( Concentration ( ( is ( ( the number ) ( of ( particles ( in ( ( a volume ) ( of space ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Concentration)) (VP (VBZ is) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS particles)) (PP (IN in) (NP (NP (DT a) (NN volume)) (PP (IN of) (NP (NN space))))))))) (. .)))
Concentration is the number of particles in a volume of space.
(ROOT (S (NP (DT The) (NN term) (NN concentration)) (VP (VBZ describes) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS items)) (PP (IN in) (NP (NP (DT a) (VBN given) (NN area)) (CC or) (NP (NN volume)))))))) (. .)))