( Sometimes ( , ( ( when ( ( two ( different solutions ) ) ( are mixed ) ) ) ( , ( ( ( the substances ) ( they contain ) ) ( ( react ( with ( each ( other ( and ( form ions ) ) ) ) ) ) . ) ) ) ) ) )
(ROOT (S (ADVP (RB Sometimes)) (, ,) (SBAR (WHADVP (WRB when)) (S (NP (CD two) (JJ different) (NNS solutions)) (VP (VBP are) (ADJP (JJ mixed))))) (, ,) (NP (NP (DT the) (NNS substances)) (SBAR (S (NP (PRP they)) (VP (VBP contain))))) (VP (VBP react) (PP (IN with) (NP (DT each) (JJ other) (CC and) (NN form) (NNS ions)))) (. .)))
Sometimes, when two different solutions are mixed, the  substances they contain react with each other and form ions.
(ROOT (S (NP (DT All) (NNS solutions)) (VP (VBP contain) (NP (QP (IN at) (JJS least) (CD two)) (NNS substances))) (. .)))