.01 g or better, while traditional beam balances require manual adjustment and reading.

For example, weighing a sample of salt before dissolving it in water requires placing the container on the balance, zeroing it (tare), then adding the salt to measure its mass accurately.

Balances must be placed on stable, level surfaces and calibrated regularly. Avoid drafts or vibrations that can affect readings.

• Balances measure mass in grams (g).
• Electronic balances offer quick, precise readings with digital displays.
• Taring the balance before weighing ensures only the sample mass is recorded.

Accurate volume measurement is critical in titrations, solution preparation, and dilution.

Burettes deliver variable volumes with high precision, allowing measurement of the volume of liquid dispensed during titrations. They have a tap to control flow and graduations to read volume to 0.05 cm³.

Volumetric pipettes measure a fixed volume very precisely, used to transfer exact amounts of liquid. For example, a 25 cm³ pipette delivers exactly 25.00 cm³ when filled to the mark and drained properly by gravity without blowing out the last drop unless specified (blow-out pipettes). They are calibrated to deliver (TD). Measuring cylinders are less precise but useful for approximate volume measurements. They come in various sizes and are graduated, typically read at the bottom of the meniscus at eye level to reduce parallax error.

• Burettes measure variable volumes with high precision, ideal for titrations.
• Volumetric pipettes deliver fixed, exact volumes, essential for solution preparation.
• Measuring cylinders provide approximate volume measurements, suitable for less precise work.
• Reading liquid volume requires eye-level observation of the meniscus.

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 volume, typically in cm³ or mL. For example, measuring the volume of hydrogen gas produced when zinc reacts with hydrochloric acid.

Gas syringes must be airtight to prevent gas leakage. The plunger should move smoothly without sticking to ensure accurate volume readings. Multiple readings can be taken to calculate an average volume for reliability.

• Gas syringes measure gas volumes accurately in cm³ or mL.
• They must be airtight to prevent gas loss during measurement.
• Smooth plunger movement ensures precise volume readings.

In chemistry, a solution is formed when one substance dissolves in another. The substance that does the dissolving is called the solvent, while the substance that dissolves is called the solute.

For example, when sugar dissolves in water, water acts as the solvent and sugar is the solute. The solvent is usually the component present in the larger amount, and it determines the physical state of the solution (usually liquid).

The process of dissolving involves the solute particles separating and dispersing evenly throughout the solvent, forming a homogeneous mixture. This means the composition is uniform throughout the solution.

• Solvent: dissolves the solute; usually the larger quantity.
• Solute: substance dissolved in the solvent.
• Solution: homogeneous mixture of solute(s) in solvent.

A solution is a homogeneous mixture where the solute is completely dissolved in the solvent. The concentration of a solution refers to how much solute is dissolved in a given volume of solvent.

A saturated solution is one where the maximum amount of solute has dissolved at a specific temperature. Beyond this point, any additional solute will remain undissolved and settle out or form a separate phase.

Temperature affects solubility: increasing temperature usually increases the amount of solute that can dissolve, so a solution saturated at one temperature may become unsaturated if heated, allowing more solute to dissolve. Conversely, cooling a saturated solution can cause excess solute to crystallise out, a process used in recrystallisation techniques.

• Solution: solute fully dissolved in solvent, uniform mixture.
• Saturated solution: contains maximum solute dissolved at given temperature.
• Temperature influences solubility and saturation level.

When separating mixtures, especially solutions containing undissolved solids, two important terms are residue and filtrate.

Residue is the solid material left behind on the filter paper after filtration or after evaporation of the solvent. For example, sand left on filter paper after filtering a sand-water mixture is residue.

Filtrate is the liquid or solution that passes through the filter paper during filtration. It contains dissolved substances but no solid particles that were trapped by the filter. For example, the clear salt solution collected after filtering sand from saltwater is the filtrate.

• Residue: solid left on filter after filtration or after evaporation.
• Filtrate: liquid or solution that passes through the filter.
• Filtration separates solids from liquids based on particle size.

Consider making a sugar solution by dissolving sugar in water. Initially, sugar dissolves until the solution becomes saturated. If you add more sugar beyond saturation, it will remain undissolved as residue.

In a laboratory, filtration is used to separate insoluble solids from liquids. For example, filtering a mixture of sand and water leaves sand as residue on the filter paper and water as filtrate.

Evaporation can be used to recover solute from a solution. When water evaporates from saltwater, salt crystals remain as residue. This method is useful for obtaining dissolved solids from solutions.

• Dissolving sugar in water illustrates solvent and solute roles.
• Filtering sand-water separates residue (sand) and filtrate (water).
• Evaporation recovers solute as residue after solvent removal.

Understanding these terms is fundamental for studying solutions and separation techniques in chemistry. Knowing the difference between solvent and solute helps explain how solutions form and behave.

Recognising what a saturated solution is aids in predicting solubility limits and crystallisation processes.

Distinguishing residue and filtrate is essential for practical laboratory work involving filtration and evaporation, ensuring correct identification of substances after separation.

• Solvent dissolves; solute is dissolved.
• Solution is a uniform mixture of solute in solvent.
• Saturated solution holds maximum solute at given temperature.
• Residue is solid left after separation; filtrate is liquid passed through filter.