The most common and most likely answer to this crossword clue is the 9 letter word REACTANTS
R
E
A
C
T
A
N
T
S
More clues leading to the result REACTANTS
Buddy explains!
The straight part
We know that "chemical experiment substances" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chemical experiment substances = reactants.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does REACTANTS mean?
REACTANTS noun- Any of the participants present at the start of a chemical reaction