( ( A solution ) ( ( is ( ( a mixture ) ( ( in which ) ( ( one substance ) ( dissolves ( in another ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN solution)) (VP (VBZ is) (NP (NP (DT a) (NN mixture)) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (CD one) (NN substance)) (VP (VBZ dissolves) (PP (IN in) (NP (DT another)))))))) (. .)))
A solution is a mixture in which one substance dissolves in another.
(ROOT (S (NP (DT A) (NN solution)) (VP (VBZ forms) (SBAR (WHADVP (WRB when)) (S (NP (CD one) (NN substance)) (VP (VBZ dissolves) (PP (IN into) (NP (DT another))))))) (. .)))
A solution forms when one substance dissolves into another.