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'Produces a gas' is not an observation — must say effervescence/bubbling
“Candidates stated that a gas was produced rather than describing the observable evidence such as effervescence or bubbling.”— 5070 Paper 2, May/June 2023
How to fix: An observation is what you SEE, HEAR, or SMELL. 'A gas is produced' is a conclusion, not an observation. Write 'effervescence/bubbling' instead. Similarly: 'heat is released' should be 'temperature increases' or 'test tube becomes warm to touch'.