Most solids objects turn black after they are burnt. But some turn white. Why is that ?
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The objects which turn black are usually carbon based/organic compounds. When burned/combusted they follow one of the following unbalanced reactions: \begin{align} \ce C_x(\ce H_y\ce O_z)_\text{(solid/liquid)} + {\ce{O2}}_\text{(gas)} &\ce{->} \ce C_\text{(solid)} + \ce{H_2O}_\text{(liquid)} + {\ce{CO_2}}_\text{(gas)}\\ \ce C_x(\ce H_y\ce O_z)_\text{(solid/liquid)} + {\ce{O2}}_\text{(gas)} &\ce{->} \ce{H2O}_\text{(liquid)} + {\ce{CO2}}_\text{(gas)}\\ \ce C_x(\ce H_y\ce O_z)_\text{(solid/liquid)} + {\ce{O2}}_\text{(gas)} &\ce{->} \ce {H2O}_\text{(liquid)} + {\ce{CO2}}_\text{(gas)} + \ce{CO}_\text{(gas)} \end{align}
You'll notice that some of the reactions will produce solid carbon, which is the black thing you see.
On the other hand, inorganic compounds begin to decompose into their ions and/or release their "trapped" water molecules. As you'll notice, most ionic compounds are white solid salts.
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$\begingroup$ Thanks for the info. How do you know (theoretically) how to do the states of the matter in the 'computation' ? $\endgroup$– mickOct 23, 2012 at 13:07
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1$\begingroup$ @mick Usually you need to check for intermolecular bonds, molecule size, etc. $\endgroup$– FiireOct 23, 2012 at 22:22
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1$\begingroup$ And if you pay close attention, you'll notice that some materials turn black when partially burned and then white (well, gray) when fully used up (e.g. wood to charcoal to ash). $\endgroup$– JABFeb 1, 2016 at 19:15