Sodium metal (Na) reacts violently with water to produce sodium hydroxide (NaOH) and hydrogen gas (H2) via the equation below.
2 Na + 2 H2O --> H2 + 2 NaOH
H2 is a gas that will dissipate into the air above the reaction. The best way to measure the amount of hydrogen gas produced by the reaction is to do the reaction is an enclosed vessel with an outlet to a gas tube at the top. The tube is then run into an inverted graduated cylinder that is filled with water. As the hydrogen gas is produced, it passes through the tube and into the cylinder where it displaces the water as it collects at the top. When the reaction is done, read where the water level is to measure the volume of gas that has collected into the tube. Be careful since hydrogen gas is highly flammable.