An apparatus diagram is a description, not a recipe: each part has a purpose, and reading it well means stating what goes in, what is measured and what could go wrong. Diagram questions appear in practical-style papers and in written questions about experiments. This lesson comes after drawing results tables in practical-data interpretation.
How do you read a diagram, step by step?
- Name each part and say what it does in one phrase, not just its label.
- Follow the path. Trace where the reactants start, where a gas or liquid moves, and where the measurement is taken.
- Identify the measured quantity and its unit, such as volume of gas / cm³ or time / s.
- Ask why this method of collecting or measuring suits the substance. Solubility, density and whether the gas reacts with water are the usual reasons.
- Look for places where material could be lost or gained, such as a gap or a delay before sealing.
- State hazards as ideas, such as flammable, corrosive or toxic, and what that means for the design of the set-up.
Worked example
Invented diagram, described in words. A flask containing magnesium and dilute hydrochloric acid is closed with a bung. A delivery tube leads from the flask to a measuring cylinder that is upside down, full of water, standing in a trough of water. Hydrogen gas is produced and pushes water out of the cylinder.
Part by part:
- Flask and bung: holds the reacting mixture and stops gas escaping before it reaches the tube.
- Delivery tube: carries the gas from the flask to the collection point.
- Inverted measuring cylinder in water: collects the gas by displacing water, and its scale shows the volume of gas.
What is measured: the volume of hydrogen collected, in cm³, possibly at set time intervals.
Why collect over water? Hydrogen is only very slightly soluble in water, so little of it is lost by dissolving. The method also shows the volume directly.
Where could error enter?
- Gas escapes if the bung is not sealed, making the volume too low.
- Some gas is produced before the bung is in place, and this is lost.
Hazard as an idea: hydrogen is flammable, so a safe design keeps ignition sources away. The acid is a corrosive liquid, so eye protection is standard in any supervised laboratory.
What mistake is easy to make?
Mistaken answer: “The measuring cylinder collects ammonia over water because the gas bubbles up.”
The student copied the set-up without asking whether the method suits the gas.
Collecting over water only works for a gas that is not very soluble in water. Ammonia and hydrogen chloride dissolve readily, so the gas would disappear into the water and the volume would be wrong. The correct reasoning links the method to the property: hydrogen, which is only slightly soluble, suits collection over water.
Check yourself
1. In the worked example, what would happen to the measured volume if the bung were loose?
Show answer
Some hydrogen would escape from the flask, so the measured volume would be lower than the true volume produced.
2. Why is a flask with a delivery tube usually drawn with an open route for gas to leave, rather than a fully sealed and heated container with no outlet?
Show answer
Gas produced in a sealed container with nowhere to go can build up pressure, which is a hazard. The design must let the gas reach the collection or measuring part. This is an idea about pressure, not a method to try.
3. A question says “state what the apparatus measures”. Which answer is better: “the gas” or “the volume of gas collected, in cm³”?
Show answer
“The volume of gas collected, in cm³”, because it names a measurable quantity with a unit, while “the gas” names only a substance.
Where does this lead next?
Once you can read what each part does, you can judge whether a reading looks believable in identifying an anomalous reading. Apparatus ideas also connect to rates of reaction, where gas volume and time are common measurements.
Teachers in online one-to-one Chemistry tuition can use your own diagram answers to show where explanation drifts from purpose to labels.