Chemical weapons changed the nature of war. Two examples of chemical weapons are mustard gas and tear gas.
Tear Gas (1914)
The earliest military use of chemicals were tear gas, which were tear inducing irritants (rather than fatal or disabling poisons like mustard gas. This was first used by the french army on august 1914. They filled up 25 mm grenades with tear gas against germans.
Mustard Gas (1917)
Mustard gas was one of the most effective gases in World War 1. Because it was heavier than air, once it touched the ground, it remained active in the soil for a couple of days, weeks, or even months, depending on the weather conditions. Mustard gases caused blisters, sore eyes, vomiting, and also attacked the bronchial tube, stripping off the mucous membrane.
Below is a chart of the top chemical weapons used in World War 1.