You have not given a word in CAPITALS. However, I assume the word to be used is "solid"; in which case the word you are seeking is "solidifies".
Use the word given in capital to form a word that fits in the gap
'The chemical substance... on contact with water' (solid)
You have not given a word in CAPITALS. However, I assume the word to be used is "solid"; in which case the word you are seeking is "solidifies".
I'm not a teacher of English, but I have spoken it for (almost) all of my life....
Please do not ask us to do your homework again, Barnej.
Rover