A mark scheme can be written as instructions to a marker rather than as a model answer, which suits questions where there is no single right response.
Writing one
Open the lesson from My Library and click into the question you want to work on.
Type your instructions into the Mark Scheme field.
What works
Say what earns each mark, and how many. "1 mark for naming the reactants, 1 mark for the products."
Say what does not earn a mark. "A generic statement with no reference to the passage gets 0."
Give the acceptable range for a calculation, not just the exact figure.
Counting rules are fine. "Any three of the following, 1 mark each" is marked as one criterion, and any three of your points earn the marks.
If the question can be answered more than one way, write each method as its own alternative. Separate them with OR or give each one a heading, and make each method add up to the full marks along with anything they share. The marker works out which method the student used and marks against that one. See Schemes with more than one method.
Keep each criterion to one idea. The marker splits your scheme into separate criteria and decides each on its own, so one idea per line marks more consistently.
See How long answers are marked for what the marker does with it.