0620 · 12.1Experimental design

Choosing apparatus for measurements

0:00Two students can investigate the same reaction and obtain different results because they use different equipment or methods. A measuring cylinder might be convenient, but another instrument could measure the volume more precisely. Good experimental design means matching your choices to the measurement you need. By the end, you will understand those choices and the vocabulary used to describe solutions. Before choosing apparatus, identify the quantity being measured. Time describes how long something takes, temperature tells us how hot something is, and mass measures the amount of matter.

0:30Volume describes the space occupied by a liquid or gas. An instrument must measure the correct quantity and suit the amount involved. Use a stop-watch to measure time, such as the interval between starting a reaction and reaching a chosen endpoint. Start timing at a clearly defined event, and use the same endpoint each time. Human reaction time can affect when you press the button. This matters especially when the interval is very short.

0:54A thermometer measures temperature, usually in degrees Celsius during school experiments. Place its bulb in the substance being measured, and allow the reading to settle. Avoid resting it against the container when measuring a liquid. Otherwise, the reading may be influenced by the container rather than representing the liquid properly. Use a balance to measure mass, usually in grams. When weighing a solid in a container, first place the empty container on the balance and set the reading to zero.

1:20Then add the solid, so the displayed value gives its mass alone. Alternatively, subtract the empty container’s mass from the combined measurement. A precise display does not remove every source of error. A balance can show very small changes in mass, but it cannot correct for spilled material. Repeating an experiment helps reveal inconsistent results, but cannot repair a method that loses material every time. Think about the whole procedure, not just the instrument.

Measuring liquid volumes accurately

1:46For liquid volume, three important choices are a measuring cylinder, a volumetric pipette and a burette. Their different shapes reflect different jobs. A measuring cylinder is convenient for measuring a range of volumes. However, it generally gives a less precise measurement than a suitable volumetric pipette or burette. Choose a measuring cylinder when speed and convenience matter more than a very precise volume. For example, it is suitable for measuring water when a small difference would not significantly affect the investigation.

2:15Use an appropriately sized cylinder with graduations you can read clearly. A very large cylinder is usually a poor choice for a small volume. A volumetric pipette is designed to deliver one fixed volume accurately, such as twenty-five cubic centimetres. Its single calibration mark identifies the correct filling level. Use a pipette filler, never your mouth, to draw liquid into it. This makes it a good choice when the same precise volume must be transferred repeatedly.

2:40The pipette’s fixed volume is also its limitation. It is not suitable when you need to deliver different amounts using the same instrument. A burette provides that flexibility because it has a graduated scale and a tap. You can release liquid gradually, including drop by drop, while measuring how much has been delivered. Water and many aqueous solutions form a curved surface inside narrow glass apparatus. This curve is called a meniscus.

3:04Read the bottom of that curve at eye level for these liquids. Looking from above or below makes its position appear different against the scale, creating a reading error. Suppose a burette reading starts at two cubic centimetres and finishes at eighteen cubic centimetres. Its scale increases downwards as liquid leaves the tube. Calculate the delivered volume by subtracting the initial reading from the final reading. Eighteen cubic centimetres minus two cubic centimetres gives sixteen cubic centimetres.

3:31The final reading alone is not necessarily the volume delivered because the burette did not have to start at zero. Also check that its outlet is filled with liquid before taking the initial reading. An air bubble could make the apparent delivered volume larger than the amount actually collected. Careful technique makes the apparatus useful.

Measuring gas volume

3:48A gas syringe measures the volume of gas collected. Gas enters its barrel and pushes the movable plunger outwards. The graduations show the volume inside, usually in cubic centimetres. Choose a syringe with enough capacity for the expected gas, and make sure the connections are airtight. A gas syringe allows the volume to be read directly without collecting the gas over water. This is useful when the gas would dissolve in water and escape measurement by that method.

4:12However, leaking connections can allow gas to escape before it reaches the syringe. A sticking plunger can also interfere with collection. Collecting a gas over water can be a suitable alternative when the gas is not significantly soluble in water. The gas displaces water from an inverted, water-filled measuring cylinder. However, some gas may dissolve, making the collected volume too small. The better method depends on the gas and the equipment available.

4:36When comparing methods, connect each advantage or disadvantage to the experiment. Saying that equipment is better is not enough. Explain whether it improves precision, makes a measurement easier, prevents loss, or suits the required volume. A precise instrument is only helpful when its design matches the task.

Solutions, saturation and filtration

4:53A solvent is a substance that dissolves a solute. A solute, in turn, is a substance that is dissolved in a solvent. A solution is a mixture of one or more solutes dissolved in a solvent. These names describe the roles of the substances rather than three unrelated types of material. When sodium chloride dissolves in water, the water is the solvent and the sodium chloride is the solute. Together they form a solution.

5:18The dissolved substance has not disappeared; it is distributed throughout the liquid. Water is a common solvent, but the definition does not require every solvent to be water. A saturated solution is a solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature. The temperature is part of the definition because changing it can change how much dissolves. For many solids, warmer water can dissolve more.

5:43That pattern is useful, but it is not universal. Imagine adding small portions of sodium chloride to water while stirring and keeping the temperature constant. Initially, each portion dissolves, provided the solution is not yet saturated. Eventually, some solid remains even after sufficient stirring and time. The liquid has reached its limit, while the extra solid is undissolved material. A residue is a substance that remains after evaporation, distillation, filtration or any similar process.

6:11A filtrate is a liquid or solution that has passed through a filter. In this example, excess undissolved sodium chloride stays on the filter paper as the residue. The solution passing through becomes the filtrate. Filtration does not remove dissolved sodium chloride because its dissolved particles pass through the filter with the water. The filtrate can therefore still contain a solute and remain saturated at the same temperature. If water is later evaporated, the sodium chloride left behind is also a residue.

6:38The word describes what remains, not exclusively material on filter paper. These ideas can be tested in written questions, the Practical Test and Alternative to Practical paper. A question might ask you to name apparatus for transferring exactly twenty-five cubic centimetres of solution. It could then ask you to suggest an advantage over a measuring cylinder. Match your answer to the fixed volume and the precision required. Use a twenty-five cubic centimetre volumetric pipette to transfer the solution.

7:05It delivers the fixed volume more precisely than a measuring cylinder. A pipette filler allows the liquid to be drawn into it safely. These separate statements identify the apparatus, justify the choice and describe an appropriate handling method. Another question might ask you to describe the filtrate obtained after filtering a saturated sodium chloride solution containing excess solid. The filtrate is the solution that has passed through the filter. It contains water and dissolved sodium chloride.

7:32Excess undissolved solid remains on the filter paper as the residue. Good experimental design connects a measurement to suitable apparatus and a careful method. Choose a stop-watch for time, a thermometer for temperature and a balance for mass. For volume, distinguish convenient cylinders, fixed-volume pipettes, adjustable burettes and gas syringes. Finally, keep dissolved solute separate from undissolved residue, and remember that saturation depends on temperature.