Which of these can be used to distinguish a mixture from a compound?

Differences between a mixture and a compound are;
A mixture can be separated by physical means such as decantantion and filtration whereas a compound can only be separated by chemical means like smelting
The proportion of elements in a mixture varies while the proportion of elements in a compound is constant
Elements retain their characteristics in a mixture while in a compound, elements never retain their characteristics

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na