The grey deposit you're describing is likely platinum metal, formed by a displacement reaction between platinum chloride (PtCl₄) and zinc (Zn).

Here's how it works:

* Platinum chloride (PtCl₄) is a soluble compound, meaning it dissolves in water to form platinum ions (Pt⁴⁺) and chloride ions (Cl^-).

* Zinc (Zn) is a more reactive metal than platinum. This means it has a stronger tendency to lose electrons and form positive ions.

* When you place a zinc strip in a solution of platinum chloride, the zinc atoms will react with the platinum ions. The zinc atoms will donate electrons to the platinum ions, reducing them to platinum metal (Pt).

* The zinc atoms become zinc ions (Zn²⁺), which dissolve in the solution.

The overall reaction can be represented as:

PtCl₄ (aq) + 2Zn (s) → Pt (s) + 2ZnCl₂ (aq)

The grey deposit is the solid platinum metal that precipitates out of the solution.

This process is a common method for extracting platinum from its compounds and is a classic example of a redox reaction, where one element is oxidized (zinc) while another is reduced (platinum).