Thulium tarnishes slowly in air and burns readily at 150 °C to form thulium(III) oxide:
4 Tm + 3 O2 → 2 Tm2O3
Thulium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form thulium hydroxide:
2 Tm (s) + 6 H2O (l) → 2 Tm(OH)3 (aq) + 3 H2 (g)
Thulium reacts with all the halogens. Reactions are slow at room temperature, but are vigorous above 200 °C:
2 Tm (s) + 3 F2 (g) → 2 TmF3 (s) (white)
2 Tm (s) + 3 Cl2 (g) → 2 TmCl3 (s) (yellow)
2 Tm (s) + 3 Br2 (g) → 2 TmBr3 (s) (white)
2 Tm (s) + 3 I2 (g) → 2 TmI3 (s) (yellow)
Thulium dissolves readily in dilute sulfuric acid to form solutions containing the pale green Tm(III) ions, which exist as [Tm(OH2)9]3+ complexes:[5]
2 Tm (s) + 3 H2SO4 (aq) → 2 Tm3+ (aq) + 3 SO2−
4 (aq) + 3 H2 (g)