In India roads are made up of a black liquid which is prepared by melting a black material(probably coal tar). After cooling the liquid becomes very rigid. Can this solid be converted back to its previous liquid state by heating?

P.S: This is a question in my book. I think that after getting solidified the chemical composition of Tar may change so it can't be converted back to its original state.

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    $\begingroup$ Chip off a little piece and try heating it. Chemistry is an experimental science. I think it would melt just fine. $\endgroup$ – Ivan Neretin Sep 29 '15 at 7:53
  • $\begingroup$ @IvanNeretin I don't have proper apparatus. Heating Tar on a cooking stove would be very risky, I guess. $\endgroup$ – user31782 Sep 29 '15 at 8:32
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    $\begingroup$ On extremely hot days I have observed this tar melt slightly. Whether it changes to the original material, I do not know. $\endgroup$ – Shailesh Sep 29 '15 at 9:02
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    $\begingroup$ If you've even seen a roof being tarred the tar is solid which is melted and then applied to the roof. $\endgroup$ – MaxW Mar 26 '16 at 19:00

Tar is mainly made up of a collection of very heavy hydrocarbons, with rather high melting temperatures. Commonly, it is mixed with some stones or gravel to make a harder surface for roads. It is possible to heat this up to soften or melt this; you can try it yourself by heating it in a metal can over a stove (don't use a container that you plan to keep clean) or in a wood or (char)coal fire.

EDIT: As MaxW pointed out, it would be a bad idea to do this inside the house because it would create a strong smell, similar to that created by whatever machine they used to make the road.

  • $\begingroup$ I wouldn't heat it indoors because it will stink up the house. $\endgroup$ – MaxW Mar 26 '16 at 18:58
  • $\begingroup$ So it means melting solid tar doesn't change it's chemical composition? $\endgroup$ – user31782 Mar 27 '16 at 6:26
  • $\begingroup$ @user31782 It depends on what other impurities are present in the tar. Pure tar will not change, but road tar might. I do not know what impurities are in it. Of course, you could always try melting some (outdoors) and letting it cool, and observe any differences. $\endgroup$ – sadljkfhalskdjfh Mar 27 '16 at 10:37

The process is reversible, and can actually happen to some extent in hot weather, see here or here. It can definitely soften on a hot day, and it melts at $150\rm~^\circ C$.


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