Predict floating and sinking from object density
Compare a whole object's average density with its surrounding liquid under stated conditions.
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
- Calculate mass divided by occupied volume.
- Compare values with matching density units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which object density is smaller than liquid density 1.00 g/cm³?
What you will learn
- Use density data to predict floating or sinking.
- Distinguish material density from whole-object average density.
Compare with the surrounding liquid
For an object free to move in a uniform liquid with no other support and no surface-tension complication, an average object density below the liquid density predicts floating. A larger object density predicts sinking. An equal density gives neutral buoyancy when fully submerged: there is no density-driven tendency to rise or sink in this simple model.
In liquid density 1.00 g/cm³, objects of average densities 0.80, 1.00 and 1.20 g/cm³ therefore float, are neutrally buoyant when submerged, and sink respectively. The diagram is a schematic comparison, not a measured trajectory. A floating object is only partly submerged once it has settled.
Use the whole object's mass and displaced-volume capacity
A sealed hollow object can float even if its wall material is denser than water. If total mass is 120 g and the closed outside volume is 200 cm³, its average density is 0.60 g/cm³. The enclosed space contributes to the object's outside volume while the wall and contents contribute to its mass.
If water enters or the object changes shape, the relevant whole-object mass and volume can change. The earlier prediction must be rechecked. Do not decide whether a ship floats from metal density alone, or claim that any low-density material always floats regardless of added loads, liquid density or external constraints.
Sources for this lesson
Pause and explain
A sealed 120 g object has outside volume 200 cm³. What does its average density predict in 1.00 g/cm³ water?
Worked example
Predict the behaviour of densities 0.80, 1.00 and 1.20 g/cm³ in liquid of density 1.00 g/cm³.
Show the worked solution
- 0.80 is below the liquid density: floats.
- 1.00 matches: neutral buoyancy if fully submerged.
- 1.20 is above: sinks.
Answer Floats; neutral buoyancy when submerged; sinks.
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 objects of densities 0.80,1.00,1.20 g/cm³ in 1.00 g/cm³ liquid.
Give me a hint
Check smaller, equal and larger average densities separately.
Compare my reasoning
- The 0.80 object floats.
- The 1.00 object is neutrally buoyant when fully submerged.
- The 1.20 object sinks in this unsupported model.
Floats; neutral when submerged; sinks.
Look for these in your work
- Compare matching units.
- State the equal-density condition.
A sealed object has mass 120 g and outside volume 200 cm³. Calculate average density and predict motion in 1.00 g/cm³ water.
Give me a hint
Include the enclosed space in the intact outside volume.
Compare my reasoning
- Average density = 120/200 = 0.60 g/cm³.
- This is below water density.
- The object can float when free from other constraints.
0.60 g/cm³; floats.
Look for these in your work
- Use whole-object density.
- Compare with the surrounding liquid.
The same 200 cm³ object takes in water so its total mass becomes 240 g without an outside-volume change. Recalculate its behaviour.
Give me a hint
Recheck both the mass and outside volume after the stated change.
Compare my reasoning
- Average density = 240/200 = 1.20 g/cm³.
- That exceeds 1.00 g/cm³ water density.
- The changed object sinks in the simple unsupported model.
1.20 g/cm³; sinks.
Look for these in your work
- Use the updated mass.
- Explain why the previous prediction changes.
Common mistakes
- Use compatible density units.
- Compare average whole-object density for hollow objects.
- Equal density is not the same as guaranteed rest at the surface.
What can you explain now?
A sealed 120 g object occupies 200 cm³. Does it float in water of density 1.00 g/cm³?
Compare with the explanation
Yes; its average density is 0.60 g/cm³.
The intact object's mass divided by outside volume is below the water density.
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Make it stick
Explain a floating hollow object and state the conditions under which a density comparison is valid.
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.