Canadian Welding Bureau (CWB) Level 3 Practice Exam

Substitutional solid solutions form when atoms of the solute replace atoms of the solvent in the crystal lattice, typically when they are similar in size and have the same crystal structure. Copper in silver fits this pattern well: both metals crystallize in the face-centered cubic lattice, and their atomic sizes are close enough that copper atoms can substitute for silver atoms in the lattice without causing significant distortion. This creates a solid solution with a wide range of copper–silver compositions. The other examples are not substitutional for these reasons. Oxygen in nickel is interstitial because oxygen is much smaller than nickel atoms and occupies the gaps between them rather than substituting nickel in the lattice. Hydrogen in palladium behaves similarly as an interstitial species, moving into interstitial sites. Carbon in iron is also interstitial, sitting in the spaces between iron atoms rather than replacing iron atoms in the lattice.