Can water oxidize zinc?
I've seen similar demonstrations with magnesium, and in both cases the explanation is the same.  The metal reacts to produce the hydroxide plus hydrogen; with zinc this would be
$\ce{Zn + 2 H2O -&gt; Zn(OH)2 + H2}$
But, the zinc hydroxide is essentially insoluble in cold water and seals up the metal before the reaction gets very far.
Boiling the water speeds things up in two ways.  First, the metal hydroxide, with either zinc or magnesium, is inherently more soluble.  Second, the turbulent action created by the boiling (the bubbles you see are actually steam, not hydrogen) acts as a stirring mechanism, speeding up the mass transfer so the slight concentration of dissolved hydroxide gets to the litmus paper faster.
Magnesium differs from zinc because the former hydroxide is somewhat more soluble.  Therefore, the basic nature of the product with magnesium can be demonstrated with phenolphthalein, which requires a slightly higher pH to react than litmus.  Some demonstrations with magnesium show a rose coloring of the magnesium metal in cold water with the phenolphthalein showing that the magnesium hydroxide remains essentially on the surface.