NORFOLK, Va. — Have you ever been to the beach on a hot summer day and wondered why the water feels so cool, but the sand feels so hot? There are a few factors that go into play and explain why certain substances heat faster than others.
There are a few differences between sand and water such as their specific heat capacities, thermal conductivity and overall interaction with sunlight. When all of these factors come into play, they can create a vast difference in temperatures.
One of the main factors is the specific heat of both substances. Specific heat is the amount of energy a substance needs to heat up by one-degree Celsius. Sand has a specific heat of about 0.84 J/g°C, while water's specific heat is around 4.18 J/g°C.
This means that sand requires less energy to increase its temperature compared to water, so when both of these substances are exposed to the same amount of energy, sand heats up more quickly and reaches higher temperatures than water.
Specific heat has a lot to do with how the molecules for each substance are arranged. For example, the molecules that make up water (H2O) have hydrogen bonding, which is an extremely strong intermolecular force between the molecules, so it takes significantly more energy to break these molecules, which results in a much higher specific heat.
Hydrogen bonding plays a crucial role in regulating the Earth's ocean temperatures. When solar radiation initially hits the water, most of the energy goes towards breaking the hydrogen bonds between water molecules before the temperature of the ocean can even start to rise.
The heat is mostly absorbed by these bonds breaking, which contributes to water’s high heat capacity, thus allowing water to absorb and release heat efficiently, helping to stabilize ocean temperatures.
Sand is mostly made up of silicon dioxide (SiO2), so the bonds between the silicon and oxygen do not create that strong of an intermolecular force like what water has, resulting in less energy to break these connections, thus having a lower specific heat.
Thermal conductivity also has a large role in how heat is distributed throughout sand and water. Thermal conductivity is much lower in sand, so it doesn't transfer heat efficiently which is why the top of the sand can become very hot but if you were to dig a bit deeper, the underlying sand is much cooler.
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Water has a much higher thermal conductivity, allowing heat to spread more evenly throughout its volume which helps to moderate the temperature of the ocean and prevents it from becoming as hot as the sand.
Albedo is another factor regarding the ability of sand and water to heat up, which is a fraction of light that is reflected by a body or surface. Sand has a relatively low albedo, meaning so it absorbs a significant portion of the sunlight that strikes it. Water has a higher albedo, so it reflects much more sunlight than the sand.