When you rotate items in Chemix, the app will lock the angles to increments of 5°. This can be helpful when you want to snap to common values such as 90° or 45°. To get there even faster, hold Shift to snap to increments of 22.5°.
At other times, you may need more precision. Maybe you’re drawing a diagram involving lasers and mirrors, or want to adjust the pouring effect on a beaker. In those cases, hold Ctrl for increments of 1°, or hold Alt for completely unconstrained rotation.