One of the properties differentiating one type of particle from another is the chemical composition of the particle. The chemical composition of the particles is determined by the chemical composition of the sole bulk species specified for the dispersed material. There can only be one bulk species specified. This determines the "chemical unit" of the particle and this "chemical unit" for the initially formed condensed "species" in the material. This "chemical unit" can be a single atom, a group of atoms, or a chemical compound. Ansys Chemkin identifies the "chemical unit" of the particle by the sole bulk species on the dispersed material. For example, Figure 6.3: Dispersed Graphite Material Declaration With Chemical Composition defines a particle type called "soot".

MATERIAL soot
DISPERSED
END
BULK/GRAPHITE/ C(B) /1.8/
END
THERMO
C(B)              121286C   1               S  0300.00   5000.00  1000.00      1
 0.14901664E+01 0.16621256E-02-0.06687204E-05 0.12908796E-09-0.09205334E-13    2
-0.07074018E+04-0.08717785E+02-0.06705661E+01 0.07181499E-01-0.05632921E-04    3
 0.02142298E-07-0.04168562E-11-0.07339498E+03 0.02601595E+02                   4
END
REACTIONS
…
END

The BULK declaration line indicates that the core of the "soot" particles consists of units of bulk species C(B) and the bulk density of the particle core is 1.8 [gm/cm³ ]. The actual chemical composition of C(B), the core of the "soot" particle, is a single carbon atom as it is defined in the Thermodynamic Data section that follows the BULK declaration.

Another example is given in Figure 6.4: Dispersed Silicon Dioxide Material Declaration With Chemical Composition , for a compound particle composed of silicon dioxide.

MATERIAL silicon
DISPERSED
END
BULK/OXIDE/ SIO2(B) /2.533/
END
THERMO
SIO2(B)            72391SI  1O   2          S    298.00   2000.00 1000.00      1
 0.48925619E+01 0.41191629E-02-0.94570083E-07-0.80073115E-09 0.25433412E-12    2
-0.11005530E+06-0.23469570E+02 0.22325585E+01 0.12478522E-01-0.28715690E-05    3
-0.96847970E-08 0.62160411E-11-0.10962063E+06-0.10594849E+02                   4
END
REACTIONS
…
END