Answer
Adding boron oxide to silicon oxide glass alters the thermal expansion of the glass, which is important for glassware intended for heating.
Work Step by Step
When glass is heated, the outer edge of the glass warms and expands more quickly than the inner edge, creating stress that can result in cracking. Adding boron oxide to glass reduces its thermal expansion, allowing the glass (called borosilicate glass or Pyrex) to be heated without cracking.
