炸 zhà means to explode, bomb, blow up or blast. Zh is pronounced like the letter j and zhà is pronounced jar
in the fourth tone. Note the radical 火 hǔo
(which means fire) on the left side of 炸, hinting that this word has something to do with flames.
药 yào means chemicals or medicines. In ancient China, many chemicals were used as medicines. In fact, gun-powder was accidentally discovered during the Tang Dynasty 1200 years ago. Some monks were searching for the elixir of immortality when they discovered that heating a mixture of potassium nitrate, sulphur and honey produced amazing effects.
Smoke and flames erupted and the whole house burned down. The monks called the mixture 火 药 hǔo yào
http://chinesecharacteraday.com/wp-content/uploads/2011/02/Huo_Yao.mp3 (fire chemical).
Increasing the proportion of potassium nitrate in the mixture made the 火 药 much more explosive and dangerous. That was how gun-powder or 炸 药 (exploding chemical) was discovered.