Complex sentence: Like all the alkali metals, it reacts exothermically with water, to the point that sufficiently large pieces melt to a sphere and may explode; this reaction produces caustic soda (sodium hydroxide) and flammable hydrogen gas.
Split into simpler sentences: Like all the alkali metals, it reacts exothermically with water, and sufficiently large pieces melt to a sphere and may explode. The reaction produces caustic soda (sodium hydroxide) and flammable hydrogen gas.