Why Does Ice in Black Bags Melt Faster?
For ice to melt it requires heat, but it will melt in broad daylight even when no heat is applied. Similarly, when ice is put in a dark bag it will melt quicker than ice in a light-colored bag. This is because of the amount of light, which is a form of energy just like heat, each bag can absorb and how well the bag passes on that heat energy.
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What is Heat?
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Heat is a specific form of energy that is stored in the motion of the atoms in an object. When an object is cold, its atoms vibrate with a lower frequency and are referred to as being in a "less excited state." As an object gains heat energy, the atoms become more excited and vibrate faster. We feel this change in energy levels as warmth.
What are Colors?
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Colors are electromagnetic waves that fall within the limits of the visible spectrum of light. This is part of the greater electromagnetic spectrum that also includes television and radio waves. The colors you see are the different wavelengths within the visible spectrum, and the color of a substance depends on which wavelengths it absorbs, so a pure blue object absorbs all but the blue wavelengths. Black absorbs all wavelengths, while white absorbs none.
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Energy Absorption
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As an object absorbs electromagnetic energy, its atoms become excited and begin to vibrate faster; warming them up. Similarly, expelling electromagnetic energy cools an object down because the atoms expelling the energy lose some of the energy they are using to vibrate. This is the principle behind the microwave oven, which emits microwave energy (electromagnetic waves with a lower frequency than light) on a frequency easily absorbed by water; causing food to heat up.
Heat Transfer
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Atoms do not exist in isolation. When one atom begins to vibrate, it has an effect on the atoms it is next to or bonded with. Some of the vibrational energy from the first atom will pass to the second atom, which begins to vibrate. This atom goes on to cause others to vibrate in turn, passing the heat energy through the substance. This is why an object will be warmer on the side closer to a heat source.
Melting Ice in a Bag
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When ice is placed in a bag, the bag must heat up before the energy can be passed to the ice. Light-colored bags are poorest at absorbing light and therefore cannot transfer heat as quickly to the ice, while black bags will heat up relatively quickly and also transfer a lot more heat energy. As a consequence, ice in a black bag gets more heat quicker than ice in a lighter bag and thus melts faster.
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References
- Photo Credit disc reflection image by Leonid Nyshko from Fotolia.com
