Sn + HNO3: When will the metastannic acid H2SnO3 appear?
It is a common belief that tin dissolves in dilute nitric acid to produce tin(II) nitrate. The reactions are addressed in a previous answer of mine. However, employing a more hot and concentrated nitric acid yields an unstable compound called hydrated stannic oxide also known as β-stannic acid. The compound also has a α-form which is later converted to β form. The relevant information can be found here:


  [...]tin dissolves slowly, in very dilute nitric acid to produce
  stannous nitrate (but) when hot and more concentrated acid is
  employed, is very unstable and quickly decomposes, yielding the form
  of hydrated stannic oxide known as β-stannic acid. Probably
  α-stannic acid is first produced [...] which then passes into the
  β-form. The product is, therefore, liable to contain both α- and
  β-forms, and the pure β-acid is obtained by dissolving this product
  in sodium hydroxide solution, and then adding excess of concentrated
  soda, which precipitates sodium p-stannate whilst the α-salt remains
  in solution. Pure β-stannic acid, or β-metastannic acid, is then
  obtained by decomposing the sodium salt with acid. It is also formed
  by the hydrolysis of its sodium salt at 60° C., and by boiling a
  dilute solution of stannic chloride, with or without the addition of
  nitric acid.[...]
  
  [...]Gelatinous, precipitated β-stannic acid has the empirical
  composition $\ce{SnO2.4H2O}$, when air-dried $\ce{SnO2.2H2O}$, and
  when dried in a vacuum, $\ce{SnO2.H2O}$; [...]