Water’s Properties: Density, Floating & Surface Effects · Grade 6 Science
Chapter: Matter, Mixtures & Water PropertiesWater’s Properties: Density, Floating & Surface Effects
Learning goals
- I can predict whether an object floats by comparing its average density with water's.
- I can explain how a steel ship floats when steel is far denser than water.
- I can explain why coastal places have smaller temperature swings than inland ones.
Readings for this unit
Before this unit
Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.
Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.
Water has a density of 1.00 g/cm3, which is not a coincidence: the gram was originally defined so that 1 cm3 of water would have a mass of 1 g. That makes water a convenient yardstick, because any material with a density below 1.00 floats in it and anything above sinks. Cork at 0.24 g/cm3 floats high, ice at 0.92 floats low, aluminium at 2.70 sinks and lead at 11.3 sinks fast.
Water is also unusually good at absorbing heat without getting much hotter. Raising the temperature of 1 kg of water by 1 degree Celsius takes about 4180 J. The same job for 1 kg of iron takes about 450 J, and for dry sand around 800 J. Water needs roughly nine times the energy iron does.
And water's surface behaves like a stretched skin, strong enough to hold up a steel paperclip laid on it flat and to let insects walk across a pond. All three oddities come from the same cause: water molecules attract each other far more strongly than their small size would suggest.
Build your understanding
Where this fits
This lesson closes the chapter on matter by looking hard at one substance. Water appears in almost every remaining chapter of this course, in the water cycle, in weather, in watersheds and in living things, and the properties set out here are the reason it matters in all of them. The specific heat figure in the last section is the one that explains coastal climates in chapter 6.
Use three flexible 40-45-minute sessions: density and floating; ships, displacement and average density; surface effects and water's heat capacity, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · What floats and what does not
An object floats in water if its density is less than 1.00 g/cm3 and sinks if its density is greater, and that one comparison settles the whole question. Cork at 0.24 g/cm3 floats with most of itself above the surface. Ice at 0.92 g/cm3 floats, but only just: the fraction submerged is the ratio of the two densities, 0.92 divided by 1.00, so about 92 per cent of an iceberg sits below the waterline and only 8 per cent shows. That is where the expression about the tip of the iceberg comes from, and it is a prediction you can make from two numbers rather than a saying you have to remember.
Oil floats on water for the same reason, with most cooking oils sitting around 0.92 g/cm3. The ordering is strict and you can build a column of layers to show it: honey at about 1.42 g/cm3 at the bottom, then water at 1.00, then oil at 0.92, each floating on the one below because each is less dense than it. Dropping objects into that column sorts them by density too, so a grape at about 1.1 g/cm3 settles between the honey and the water while a cork rides on the very top. Density also sorts things on a planetary scale, which is worth noticing now because chapter 5 depends on it. Earth's interior is layered with the densest material at the centre and the lightest at the surface, for exactly the reason the liquid column layers itself, and the ocean itself is layered too, with colder and saltier water sitting beneath warmer, fresher water.
Pause and explain
Ice has a density of 0.92 g/cm3 and water 1.00. What fraction of a floating iceberg is below the surface, and how do you know?
Check your reasoning after answering
About 92 per cent, because the submerged fraction equals the ratio of the two densities, 0.92 divided by 1.00. The ice must push aside enough water to match its own mass, and since ice is only slightly less dense than water it has to sink almost entirely to do so, leaving roughly 8 per cent visible above the waterline.
Part 2 · How steel floats
Steel has a density of about 7.85 g/cm3, nearly eight times that of water, so a solid steel block sinks immediately. A steel ship floats because what matters is the average density of the whole vessel, hull and cargo and the enormous volume of air enclosed inside it, and that average comes out below 1.00 g/cm3. Flatten the same steel into a sheet and it sinks; fold it into a box and it floats, with no change in the amount of steel. Shape is doing the work by increasing the volume while leaving the mass alone.
The same idea stated the other way round is Archimedes' principle: a floating object pushes aside a mass of water equal to its own mass. A ship with a mass of 50,000 tonnes floats at the depth where its hull has displaced exactly 50,000 tonnes of water, which is why loading cargo makes a ship sit lower rather than making it sink. Ships carry a painted line on the hull marking the safe limit, and a vessel loaded past it has run out of hull to displace with and is genuinely in danger. Submarines use the same principle deliberately in both directions. They carry tanks that can be flooded with sea water to raise the vessel's average density above that of the surrounding water, which makes it sink, and then blown clear with compressed air to lower the average density again so it rises. Nothing about the steel changes; only the proportion of the enclosed volume filled with water rather than air.
Pause and explain
Steel is about 7.85 g/cm3, so why does a steel ship float?
Check your reasoning after answering
Because floating depends on the average density of the whole vessel, not of the steel alone. A hull encloses a very large volume of air, so the total mass divided by the total volume comes out below 1.00 g/cm3. The same steel rolled flat sinks, because the volume collapses while the mass stays the same.
Part 3 · The skin on the surface
Water molecules pull on each other in all directions inside the liquid, but a molecule at the surface has neighbours only below and beside it, so the net pull drags it inward. The result is a surface that behaves like a stretched elastic sheet, which is called surface tension. It is strong enough to support a steel paperclip laid flat, even though steel at 7.85 g/cm3 should sink instantly, because the clip is resting on the surface rather than floating in the water. Push it through the surface and it drops at once, which shows that the skin, not buoyancy, was holding it.
The same effect shapes every water droplet into a sphere, since a sphere has the smallest surface for a given volume and the surface is pulling itself as small as it can. It is why water forms a curved meniscus in a measuring cylinder, which is why the last lesson insisted on reading the bottom of the curve. It is also why adding detergent matters: detergent molecules get between the water molecules and weaken their grip on each other, the surface tension falls, and water that used to bead on a greasy plate now spreads and wets it. Surface tension is not a small force at small scales. A water strider's legs rest in dimples on the surface without breaking through, and the insect can stand on water many times its own mass because its weight is spread across several contact points. Gravity dominates for large objects and surface tension dominates for very small ones, which is why a paperclip can be floated and a spoon cannot.
Pause and explain
Why does a paperclip rest on water when steel is nearly eight times denser than water?
Check your reasoning after answering
Because it is held by the surface rather than floating in the liquid. Molecules at the surface are pulled inward by their neighbours below, producing a skin under tension that can support a small flat object. The clip is not displacing its own mass of water, which is why pushing it through the surface makes it sink immediately.
Part 4 · Water holds on to heat
It takes about 4180 J to raise the temperature of 1 kg of water by 1 degree Celsius, against about 450 J for iron and around 800 J for dry sand. Water therefore warms slowly and cools slowly, while land warms quickly and cools quickly. Anyone who has walked on a beach on a hot afternoon has measured this already: the dry sand is painful underfoot while the sea a few metres away is still cold, even though both have been under the same sun all day.
Scaled up, this governs climate. A town on the coast has a large body of water beside it that changes temperature only slowly, so its summers are cooler and its winters milder than an inland town at the same latitude; the yearly range might be 15 degrees Celsius on the coast against 30 degrees Celsius well inland. It is also why the sea breeze exists, since land heats faster during the day, the air above it rises, and cooler air flows in off the water to replace it. Chapter 6 returns to both of these. The same property does quiet work inside living things. A human body is roughly 60 per cent water by mass, and that water resists sudden temperature changes in exactly the way the sea does, which helps keep internal temperature near 37 degrees Celsius through a hot afternoon or a cold morning. Water's reluctance to change temperature is doing a job in a pond, in a climate and in you.
Pause and explain
Why does a coastal town have a smaller annual temperature range than an inland town at the same latitude?
Check your reasoning after answering
Because water takes about 4180 J to warm 1 kg by 1 degree Celsius against roughly 800 J for sand, so the sea changes temperature far more slowly than land. A large body of water beside the town absorbs heat in summer without warming much and releases it in winter, damping both extremes and leaving a range perhaps half that of an inland site.
Apply the whole unit · Float or sink
A sealed plastic box measures 10.0 cm by 8.0 cm by 5.0 cm and has a mass of 320 g. A student then puts 150 g of sand inside it and seals it again.
Your task
Decide whether the box floats before and after the sand is added, and explain the change.
What your response should show
- Calculates volume and both densities with correct units
- Compares each density against 1.00 g/cm3 to reach the float or sink decision
- Explains that mass changed while volume did not
- States an assumption the prediction depends on
Response guidance — after your attempt
Calculate the volume of the box and its density before the sand is added.
Recalculate the density with the sand inside and compare both with water.
Explain what has changed and what has not, and state one assumption you have made.
The volume is 10.0 times 8.0 times 5.0, which is 400 cm3. Before the sand, the density is 320 divided by 400, or 0.80 g/cm3, which is below 1.00 g/cm3, so the box floats, with about 80 per cent of it submerged. With 150 g of sand inside, the mass rises to 470 g while the volume stays at 400 cm3, because the sand goes into space that was already inside the box. The density is now 470 divided by 400, which is 1.175 g/cm3, above 1.00, so the box sinks. What changed is the mass and therefore the average density; what did not change is the volume, which is why adding contents can sink a container without making it any bigger. I have assumed the box stays sealed and watertight, since a leak would let water in, raise the mass further and sink it regardless of the arithmetic.
Scientific references for this unit
- US Geological Survey Water Science School: water density and its properties
- National Oceanic and Atmospheric Administration: why coasts have milder climates
- National Institute of Standards and Technology: specific heat capacity reference data
- Royal Society of Chemistry: surface tension and the behaviour of water
Parent / teacher teaching notes
Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.
The key idea
Water behaves oddly in several ways that matter enormously: its solid floats on its liquid, it holds more heat than almost any common substance, and its surface behaves as though it has a skin. Each of these follows from the same thing, which is how strongly water molecules hold on to one another.
Worked examples
Example 1 · Compare an ice model and a floating hull
- Divide ice mass by its volume.
- Find the mass of displaced water for the floating hull.
- Distinguish material density from whole-object geometry.
Example 2 · Transfer to a different case
- Identify the distinction and conditions in this new case.
- Use the relevant mechanism or quantity; preserve units and the chosen boundary.
- Explain why the answer fits and what further evidence would be needed.
Example 3 · Explain displacement and shape · a focused example
- State the literal condition or observation in the example.
- Apply the distinction explained in this teaching part.
- Check whether the claim stays within the given evidence.
Avoid these mistakes
- Misleading claim: The heaviest or largest object must always sink.
- Misleading claim: Ice floats because freezing removes its mass.
- Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
- Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.
Check your understanding
Which is less dense than the modeled water?
Discuss the answer
The 20 mL object: 12 ÷ 20 = 0.6 g/mL. The second is 2 g/mL; the equal masses do not imply equal floating behavior.
Why can reshaping affect floating without changing the metal?
Discuss the answer
It changes the object’s geometry and enclosed air, allowing different displacement and average density.
Did the freezing model lose mass?
Discuss the answer
No. Mass stays 90 g while volume and density change with arrangement.
Does surface support prove the needle is less dense than water?
Discuss the answer
No. A surface force can matter for a small object; density and buoyancy must be considered separately.
Sustained scientific explanation and revision
These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.
Activity 1 · Water’s Properties: Density, Floating & Surface Effects · Model and explain
Read The Clear Liquid and the Missing Salt and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Water’s Properties: Density, Floating & Surface Effects, study Parts 3–4 and attempt the worked case: Calculate ice density and explain the hull’s displacement without claiming that every metal floats. Explain the steps before opening the model.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Ice density is 0.9 g/mL. The hull displaces 60 g of modeled water, consistent with buoyancy balancing its weight. A hollow hull can float because of its overall geometry and average density; a compact piece of the same material may behave differently. Neither result is a rule for every temperature or liquid.
Activity 2 · Water’s Properties: Density, Floating & Surface Effects · Practice, revise and exit
Read The Clear Liquid and the Missing Salt and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Water’s Properties: Density, Floating & Surface Effects, complete practice questions 1–6 and both choice checks, then answer independently: A learner says the largest object must sink first. What measurements would you ask for? Save a correction explaining one changed claim or method.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Ask for mass, volume or displacement, shape, enclosed air, and the liquid’s properties. Size alone does not determine average density or buoyancy.
Words to know
- buoyancy
- The upward force a fluid exerts on an immersed or partly immersed object.
- surface tension
- An effect of particle interactions at a liquid surface that can influence small surface objects.
- evidence
- A relevant observation, measurement, or record used to evaluate a claim.
- system boundary
- The chosen limit specifying what is included and which exchanges cross it.
- uncertainty
- A stated limit in measurement or knowledge, distinct from a careless mistake.
In the real world
Exit check
A learner says the largest object must sink first. What measurements would you ask for?
Check your explanation after answering
Ask for mass, volume or displacement, shape, enclosed air, and the liquid’s properties. Size alone does not determine average density or buoyancy.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.