( They ( ( ( can also ) ( be ( caused ( ( ( by ( ( chemical reactions ) ( -LRB- ( ( for ( ( instance , ) ( when ( ( two ( or ( more incompatible ) ) ) ( ( substances and ) combined ) ) ) ) ) -RRB- ) ) ) ) or ) ( when ( ( certain chemicals ) ( are ( exposed ( to ( ( ( ( ( heat , ) water ) , ) or ) air ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (MD can) (ADVP (RB also)) (VP (VB be) (VP (VBN caused) (UCP (PP (IN by) (NP (NP (NN chemical) (NNS reactions)) (PRN (-LRB- -LRB-) (PP (IN for) (NP (NP (NN instance)) (, ,) (SBAR (WHADVP (WRB when)) (S (NP (CD two) (CC or) (JJR more) (NNS incompatible)) (VP (VP (VBZ substances)) (CC and) (VP (VBD combined))))))) (-RRB- -RRB-)))) (CC or) (SBAR (WHADVP (WRB when)) (S (NP (JJ certain) (NNS chemicals)) (VP (VBP are) (VP (VBN exposed) (PP (TO to) (NP (NP (NN heat)) (, ,) (NP (NN water)) (, ,) (CC or) (NP (NN air)))))))))))) (. .)))
They can also be caused by chemical reactions (for instance, when two or more incompatible substances and combined) or when certain chemicals are exposed to heat, water, or air.
(ROOT (S (NP (DT A) (NN combination) (NN reaction)) (VP (VBZ is) (NP (NP (DT a) (NN reaction)) (SBAR (WHPP (IN in) (WHNP (WDT which))) (S (NP (CD two) (CC or) (JJR more) (NNS substances)) (VP (VBP combine) (S (VP (TO to) (VP (VB form) (NP (DT a) (JJ single) (JJ new) (NN substance)))))))))) (. .)))