Physical phosphorous acid generally decomposes at around 190℃. Above this temperature, a disproportionation reaction occurs, primarily producing H3PO4 and PH3. The reaction equation is: 4H3PO3 (heated at 190℃) == 3H3PO4 + PH3↑. From this reaction, it can be seen that phosphorous acid should not be heated above 180℃ during the synthesis of other chemicals or during storage, otherwise, toxic and harmful PH3 gas may be produced.

When H3PO3 is in its crystalline state, due to its very high purity, its temperature resistance is much lower than that of its aqueous solution. The oxidation state of H3PO3 is +3. Upon heating, some of the +3 phosphorus is oxidized to +5, forming phosphoric acid, while the rest is reduced to -3, forming phosphine (PH3). This is a self-redox reaction. Aqueous solutions of phosphorous acid range from 1% to 70%, and generally, conditions for decomposition do not exist in aqueous solutions. The boiling point of an aqueous solution is at most 100℃, while the decomposition temperature must reach above 180℃ to occur. Therefore, decomposition cannot occur in aqueous solutions. The decomposition reaction process is roughly as follows:

1. When the temperature gradually rises above 180℃, the decomposition reaction begins to proceed significantly.

2. Solid H3PO3 melts (melting point ~73℃), forming a viscous liquid, and then decomposes within this liquid.

3. If rapidly heated to higher temperatures (e.g., >250℃), the reaction becomes very vigorous, producing a large amount of PH3 bubbles.

4. After the above reaction is complete, the main residue in the container is phosphoric acid or pyrophosphoric acid. If heated above 300℃, H3PO4 itself will further dehydrate to form glassy solids such as metaphosphoric acid.

Therefore, the final solid residue produced by the decomposition of phosphorous acid is usually a highly hygroscopic glassy substance, mainly composed of phosphoric acid or metaphosphoric acid. A certain amount of harmful PH3 gas is also produced.