Compare liquid pressure by depth and density
Reason qualitatively about stationary liquids under matching surface conditions.
Section P1.5.4 · Pressure
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Distinguish density from total mass.
- Measure depth vertically below a liquid surface.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is density?
What you will learn
- Explain greater liquid pressure at greater depth.
- Compare different liquid densities at the same depth and surface pressure.
Greater depth means a greater liquid contribution
In a stationary liquid, pressure beneath the surface increases with vertical depth because the liquid above must be supported against gravity. A deeper point in the same uniform liquid under the same surface conditions has a larger pressure. Depth is measured vertically from the liquid surface, not along a sloping wall, a pipe or a swimming route.
The qualitative comparison does not imply zero total pressure at the surface. Any pressure applied at the surface contributes to the pressure below it. If two vessels have different surface pressures, depth alone does not settle their total-pressure comparison. The Extended lessons calculate the additional liquid contribution with a stated boundary.
Density matters when other conditions match
At the same vertical depth, equal surface pressure and equal gravitational field, a denser stationary uniform liquid gives a greater pressure contribution. It has more mass in each equal volume of the vertical liquid column. Compare density rather than assuming that a wider container's greater total liquid mass automatically raises pressure at a given depth.
A point at greater depth in a less dense liquid and a shallower point in a denser liquid cannot be ranked from only one changed factor. The effects compete, so state that further quantitative information is needed when the supplied descriptions do not settle the comparison. A flowing liquid may need a different model; do not apply the stationary account without its conditions.
Sources for this lesson
Pause and explain
At equal depth with matching surface pressure and g, which stationary uniform liquid has greater pressure: the denser or less dense one?
Worked example
Compare a shallow and deep point in the same stationary uniform liquid, then equal-depth points in two different liquids with matching surface pressure and g.
Show the worked solution
- The deeper point in the same liquid has greater pressure.
- At equal depth the denser liquid has the greater liquid contribution.
- The surface and gravitational conditions must match for those comparisons.
Answer Greater depth increases pressure; at equal depth, greater density increases pressure under matched conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare shallow point A and deeper point B in one stationary uniform liquid with the same surface.
Give me a hint
Use vertical depth and keep the liquid's density fixed.
Compare my reasoning
- A and B share the same liquid density.
- B has greater vertical depth below the surface.
- The liquid contribution and total pressure are larger at B.
B has greater pressure.
Look for these in your work
- Use vertical depth.
- Keep the surface condition common.
Compare equal-depth points in liquids labelled density 800 and 1000 kg/m³ with equal surface pressure and g, then compare two equal-depth points in one uniform liquid.
Give me a hint
Control depth before using density, and control both for the same-liquid comparison.
Compare my reasoning
- The 1000 kg/m³ liquid is denser.
- Its equal-depth point has greater pressure under the supplied conditions.
- Equal-depth points in one stationary uniform liquid have equal pressure.
Greater pressure in the 1000 kg/m³ liquid; equal pressure at equal depths in the same liquid.
Look for these in your work
- Use density rather than whole vessel mass.
- Apply the matching conditions.
Point A is deeper in a less dense liquid, while B is shallower in a denser liquid. No quantitative relationship or matching surface pressure is supplied. Does either description alone settle total pressure?
Give me a hint
Identify both competing factors and the missing boundary condition.
Compare my reasoning
- Greater depth tends to increase the liquid contribution.
- Greater density also tends to increase it, so the two changes compete.
- The quantitative balance and surface pressures are needed for a definite total comparison.
No definite ranking follows from the descriptions alone.
Look for these in your work
- Do not ignore a changed factor.
- State the missing information.
Common mistakes
- Use vertical depth from the surface.
- Control surface pressure and g.
- Greater container mass is not the same as greater density.
What can you explain now?
Must a longer sloping route through a tank mean a greater pressure increase?
Compare with the explanation
No.
The stationary liquid comparison concerns vertical depth, not the route length.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain depth and density effects qualitatively and recognise when unmatched conditions prevent a definite comparison.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.