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Star formation activity in molecular clouds is often found to be correlated with the amount of material above a column density threshold of $\sim 10^{22} \, {\rm cm^{-2}}$. Attempts to connect this column density threshold to a ${\it volume}$ density above which star formation can occur are limited by the fact that the volume density of gas is difficult to reliably measure from observations. We post-process hydrodynamical simulations of molecular clouds with a time-dependent chemical network, and investigate the connection between commonly-observed molecular species and star formation activity. We find that many molecules widely assumed to specifically trace the dense, star-forming component of molecular clouds (e.g. HCN, HCO$^+$, CS) actually also exist in substantial quantities in material only transiently enhanced in density, which will eventually return to a more diffuse state without forming any stars. By contrast, N$_2$H$^+$ only exists in detectable quantities above a volume density of $10^4 \, {\rm cm^{-3}}$, the point at which CO, which reacts destructively with N$_2$H$^+$, begins to deplete out of the gas phase onto grain surfaces. This density threshold for detectable quantities of N$_2$H$^+$ corresponds very closely to the volume density at which gas becomes irreversibly gravitationally bound in the simulations: the material traced by N$_2$H$^+$ never reverts to lower densities, and quiescent regions of molecular clouds with visible N$_2$H$^+$ emission are destined to eventually form stars. The N$_2$H$^+$ line intensity is likely to directly correlate with the star formation rate averaged over timescales of around a Myr., Comment: 10 pages, 10 figures. MNRAS accepted

Much of what we know about molecular clouds, and by extension star formation, comes from molecular line observations. Interpreting these correctly requires knowledge of the underlying molecular abundances. Simulations of molecular clouds typically only model species that are important for the gas thermodynamics, which tend to be poor tracers of the denser material where stars form. We construct a framework for post-processing these simulations with a full time-dependent chemical network, allowing us to model the behaviour of observationally-important species not present in the reduced network used for the thermodynamics. We use this to investigate the chemical evolution of molecular gas under realistic physical conditions. We find that molecules can be divided into those which reach peak abundances at moderate densities ($10^3 \, {\rm cm^{-3}}$) and decline sharply thereafter (such as CO and HCN), and those which peak at higher densities and then remain roughly constant (e.g. NH$_3$, N$_2$H$^+$). Evolving the chemistry with physical properties held constant at their final values results in a significant overestimation of gas-phase abundances for all molecules, and does not capture the drastic variations in abundance caused by different evolutionary histories. The dynamical evolution of molecular gas cannot be neglected when modelling its chemistry., Comment: 14 pages, 13 figures. MNRAS accepted