Silver hardening by alloying: Soft pure silver can be hardened for service by adding a small amount of which alloying elements?

Introduction / Context:
Pure silver is soft and ductile. For jewelry, contacts, and utensils, it is commonly alloyed to raise hardness and strength without losing too much conductivity or tarnish resistance.

Given Data / Assumptions:

• Target is modest hardening with small alloy additions.
• Service includes wear and handling; thermal extremes are not primary.
• Electrical properties may remain important.

Concept / Approach:
Copper is the classic hardener for silver (e.g., sterling silver Ag–Cu ~92.5/7.5). Small nickel additions can further improve hardness and tarnish resistance in certain silver alloys and contact materials, balancing mechanical strength with acceptable conductivity.

Step-by-Step Solution:
Identify principal hardener: copper in small percentages meaningfully raises hardness.Consider nickel: often used in combination to enhance strength/wear in specialty silver alloys.Choose the combined alloying option reflecting standard practices.

Verification / Alternative check:
Standards for sterling and coin silver focus on copper; silver–copper–nickel options exist for contacts and decorative applications with enhanced strength.

Why Other Options Are Wrong:
Zinc, aluminium, tin: can be present in small amounts in some brasses/bronzes or solders but are not the typical minor additions chosen primarily to harden silver in standard practice.

Common Pitfalls:
Confusing German silver (Cu–Ni–Zn, no silver) with silver alloys; the hardening strategy for true silver is dominated by copper, sometimes with nickel.

Final Answer:
Copper and nickel