What is indicated by a letter in parentheses after a chemical formula, such as (s)?
a) whether the reaction is fast or slow
b) the purity of the substance
c) the physical state of the substance
d) that the substance is a catalyst

The letter in parentheses after a chemical formula indicates c) the physical state of the substance.

The letters can be:

→ (s) for solid.

→ (l) for liquid.

→ (g) for gazeous.

→ (aq) for an aqueous solution (like NaCl (aq) - NaCl is dissolved in water)

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explanation: no but i do not usually comment