When formaldehyde (HCHO) reacts with phenol (C₆H₅OH) in the presence of a base (alkali), it undergoes a polymerization reaction. This is a classic example of a condensation polymerization, where small molecules like water are eliminated. The initial product is a thermosetting polymer known as Bakelite. The reaction involves the formation of methylene bridges between phenol molecules.

Key reaction step: $C{H}_{2}O+2C_6{H}_{5}OH\stackrel{\text{alkali}}{\to }{C}_6{H}_{4}OH-C{H}_2-C_6{H}_{4}OH+H_{2}O$

Final Answer: bakelite