Guided lesson: Core Practical: Determining Density

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2.Irregular objects: the displacement technique

5.4

An irregular object hides its volume from any ruler. The eureka can solves that: submerged, the object pushes out its own volume of water, and the displaced water read in a measuring cylinder becomes the volume the equation needs.

Eureka-can, IGCSE & GCSE Physics revision notes — Apparatus for measuring the density of irregular objects

Key points

The experiment finds the densities of irregular objects through a displacement technique.
The eureka can overflows until the water settles; the object then displaces its own volume into the measuring cylinder below the spout.
The volume of displaced water equals the volume of the object.
The object is lowered in gently — dropping splashes water and corrupts the volume reading.

First pass

Fill the can until it just stops overflowing, empty the cylinder, then lower the odd-shaped treasure in. Whatever spills through the spout is exactly the treasure's volume — read it, average repeats, divide the mass by it.

Analogy — Climbing into a full bathtub: the slop over the sides is your own volume, measured where it lands.

Exam pointer — State the displacement equality explicitly — displaced volume equals object volume — and warn against splashing.

Going deeper

Displacement converts an unmeasurable shape into a readable liquid volume: the can's spout delivers exactly the submerged object's volume into the cylinder, since displaced volume equals object volume. Gentle lowering protects the reading from splash losses, and repeats with averaging finish the discipline before the density equation divides.

Analogy — A slot machine paying out in water: whatever you feed it, it returns the object's volume in a readable column — unless you jolt the machine and spill extra.

Exam pointer — In the write-up, the displaced volume step is where marks live: name the eureka can, the cylinder below the spout, and the gentle lowering.

Checks

0 of 9 attempted · 0 correct

1.What does the volume of displaced water equal?
2.Why must the measuring cylinder sit below the spout before the object goes in?
3.What is the dependent variable in the irregular-objects experiment?
4.Why lower the object gently rather than drop it?
5.Which extra step does the irregular-object method need that the regular-object method does not?
6.State the aim of the irregular-objects experiment.
Written response · recall
Your answer
7.State what must be done with the measuring cylinder just before lowering the object in.
Written response · recall
Your answer
8.Explain why the eureka can is filled until water overflows and then allowed to settle before the object is added.
Written response · explain why
Your answer
9.An irregular object of mass 240 g displaces 30 cm³ of water. Calculate its density in g/cm³.
Written response · application
Your answer

3.Liquids and the evaluation discipline

5.4

A liquid's volume reads directly off the cylinder, but its mass needs subtraction: weigh the empty cylinder, add liquid, weigh again, and the difference is the liquid's mass. Zeroing the balance and steady handling carry the evaluation marks.

density-liquid, IGCSE & GCSE Physics revision notes — Apparatus for determining the density of a liquid

Key points

The liquid experiment finds the liquid's mass by subtracting the empty cylinder's mass from the filled reading.
The volume is read from the cylinder's scale when the liquid is added.
The balance is zeroed with the empty cylinder aboard before liquid is added — a systematic-error guard.
Repeats and averages limit random error, and glassware plus electrical balance demand careful handling.

First pass

Weigh the empty cylinder, pour to a chosen volume, weigh again; the difference is purely the liquid. Set the balance to zero first or the cylinder's own mass sneaks into every reading.

Analogy — Packing a suitcase on a scale: weigh the case, pack, weigh again — the contents' weight is the difference, never the total.

Exam pointer — For evaluation marks, name the zeroing as a systematic-error fix and the repeats as a random-error fix — the two error families are examined separately.

Going deeper

Experiment 3's mass is a difference: filled-cylinder reading minus empty-cylinder reading, with the balance zeroed aboard the empty cylinder so the liquid's mass alone survives. Volume comes from the cylinder's scale; repeats average the random scatter of pouring and reading, while the zeroing kills the systematic offset that repeats cannot touch.

Analogy — A checkout scale: the cashier tares the scales with the container first — the tare is the zeroing, the filled reading is the sale.

Exam pointer — Write the subtraction sentence — mass of liquid equals mass of cylinder with liquid minus mass of cylinder — and attribute each error fix to its family.

Checks

0 of 9 attempted · 0 correct

1.How is the liquid's mass obtained?
2.Why is the balance zeroed with the empty cylinder on it?
3.Which hazard belongs in this practical's safety notes alongside its glassware?
4.Which error family do repeat-and-average fixes address?
5.A student records 50 g for the empty cylinder and 130 g with liquid aboard. The liquid's mass is
6.State the mass-difference sentence used in the liquid experiment.
Written response · recall
Your answer
7.State one systematic-error fix and one random-error fix from these experiments.
Written response · recall
Your answer
8.Explain why the measuring cylinder must never sit on the balance while water is poured into it.
Written response · explain why
Your answer
9.A student obtains liquid masses of 78 g, 82 g and 80 g across three repeats. State the averaged mass and the density if the poured volume was 100 cm³.
Written response · application
Your answer