When a zinc plate is put into copper(II) sulfate solution, reddish brown copper appears on the surface of the zinc, and the blue colour of the solution gradually fades.
\mathrm{Zn} + \mathrm{Cu^{2+}} \longrightarrow \mathrm{Zn^{2+}} + \mathrm{Cu}
This is because zinc loses electrons to become \mathrm{Zn}^{2+}, and \mathrm{Cu}^{2+} gains those electrons to become copper.
On the other hand, nothing happens when a copper plate is put into zinc sulfate solution. This is because zinc forms ions more readily than copper.
Listing metals in order of how readily they form ions, starting with the most readily, gives the electrochemical series.
Of the metals that form ions most readily, lithium, potassium, calcium and sodium react with water at room temperature, magnesium reacts with hot water, and aluminium, zinc and iron react with steam at high temperature, giving off hydrogen.
Copper, mercury and silver dissolve only in acids that are strong oxidizing agents, such as nitric acid and hot concentrated sulfuric acid, and platinum and gold dissolve only in aqua regia.
Metals that come before hydrogen in the electrochemical series dissolve in hydrochloric acid or dilute sulfuric acid, giving off hydrogen. The earlier a metal comes in the series, the more vigorously it reacts: the bubbling becomes gentler from magnesium to zinc to iron.
Copper, which comes after hydrogen, does not dissolve in hydrochloric acid or dilute sulfuric acid.
When two metals that differ in how readily they form ions are placed in electrolytes and connected by a wire, they make a cell.
In the Daniell cell, zinc in zinc sulfate solution is the negative electrode and copper in copper(II) sulfate solution is the positive electrode, and electrons flow through the wire from the zinc to the copper. The two solutions are separated by a porous partition of unglazed porcelain, which lets ions through but keeps the solutions from mixing easily.
The voltage of the Daniell cell is about 1.1 V.
Gold, silver and copper, which form ions reluctantly, are found in nature as metals or can be extracted easily from their ores, so they have been used for thousands of years.
Iron has to be extracted by reducing iron ore with charcoal, and it came into wide use only about 3000 years ago.
Aluminium, which forms ions very readily, can be extracted only by electrolysis, and it has been made in large amounts only since 1886, about 140 years ago.