How would I be able to get the identity of a substance (for example, aluminium)?
What do I have to measure? Mass, volume and what else? I HAVE to use the q = m * c * Delta T method.
Any suggestions?
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$\begingroup$ Can you please elaborate on the question a little more? Do you want to identify the substance using laboratory methods, or by thermodynamic properties, or something else? $\endgroup$– Todd MinehardtCommented Sep 23, 2015 at 1:44
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$\begingroup$ laboratory methods only $\endgroup$– Imagine DragonsCommented Sep 23, 2015 at 1:56
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$\begingroup$ @ToddMinehardt ^^^ $\endgroup$– Imagine DragonsCommented Sep 23, 2015 at 1:58
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1 Answer
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Identifying an unknown substance usually involves measuring an intensive property - some property that depends on the identity and composition of the sample and not on its size - and then comparing it with known values.
Density is one of these properties. The density of a substance depends only on the identity of the substance (and the temperature).
Another such property is the specific heat capacity, the amount of heat required to raise 1 gram of the substance 1 degree Celsius. For example, the specific heat capacity for water is $4.184\ \mathrm{\frac{J}{g\cdot ^\circ C}}$. The heat capacity world is a little more complicated than this (for example, there are different heat capacities for constant pressure conditions than there are for constant volume conditions), but that is not something to we need to worry about it.
If we know the heat capacity $c$ and the mass $m$ of an object, we can determine the amount of heat transferred $q$ as a result of a temperature change $\Delta T$.
$$q=mc\Delta T$$
However, if we do not know the heat capacity, we can rearrange this equation to solve for it.
$$c = \dfrac{q}{m\Delta T}$$
Thus, if we can measure $q$, $m$, and $\Delta T$, we can determine $c$. Measuring $q$ is the trickiest part. In lab we usually use a calorimeter. The type of calorimeter used in introductory courses is often a coffee cup calorimeter made from two nested polystyrene foam cups, which insulate the system from the outside. For a quick approximation, you can fill the cups with a know mass of water at a certain cold temperature $T_C$ and plunge a known mass of your unknown sample at a certain hot temperature $T_H$ into the water. The water and the sample reach thermal equilibrium at some final temperature $T_F$. You can determine the heat absorbed by the water using its temperature change, mass and specific heat capacity. Since the heat gained by the water was lost by the sample, you can then calculate the heat capacity of the sample.
answered Sep 23, 2015 at 11:49