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In page Chemical warfare:

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The chemical used in warfare is called a chemical warfare agent (CWA). About 70 different chemicals have been used or stockpiled as chemical warfare agents during the 20th and 21st centuries. These agents may be in liquid, gas or solid form. Liquid agents that evaporate quickly are said to be volatile or have a high vapor pressure. Many chemical agents are volatile organic compounds so they can be dispersed over a large region quickly.[citation needed][1]