  We investigate the contribution that instantons make to the QCD chiral
condensate for Nf= 0, 1 and 2 quark flavours. We use a simplified model: the
instantons are a (weighted) gas, the fermionic integrations are restricted to
the subspace spanned by the would-be zero-modes and only the dimensional and
chiral features of the Dirac operator are retained. Previous work along these
lines found a power-like divergence of the Dirac spectral density at zero
eigenvalue. By changing the various components of the model, we show that a
power divergence appears to be a generic feature of instanton mixing for both
Nf=0 and Nf=/=0. In the latter case the divergence disappears as m->0 in such a
way that psibar-psi remains finite. We find that the exponent of the power
decreases for larger instanton densities, becoming negligible for very dense
instanton `gases'. We reproduce the expected m-dependence of the topological
susceptibility, we investigate the space-time structure of the eigenfunctions,
as a function of the eigenvalue, and we calculate the eta-prime mass. In
summary: instantons appear to provide a natural mechanism for the spontaneous
breaking of chiral symmetry in QCD, but they typically produce a power-like
divergence in the Dirac spectral density. This divergence is non-standard but
`harmless' in the case of full QCD, and implies a pathology in the case of
quenched QCD.
