Use spoken observation and reasoning to pin down the signs of a new substance.
Observation round-robin
Straight after the reaction, go round the group with each student naming one thing they observed aloud — "I heard fizzing", "I saw bubbles", "the cup felt cooler". Collecting spoken observations builds a rich shared list of the signs before anyone writes.
The "new substance?" frame
Each classification is justified aloud first: "This is irreversible because I saw/heard/felt ___, which is a sign a new substance was made." Listen for students linking a specific observed sign to the conclusion, not just guessing.
Sort-and-defend
For the exit-ticket sort, students say each change and its category to a partner with a reason, and the partner may challenge: "are you sure that's irreversible?" The spoken defence is the assessable moment.