The phrase "to use chemical weapons" refers to the act of employing or deploying substances that are capable of causing harm, injury or death through their toxic properties. These substances can include gases such as sarin and mustard gas, as well as liquids like chlorine and other agents that can cause severe physical damage when used in a military conflict or during an act of terrorism. The use of chemical weapons is considered a serious violation of international law and human rights, and those who engage in their production, stockpiling or deployment may be held accountable for war crimes.