Name common laboratory apparatus and select the correct instrument to measure mass, volume of liquids and gases, time and temperature, reading each to an appropriate precision

What this dot point is asking

SEAB wants you to name the common pieces of laboratory apparatus, choose the right instrument for measuring mass, the volume of liquids and gases, time and temperature, and read each scale to a sensible precision. This is the bedrock of every practical paper: a titration is only as good as the burette reading, and a rate experiment is only as good as the timing. The skill is matching the instrument to how exact the measurement needs to be.

The answer

Measuring mass

A balance measures mass in grams. An electronic balance reads to $0.01\ \text{g}$ or $0.001\ \text{g}$ and is used for weighing solids when preparing a solution of known concentration. To find the mass of a solid accurately, weigh the container, add the solid, and weigh again; the difference is the mass of solid. This method by difference avoids the error of solid sticking to the weighing boat.

Measuring the volume of liquids

The choice depends on how exact the volume must be:

• A beaker or conical flask only holds liquids; its markings are rough.
• A measuring cylinder gives an approximate volume, read to about $\pm 1\ \text{cm}^3$, suitable for adding a known but not critical amount.
• A pipette delivers one fixed volume (such as $25.0\ \text{cm}^3$) very precisely, read to $\pm 0.05\ \text{cm}^3$.
• A burette delivers a variable, precisely known volume and is read to $\pm 0.05\ \text{cm}^3$, which is why it is used in titrations.
• A volumetric flask makes up a solution to one exact total volume.

Always read the bottom of the meniscus with your eye level with the scale to avoid a parallax error.

Measuring the volume of a gas

A gas syringe collects a gas and reads its volume directly, typically to $\pm 1\ \text{cm}^3$. It is the standard apparatus for following the rate of a reaction that gives off a gas. An inverted measuring cylinder over water can also collect a gas that is not very soluble.

Measuring time and temperature

A stopwatch measures time, usually to $0.1\ \text{s}$, for rate experiments. A thermometer measures temperature in degrees Celsius, read to $0.5\ ^\circ\text{C}$ or $1\ ^\circ\text{C}$, for energetics experiments and to monitor heating.

Precision and accuracy

Precision is how finely an instrument can be read (a burette is more precise than a measuring cylinder). Accuracy is how close a reading is to the true value, which also depends on technique. A precise instrument read carelessly can still give an inaccurate result.

Examples in context

Example 1. Following a reaction rate. To measure how fast marble chips react with acid, the flask is placed on a balance and the loss of mass (as carbon dioxide escapes) is timed with a stopwatch, or the gas is collected in a gas syringe and its volume read every few seconds. The choice of apparatus determines what quantity is plotted against time.

Example 2. A standard titration setup. A pipette delivers exactly $25.0\ \text{cm}^3$ of one solution into a conical flask, and a burette delivers the other solution drop by drop until the indicator changes. The precise volumes from the pipette and burette are what make the calculated concentration trustworthy.

Try this

Q1. State the most suitable apparatus to deliver exactly $25.0\ \text{cm}^3$ of a solution into a flask. [1 mark]

• Cue. A (volumetric) pipette, which is calibrated to deliver one fixed, precise volume.

Q2. Explain why a student weighs a solid by difference rather than directly on the balance pan. [2 marks]

• Cue. Weighing the container before and after avoids loss of solid stuck to the pan or boat, so the recorded mass of solid is accurate.

Q3. A thermometer reads $21\ ^\circ\text{C}$ at the start and $34\ ^\circ\text{C}$ at the end of a reaction. State the temperature change and the precision to which a school thermometer is usually read. [2 marks]

• Cue. Temperature change is $34 - 21 = 13\ ^\circ\text{C}$; a school thermometer is read to about $0.5\ ^\circ\text{C}$ or $1\ ^\circ\text{C}$.