Physics IA · B · The particulate nature of matter · topic 6 of 30

Salt and ice: how fast does a block melt in brine? · IB Physics IA idea: guide, sample graph and step-by-step plan

simple kit, real physics. good first IA!

Every winter, councils throw salt on icy roads. You're going to find out what the salt does to the clock, and why.

Difficulty●○○ Gentle
KitBasic: beakers, scales, thermometer, phone
TimeAbout 8 hours
FitsSL and HL
  • ✓ you like kitchen-table physics
  • ✓ you can weigh things carefully
  • ✓ you want a first IA that is hard to fail

The big idea

← the particle model is your hook

Ice melts at 0 °C, but dissolved salt lowers the freezing point, so the ice surface in brine sits below 0 °C and heat should flow in faster as the concentration rises. But there is a rival effect: cold fresh meltwater is less dense than brine, so in still water it can float round the ice as an insulating layer. Which effect wins depends on your set-up, especially stirring. Decide what "melted" means and keep the ice identical, and you have a clean, measurable investigation with room for real analysis.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Change the salt

Compare table salt with another solute, such as sugar or magnesium sulfate, at the same mass or the same moles.

→ Brings in the number of dissolved particles per formula unit; your x-axis becomes moles or molality instead of grams.
Measure mass lost instead of time

Take the ice out at fixed times, dry it and weigh it, or use a balance with the ice on a mesh.

→ Your graph becomes mass against time; the gradient is a melting rate, and you can compare rates directly.
Track the bath temperature

Log the temperature of the solution while the ice melts, with a probe or by filming a thermometer.

→ You see the cooling curve that goes with the melting and can link it to latent heat of fusion.
Stirred or still

Run the same concentrations twice: once with gentle, steady stirring and once in completely still liquid.

→ Separates the freezing-point effect from the meltwater layer; your graph has two series, and the conclusion explains why they differ (or don't).
Insulate the beaker

Repeat a key concentration inside a polystyrene cup or a lagged beaker.

→ Separates heat from the room from heat from the bath, and tests your assumption about where the energy comes from.
Look at the particle model

Compare your trend with the predicted freezing-point depression for each concentration.

→ Gives you a theory line to test, not just a trend; the result is a stronger conclusion.

Your Physics IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names one independent variable and its range (which concentrations, in what unit, and how many values).
  • Names one dependent variable and a clear end-point (what exactly counts as "melted"?).
  • Says what stays fixed: ice mass and shape, starting temperatures, volume of liquid, container, stirring.
  • Says what you will compare your results with (a thermal or freezing-point model, or a no-effect baseline).
  • Is narrow enough to finish with 5 to 7 concentrations and repeats at each one.

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The set-up

One way to set it up. Draw your own version, with your real masses, volumes and temperatures, for the Method section.

ice block: same mass,same shape every runthermometer: start everyrun at the same temp.salt solution: weighthe salt, fixed volumephone video: timeto the last ice

How your Physics IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; why salt on ice matters for roads, ice cream or cooling baths; what you expect and why.
Background physics~450
Freezing point depression in terms of particles; latent heat of fusion; the idea that heat flows in faster when the temperature difference is bigger; what your graph should look like if this holds.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant, and why it matters.
Method~400
How you make each concentration; how you prepare identical ice blocks; how you start the clock and decide the end-point; repeats; enough detail to copy.
Safety, ethics, environment~60
Short and specific: cold burns, slippery spills, and what you do with the salty water afterwards.
Raw datatable
One table: concentration, each melting time, temperatures at the start, units and uncertainties in the headers.
Processing~250
How you get the mean melting time and its uncertainty at each concentration; how you convert grams of salt to concentration; one worked example of each.
Graphs and analysis~350
The graph you choose and why, error bars from your spread, and what the shape of the trend says about heat flow.
Conclusion~450
Answer the question with numbers: how much the melting time changes across your range, and whether the trend matches your model.
Evaluation~650, the biggest part!
Strengths, weaknesses ranked by their effect on your result (ice shape, room heating, end-point judgement), and improvements tied to each.
Referencesnot counted
Every source you used for theory or method.

What your Physics IA graph could look like

error bars on EVERY point ✓

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different, and it may not follow this line at all. example only

048121620240280320360400440480520560Salt concentration c / g per 100 g water (± 0.1 g)Melting time t / s (± 12 s)heat-flow model, stirred bathno effect from saltyour data, with error barsis the drop between your endsbigger than the scatter?ILLUSTRATIVE · MADE-UP DATA
  • Axis labels name the quantity and its unit, with the uncertainty of the value you set.
  • Error bars on every point. Here the spread between repeats, not the stopwatch, would set the length of the bar.
  • A model line and a no-effect line are both drawn, so the reader can see which one your data supports.
  • The scale starts near the data, not at zero. Say so in the text, or the drop will look bigger than it is.
  • Another graph to consider: melting time against the predicted freezing-point depression. If heat flow is driven by temperature difference, it should be closer to a straight line.

Which uncertainty matters most?

Rank by the effect on your melting time, not on the raw readings. Differences in ice mass often beat the stopwatch by a long way. A chart with your numbers shows where to spend your effort.

Ice mass differs by ± 1.5 g of 25 g6%Bath temp. drift (± 1 K at 20 °C)4.5%Judging the end-point (± 10 s)2.5%Stopwatch / video timing (± 1.5 s)0.4%Salt mass (± 0.01 g of 10 g)0.1%biggest effect: discuss this first!% uncertainty in one melting time of about 400 s (stirred)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

the trial run decides your end-point

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictFind out how dissolved salt changes the freezing point and why that should matter for heat flow. Write down your prediction before you touch the ice, including whether you expect the effect to level off.
  2. Make identical iceFreeze ice in the same mould and container every time. Weigh each block. How will you get the same shape and starting temperature, and what will you do with a block that is too far off?
  3. Trial run (1 lesson)Melt blocks in plain water and in one strong solution. Is the end-point clear? Does a block tilt, crack or stick to the side? Does the room's heat dominate? Fix these now.
  4. Decide range, repeats and end-pointChoose 5 to 7 concentrations spread widely, with a zero. Decide how many repeats, and write down your definition of "melted" and how you will apply it every time.
  5. Collect the dataRandomise the order of the concentrations so a warming room does not look like a salt effect. Record everything straight into your table: masses, temperatures, times.
  6. ProcessMean time and uncertainty for each concentration. Convert grams of salt to a concentration you can compare with theory, and decide how its uncertainty is treated.
  7. Graph and comparePlot time against concentration, with error bars. Add the model line if you have one. Where do data and model disagree, and by more than your error bars?
  8. EvaluateList weaknesses (ice shape, room heating, end-point, bath cooling) and rank them by how much they change your answer. Link each improvement to one weakness.
  9. Write, then cutWrite in the order of the layout above. Tables, graphs and equations do not count towards the 3,000 words, so let them carry detail.
  10. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Physics you'll need

Traps to avoid in this Physics IA

  • Uneven ice blocks: a bigger or odd-shaped block melts at a different rate, and it hides the salt effect.
  • Ignoring stirring: in still brine the fresh meltwater can float round the ice and change the trend, so decide, control and report it.
  • Not controlling the starting temperature of the liquid: a warmer bath shortens the time all by itself.
  • A vague end-point ("when it looks gone") that changes from run to run, or from person to person.
  • Running all the low concentrations in the morning and the high ones in the afternoon, as the room warms up.
  • Reporting grams of salt but never saying per what: mass of water, volume of solution, or moles.
  • Saying "heat from the room" in the evaluation without estimating how big it is.

More Physics IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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