  Every finite-dimensional representation of an algebraic group G gives a trace
symmetric bilinear form on the Lie algebra of G. We give criteria in terms of
root system data for the existence of a representation such that this form is
nonzero or nondegenerate. As a corollary, we show that a Lie algebra of type E8
over a field of characteristic 5 does not have a so-called "quotient trace
form", answering a question posed in the 1960s.
