Solder is an alloy of $\mathrm{Pb}$ and _______?

Alloy:

• An alloy is defined as a combination of two or more metals, or a metal combined with one or more elements.
• The resulting alloy has different properties than the original elements altogether, like increased strength and hardness.
• In these elements, the major component is covered by the metals only.

Solder alloy:

• Solder is also an alloy that is made up of the Tin ($\mathrm{Sn}$) and Lead($\mathrm{Pb}$).
• In Solder alloy, the quantity of Tin and Lead is 60% and 40% respectively.
• These both have a low melting point that's why used in the composition of Solder.

Therefore, Solder is an alloy of Lead ($\mathrm{Pb}$) and Tin ($\mathrm{Sn}$).