Name common laboratory apparatus and choose suitable apparatus to measure volume, mass, temperature and time, and read each scale correctly

What this dot point is asking

The syllabus wants you to recognise the common pieces of laboratory apparatus, to know which instrument measures volume, mass, temperature, and time, and to choose the most suitable one for a particular job. You also need to read each scale correctly, including reading a measuring cylinder at the bottom of the meniscus and avoiding a parallax error. These are basic practical skills, and because N(A) Chemistry has no separate practical paper, they are tested as written questions in Paper 4.

The answer

Apparatus for measuring volume

Liquids are measured by volume, usually in cubic centimetres. Different apparatus gives different accuracy:

• A beaker or conical flask has rough markings and is used to hold or mix liquids, not to measure them accurately.
• A measuring cylinder measures a chosen volume reasonably accurately, to about $1\ \text{cm}^3$.
• A pipette delivers one fixed volume, such as $25.0\ \text{cm}^3$, very accurately.
• A burette delivers any volume up to $50.0\ \text{cm}^3$ and is read to $0.1\ \text{cm}^3$, used in titrations.

The more accurate the volume you need, the finer the apparatus you choose.

Apparatus for measuring mass

Mass is measured with an electronic balance in grams. To find the mass of a solid such as a powder, you weigh a container first, add the solid, weigh again, and subtract. This is called weighing by difference and it avoids losing any of the solid stuck to the container.

Apparatus for measuring temperature and time

Temperature is measured with a thermometer in degrees Celsius. Read it at eye level and wait for the reading to settle. Time is measured with a stopwatch in seconds, for example to time how long a reaction takes.

Reading a scale correctly

When you read any scale with a liquid, the surface curves into a shape called the meniscus. For water and most liquids the meniscus curves down, so you read the bottom of the meniscus. Keep your eye level with the mark and look straight across. If you look from above or below, you get a wrong reading called a parallax error.

Examples in context

Example 1. A titration setup. In a titration you pipette an accurate $25.0\ \text{cm}^3$ of one solution into a conical flask, then run a second solution from a burette until the reaction is complete. The pipette and burette are chosen because they are accurate, while the conical flask just holds the mixture and is swirled. The choice of apparatus directly controls how accurate the result is.

Example 2. Following a temperature change. To measure the heat given out when an acid neutralises an alkali, you measure both volumes with a measuring cylinder, mix them in an insulated cup, and follow the temperature with a thermometer, recording the highest reading. The right apparatus for each quantity, volume, and temperature, makes the experiment work.

Try this

Q1. A student needs to add exactly $10.0\ \text{cm}^3$ of acid to a flask. State the most suitable apparatus. [1 mark]

• Cue. A pipette (or a burette), because it measures a small volume accurately, rather than a beaker.

Q2. Explain why an electronic balance is used to measure the mass of a powder rather than judging it by eye. [2 marks]

• Cue. The balance gives an accurate reading in grams, so the amount used is known precisely; judging by eye would be only a rough guess and the experiment would not be repeatable.

Q3. A liquid level in a measuring cylinder appears to be at $30\ \text{cm}^3$ when viewed from above. State what kind of error this is and how to avoid it. [2 marks]

• Cue. It is a parallax error caused by viewing from an angle; avoid it by looking straight across with your eye level with the bottom of the meniscus.