Why is sulfuric acid used to seal flask with highly reactive chemicals?
Sulphuric acid reacts with dichromate and table salt, forming volatile and harmful chromyl chloride.
\begin{multline}
\ce{K2Cr2O7(s) + 4 NaCl(s) + 6 H2SO4 -&gt;}\\
\ce{2 KHSO4(s) + 4 NaHSO4(s) + 2 CrO2Cl2(l,g) + 3 H2O}
\end{multline}
Therefore it serves as a reagent and at the same time as protection against chromyl chloride escape.

Safety considerations
$\ce{CrO2Cl2}$ hydrolyzes to release $\ce{HCl}$ and hexavalent chromium $\ce{Cr^{\mathrm{VI}}}$.
Acute: Exposure to chromyl chloride vapour irritates the respiratory system and severely irritates the eyes, and the liquid burns the skin and eyes. Ingestion would cause severe internal damage.
Chronic: $\ce{CrO2Cl2}$ can produce chromosome abnormality/aberrations and is a human carcinogen via inhalation. Frequent exposure of the skin to chromyl chloride may result in skin ulcer/ulceration.