Ineed to know what is the blank answer is and me learn it ?

chemical reaction and google is the best way to learn its definition witch is

"A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another."

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

answer: e.   sour taste

explanation: acids are those substances which dissociates in water to give ions. as they dissociate to give ions, they act as electrolytes and thus are electrically conductive. acids are sour in taste. example: hcl

bases are those substances which dissociates in water to give ions or accepts ions. as they dissociate to give ions, they act as electrolytes and thus are electrically conductive. bases are bitter in taste. bases are slippery in nature.