In chemistry, Criterion A is about connecting what you can observe to what the particles are doing. The best answers move between the visible change, the particle explanation and the symbols.
You use correct terms and explain why something happens, not just that it happens.
You pick the right idea and use it on a situation you have not seen before.
You weigh data, claims or sources and reach a judgement you can support.
A little of the criterion is addressed. Ideas are basic, partly missing, or not linked to the task.
The main points are there but thin. Some explanations are correct; links to the task or evidence are incomplete.
Mostly accurate, well explained and consistently tied to the task, with only small gaps.
Detailed, precise and justified throughout, with ideas connected and applied to new situations or evidence.
A plain-language guide only. Your teacher marks against the official IB level descriptors.
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
Because when it melts it can carry electricity.
Sodium chloride is a giant ionic lattice. In the solid, the Na⁺ and Cl⁻ ions are held in fixed positions by strong electrostatic attraction, so no charged particles are free to move. When it melts, the lattice breaks down and the ions can move. These moving charged particles carry the current.
Criterion A is Knowing and understanding. It is marked out of 8 across 3 strands (A.i, A.ii, A.iii). In chemistry, Criterion A is about connecting what you can observe to what the particles are doing. The best answers move between the visible change, the particle explanation and the symbols.
Explain at the particle level: what the ions, atoms or molecules are doing and why. Write balanced equations with state symbols when asked, and check the atoms on both sides. Use the right word for the right idea: atom, ion and molecule are not interchangeable.
Explaining a property of a substance by describing the whole substance instead of its particles. Equations that are not balanced, or that change a formula to make them balance. Confusing intermolecular forces with the covalent bonds inside a molecule.
Written by CritABCD for study and revision, in our own words. The official MYP Chemistry guide and assessment criteria are published by the International Baccalaureate. Official IB guide ↗