Final answer:
A cold pack is applied over a splinted open injury to reduce swelling and pain, using an endothermic reaction from chemicals like ammonium nitrate to absorb heat and cool the affected area.
Step-by-step explanation:
A cold pack should be applied to a splinted open injury to reduce swelling and pain. It should be placed around the injured area, over the splint, without applying undue pressure. Such packs typically contain ammonium nitrate, which when activated by breaking an inner seal within the pack, dissolves in water causing an endothermic reaction. This reaction absorbs heat from the surrounding area, including the injury, creating a cooling effect. Be cautious not to apply the cold pack directly to the skin to prevent frostbite and ensure it is wrapped in a cloth if necessary.