Alloys have variable compositions

Steps:

• Define alloy as a homogeneous mixture of two or more metals or a metal and non-metal.
• Evaluate option A: Alloys like steel conduct electricity and heat well.
• Evaluate option B: Alloys are typically harder than pure metals due to lattice distortion.
• Evaluate option C: Brass is specifically copper and zinc, not the listed elements.
• Confirm option D: Alloy compositions can be adjusted for desired properties.

Why D is correct:

• Alloys are mixtures, not fixed compounds, so metal percentages can vary widely while maintaining homogeneity, as per the definition of solid solutions.

Why the others are wrong:

• A: Alloys are good conductors of electricity and heat, similar to pure metals.
• B: Alloys are harder and stronger because solute atoms disrupt easy dislocation movement in the crystal lattice.
• C: Brass is an alloy of copper and zinc only, without chromium, nickel, or carbon.

Final answer: D