How purchasing power reduces when currency is commodity money(say gold)
Here is a chart from The Economist in July 2010 showing the nominal and real (i.e. inflation adjusted) price of gold in US dollars back to 1970.  The nominal price later continued to rise until 2012 and then fell, and it is currently at about its 2010 level; the real price also rose and then fell and is currently below its 2010 level.  The lines would look different in other currencies



So while the purchasing power of most currencies including the US dollar tends to fall over ten-year periods due to inflation, the purchasing power of gold (shown by the real line) may rise or fall depending on precisely which ten-year period you choose.  For example it rose from 2000 to 2010 and from 1970 to 1980 but fell from 1980 to 1990 and from 1990 to 2000; indeed from 1980 to 2000 the purchasing power of gold fell faster than the purchasing power of the dollar

An additional consideration is that you can store your currency in a bank by depositing it, and you can then receive interest on it, which may offset the decline in the purchasing power. If you store gold in a bank, the bank is likely to charge you a storage fee and you will not receive interest