  We study two body decays of a scalar glueball. We show that in QCD a spin-0
pure glueball (a state only with gluons) cannot decay into a pair of light
quarks if chiral symmetry holds exactly, i.e., the decay amplitude is chirally
suppressed. However, this chiral suppression does not materialize itself at the
hadron level such as in decays into $\pi^+\pi^-$ and $K^+K^-$, because in
perturbative QCD the glueball couples to two (but not one) light quark pairs
that hadronize to two mesons. Using QCD factorization based on an effective
Lagrangian, we show that the difference of hadronization into $\pi\pi$ and $KK$
already leads to a large difference between ${\rm Br} (\pi^+\pi^-)$ and ${\rm
Br}(K^+K^-)$, even the decay amplitude is not chirally suppressed. Moreover,
the small ratio of $R={\rm Br}(\pi\pi)/{\rm Br}(K\bar K)$ of $f_0(1710)$
measured in experiment does not imply $f_0(1710)$ to be a pure glueball. With
our results it is helpful to understand the partonic contents if ${\rm
Br}(\pi\pi)$ or ${\rm Br}(K\bar K)$ is measured reliably.
