The activity series is an arrangement of metals in order of decreasing reactivity. Metals that are higher up in the series displace metals that are lower in the series from dilute solutions. Hence, when the ion of a metal that is lower in the series reacts with a metal that is higher up in the series, the latter is oxidized.
Dilute acids dissolve metals above hydrogen in the activity series such as as zinc and lead. Platinum is much lower than sodium in the activity series hence platinum does not react with sodium nitrate.
