A. When weighing drugs with a tray balance, the drugs cannot be weighed directly on the tray. Generally, drugs are weighed on paper, and drugs that are easily deliquescent or corrosive are weighed in glassware. ; Sodium hydroxide is highly corrosive, so it can be weighed in a beaker. Therefore, the option statement is wrong.

B. Hydrogen is flammable, so the purity must be checked before igniting, otherwise it will easily explode; therefore, the option statement is correct.

C. The leftover medicines must be kept in the "three no-ones": do not put them back into the original bottles; do not throw them away; do not take them out of the laboratory; and put them into designated containers; therefore, the option statement is wrong.

D. When heating the liquid in the test tube, the liquid does not exceed one third of the test tube volume, so the option statement is wrong.

So choose: B.