Abeaker contains a mixture of sand and small pebbles. what kind of mixture is this?

Answer is: a beaker contains heterogeneous mixture.

A heterogeneous mixture have compounds that remain separate in the sample.

Heterogeneous mixture is not uniform in composition (in this mixture different sand and small pebbles), but proportions of its components (in this mixture particles of different colors and size) vary throughout the sample.

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