MYP Chemistry: Water & Solubility FAQs

Water & solubility is the MYP 4-5 unit covering why water dissolves some substances but not others, based on molecular polarity and intermolecular forces. It sits under the Interactions or Systems key concept and links directly to solutions, concentration, and later to DP Chemistry's structure and bonding topics.

Water is a polar molecule — its bent shape gives it a slightly negative oxygen end and slightly positive hydrogen ends. Polar or ionic substances (salt, sugar) dissolve because water molecules surround and separate their particles. Non-polar substances (oil, wax) don't, because there's no attraction strong enough to pull them apart.

A solute is the substance being dissolved (like salt), the solvent is what does the dissolving (usually water), and a solution is the uniform mixture you get once the solute is fully dispersed. Get these three terms exact — MYP assessment criteria specifically reward precise scientific vocabulary in Criterion A.

An unsaturated solution can still dissolve more solute at that temperature. A saturated solution has dissolved the maximum amount possible and any extra solute sits undissolved at the bottom. A supersaturated solution holds more dissolved solute than should be possible at that temperature — an unstable state usually made by cooling a hot saturated solution slowly.

At MYP 4-5, concentration is usually expressed as mass of solute per volume of solution, often in grams per decimetre cubed (g/dm³), sometimes introduced alongside percentage concentration. The formula is: concentration = mass of solute ÷ volume of solution. Some schools introduce molar concentration (mol/dm³) as a bridge into DP Chemistry.

Temperature, particle size, stirring/agitation and pressure (for gases specifically) all affect solubility and dissolving rate. Higher temperature generally increases solubility of solids but decreases solubility of gases — that's the exact opposite trend, and it's one examiners test deliberately.

Criterion C (processing and evaluating) wants a controlled comparison — for example, how temperature affects the mass of salt dissolved in a fixed volume of water. State one independent variable, one dependent variable, and list at least three controlled variables explicitly, then present data in a table before graphing it.

Expect 'state' (a saturated solution's definition), 'explain' (why polar and non-polar substances behave differently), 'describe' (a solubility curve's trend), and 'predict' (what happens if you cool a supersaturated solution). Each demands a different depth — 'state' wants one line, 'explain' wants a mechanism with reasoning.

Not conceptually hard, but it's easy to lose marks on vocabulary precision and graph interpretation. Most students who struggle aren't confused by the chemistry — they mix up solute/solvent, misread solubility curves, or forget units on concentration calculations. Nail the terminology early and this topic becomes one of the more reliable ones for a strong grade.

Water & solubility feeds directly into DP Chemistry's structure and bonding, and later into solution stoichiometry and equilibrium topics. According to the IB, first exams for the current DP Chemistry guide were 2025, and it builds molarity, solubility product (Ksp) and intermolecular force reasoning straight from MYP foundations — so gaps here compound later.

You don't need a chemistry background — ask your child to explain 'why does salt dissolve in water but oil doesn't' in their own words. If they can't explain it simply, they haven't fully understood it yet, whatever grade they're getting on worksheets. That's usually a more honest check than the mark on a quiz.

Focus on resources with worked solubility-curve questions and concentration calculations, not just definitions — this topic is assessed through application, not recall. Look for materials that pair a short concept explanation with practice questions and mark schemes, so your child can self-check where marks are actually lost.

No — MYP 4-5 doesn't require memorising ionic solubility rules (like 'all nitrates are soluble'). That level of detail belongs to DP Chemistry and beyond. At MYP, you need conceptual understanding of polarity, plus the ability to read and interpret solubility curves and calculate concentration.