soft
Like you do when you pronounce the "s" in "soft".
It's either referred to as the starting material(s) or reactant.
That depends on the starting temperature, the substance being heated, and how well insulated it is from its surroundings.
soft
Please clarify and rephrase the question. The two "are"'s and the two "with"'s make the meaning muddy.
Enid Anderson has written: 'Making nursery rhyme toys' -- subject(s): English Nursery rhymes, Soft toy making 'The Great Soft Cat and Kitten Book' 'The techniques of soft toymaking' -- subject(s): Soft toy making 'Patterns for soft toys' -- subject(s): Soft toy making
Seven Up, Shasta Cola, Sprite and Squirt are soft drinks. They begin with S.
Alison Pelegrin has written: 'Big Muddy River of Stars' 'Hurricane party' -- subject(s): Women authors, Poetry, American poetry 'Big muddy river of stars'
The substance (or substances) initially involved in a chemical reaction are called reactants or reagents
Soft Musics like 80's or 90's soft English songs
Thomas A. Ryer has written: 'The Muddy and Quitchupah projects' -- subject(s): Coal mines and mining