Ask A Scientist© Chemistry Archive

name       Brent
status     student
age        10

Question - Which will heat faster, sand soil or water? And why?
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Water [liquid] will heat faster than dry sand for two reasons:
1. Water has less capacity [heat capacity, Cp] to hold heat than dry
sand/air.
2. Water transfers heat faster than sand/air. That is, the [thermal
conductivity, Q'] of water is            larger than dry sand/air.

Here are the numbers:

SUBSTANCE               Cp(cal/gm*K)
Q'(cal/(cm^2*sec*(K/cm))

    SiO2                           45*10^-3                       32*10^-4
    air                               389*10^-3
0.57*10^-4

    H2O(liq)                     56*10^-3                        15*10^-4

1.   While the heat capacity, Cp, of water and sand are about the same, the
heat capacity of air, a gas, is about 10 times larger. The Cp of gases is
generally larger than the Cp of condensed phases because the heat energy is
used to expand the gas, as well as increasing the temperature of the gas. In
contrast, in condensed phases (solids and liquids), almost all of the heat
is used to increase the temperature of the substance, since solids and
liquids expand very little when heated. That means Cp is a smaller number.

2.   More importantly, the sand particles are "point contacts", so the
effective cross sectional area available to transfer heat from
particle-to-particle is very small and circuitous. This slows the transfer
of heat. In addition, the interstitial air phase is no help because it is a
poor conductor of heat.

Neither of these limitations are present in the case of a continuous liquid
water phase.

Vince Calder
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