  We present a comprehensive suite of high-resolution (parsec-scale), idealised
(non-cosmological) galaxy merger simulations (24 runs, stellar mass ratio
~2.5:1) to investigate the connection between interaction-induced star
formation and the evolution of the interstellar medium (ISM) in various
temperature-density regimes. We use the GIZMO code and the second version of
the 'Feedback in Realistic Environments' model (FIRE-2), which captures the
multi-phase structure of the ISM. Our simulations are designed to represent
galaxy mergers in the local Universe. In this work, we focus on the
'galaxy-pair period' between first and second pericentric passage. We split the
ISM into four regimes: hot, warm, cool and cold-dense, motivated by the hot,
ionised, atomic and molecular gas phases observed in real galaxies. We find
that, on average, interactions enhance the star formation rate of the pair
(~30%, merger-suite sample average) and elevate their cold-dense gas content
(~18%). This is accompanied by a decrease in warm gas (~11%), a negligible
change in cool gas (~4% increase), and a substantial increase in hot gas
(~400%). The amount of cold-dense gas with densities above 1000 cm^3 (the cold
ultra-dense regime) is elevated significantly (~240%), but only accounts for
0.15% (on average) of the cold-dense gas budget.
