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H2TeO3 has been synthesized as a solid but it is not well characterized. If tellurium dioxide, TeO2, is added to water, tellurous acid does not easily form, but it can be made other ways. Its CAS number is 10049-23-7.
Tellurous acid can be formed from the chlorides. Two chlorides are known, the dichloride, TeCl2, and the tetrachloride, TeCl4. They are both obtained by passing chlorine over tellurium, the product being separated by distillation (the tetrachloride is the less volatile). The dichloride is an amorphous, readily fusible, almost black solid. It is decomposed by water with formation of tellurium and tellurous acid:
2TeCl2+ 3H2O Te+H2TeO3+ 4HCl
However, pure tellurous acid, H2TeO3, can be obtained when the tetrachloride is decomposed by water, or on dissolving tellurium in nitric acid and pouring the solution into water. It is a colorless solid and behaves as a dibasic acid. The alkali tellurites are soluble in water. They also gives rise to super acid salts, such as KHTeO3·H2TeO3.

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White, amorphous substance of varying composition; H2O content is frequently less than that corresponding to the formula HsTeO3; loses water when dry but can be stored under H2O for several days without change. On heating above room temperature, extensively loses water with conversion to TeO3. Very slightly soluble in water.

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As for tellurium.

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