Float or sink — questions

1. identify

Water has a density of 1.00 g/cm³. Which of these objects float in water?

Tick every correct statement.

• a cork (0.24 g/cm³)
• a candle (0.90 g/cm³)
• a glass marble (2.5 g/cm³)
• a block of oak (0.75 g/cm³)
• a small aluminium washer (2.7 g/cm³)

What did you compare?

Nudge (only if you are stuck): Compare each density with 1.00 g/cm³. Mass and size are not given because they don’t decide it.

2. identify

Three liquids that do not mix are poured gently into a tall jar: liquid P (1.60 g/cm³), water (1.00 g/cm³) and liquid Q (0.80 g/cm³). Which liquid ends up in each layer?

Write the number of the matching option.

1. liquid P
2. water
3. liquid Q

• Top layer: ______
• Middle layer: ______
• Bottom layer: ______

Nudge (only if you are stuck): The densest liquid sinks to the bottom.

3. calculate / predict

A plastic toy has a mass of 120 g and a volume of 150 cm³. (a) Calculate its density, to 2 significant figures. (b) Will it float or sink in water? Explain.

(a) Density of the toy: ______________ unit ________

(b) Float or sink, and why?

Nudge (only if you are stuck): Find the density first, then compare it with water’s 1.00 g/cm³.

4. predict / explain

Steel has a density of 7.9 g/cm³. Mercury is a liquid metal with a density of 13.6 g/cm³. A small steel ball bearing is dropped into a beaker of mercury. What happens?

Circle one answer.

• It floats on the mercury
• It sinks to the bottom
• It dissolves in the mercury
• It floats only if it is very small

Explain using the densities.

Nudge (only if you are stuck): Compare the ball’s density with the density of the liquid it is in, not with water.

5. predict / explain

A beaker holds a layer of cooking oil (0.92 g/cm³) floating on water (1.00 g/cm³). A plastic bead with a density of 0.95 g/cm³ is dropped in from above. Where does it end up?

Circle one answer.

• On top of the oil
• In the middle of the oil layer
• At the boundary between the oil and the water
• On the bottom of the beaker

Explain using the three densities.

Nudge (only if you are stuck): Check the bead against each liquid in turn: first the oil it falls into, then the water below.

6. explain

Oil leaking from a damaged ship spreads out on top of the sea instead of sinking. The oil has a density of 0.88 g/cm³ and sea water 1.03 g/cm³. Explain why, using these densities and one other property of oil and water.

Nudge (only if you are stuck): Two things are needed: how the densities compare, and whether oil and water mix.

7. explain / predict

A submarine is a steel hull with large tanks inside. With the tanks full of air it floats at the surface. To dive, the crew opens valves and lets sea water into the tanks. To come up again, compressed air pushes the water out. (a) Explain, using density, why the submarine floats when its tanks are full of air, although steel is 7.9 g/cm³. (b) Explain why letting water into the tanks makes it sink. (c) Explain why pushing the water out makes it rise again.

(a) Why it floats with air in its tanks

(b) Why it sinks with water in its tanks

(c) Why it rises again

Nudge (only if you are stuck): Treat the whole submarine, with whatever is in its tanks, as one object. What happens to its total mass and its total volume?

8. predict / explain

A salad dressing is made of vinegar (1.01 g/cm³) and olive oil (0.91 g/cm³), which do not mix. (a) After the bottle is shaken and left to stand, which liquid is on top? Explain. (b) Milk (1.03 g/cm³) poured gently onto water does not stay as a layer on the bottom. Explain why the density rule cannot be used for milk and water.

(a) Which is on top, and why

(b) Why the rule fails for milk and water

Nudge (only if you are stuck): The rule about layers only works for one kind of pair of liquids. Which kind?