An unknown substance is heated from 10 °c to 25 °c. the table below shows the state of the substance at different temperatures.

time (minutes) temperature (degrees celsius) state of matter
0 10 liquid
2 15 liquid
4 20
6 20
8 20
10 23 gas
12 25 gas

which statement is correct about the unknown substance?
its boiling point is 20 °c.
its boiling point is 15 °c.
the substance sublimes at 15 °c.
the substance exists as a solid at 20 °c.

The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

At the boiling point all the energy that is supplied to the system is used to change the state of the liquid substance to gas, so the temperature remains constant, since this heat is not used to increase the temperature of the substance.

As the temperature remains constant at 20 ºC for 4 minutes and then the substance changes from liquid to gas then the boiling point of the substance corresponds to 20 ºC. So, the correct statement about the unknown substance is that its boiling point is 20ºC.

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