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Melting

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The fiery red-orange “river” flowing downhill in this photo isn’t water. It’s molten (melted) rock, and it’s erupting from a volcano. Most rocks on Earth’s surface are solid objects. Why is the rock that erupts from a volcano in a liquid state? How does rock change from a solid to a liquid?

From Solid to Liquid

The process in which rocks or other solids change to liquids is called melting. Melting occurs when particles of a solid absorb enough energy to partly overcome the force of attraction holding them together. This allows them to move out of their fixed positions and slip over one another. Melting, like other changes of state, is a physical change in matter, so it doesn’t change the chemical makeup or chemical properties of matter.

Q: The molten rock that erupts from a volcano comes from deep underground. How is this related to its liquid state?

A: It is always very hot deep underground where molten rock originates. The high temperatures give rock enough energy to melt and remain in a molten state. Underground rock in this state is called magma.

Q: What happens to magma after it erupts and starts flowing over the surface of the ground?

A: After magma erupts, it is called lava. On the surface, lava eventually cools and hardens to form solid rock.

Other substances that are normally solids on Earth can also be heated until they melt. You can see an example in the Figure below . The photo shows molten gold being poured into a mold. When the gold cools, it will harden into a solid gold bar that has the same shape as the mold.

Melting Point

The temperature at which a substance melts is called its melting point. Melting point is a physical property of matter. The gold pictured above, for example, has a melting point of 1064°C. This is a high melting point, and most other metals also have high melting points. The melting point of ice, in comparison, is much lower at 0°C. Many substances have even lower melting points. For example, the melting point of oxygen is -222°C.

Melting and Global Climate Change

Because of global climate change, temperatures all over Earth are rising. However, the melting points of Earth’s substances, including ice, are constant. The result? Glaciers are melting at an alarming rate. Melting glaciers cause rising sea levels and the risk of dangerous river flooding on land. You can learn more about these effects of melting at this URL: http://video.nationalgeographic.com/video/player/environment/global-warming-environment/glacier-melt.html .

Summary

  • Melting occurs when particles of a solid absorb enough energy to partly overcome the force of attraction holding them together. This allows them to move out of their fixed positions and slip over one another, forming a liquid.
  • The temperature at which a substance melts is called its melting point. The melting point of ice is 0°C.

Vocabulary

  • melting : Process in which a solid changes to a liquid.

Practice

An alloy is a mixture of a metal with one or more other substances. The following Table below lists melting points of two metals and three alloys that they form together. Examine the table and then answer the questions below.

Metal or Alloy Melting Point (°C)
Iridium metal (pure) 2454
Platinum metal (pure) 1773
Platinum-iridium alloy (15% iridium) 1821
Platinum-iridium alloy (10% iridium) 1788
Platinum-iridium alloy (5% iridium) 1779
  1. Based on the information in the table, what conclusion might you draw about the melting points of alloys relative to the melting points of the metals they contain?
  2. Bronze is another alloy. It is a mixture that contains mainly copper with some added tin. The melting point of copper is 1084°C, and the melting point of tin is 232°C. What might be a reasonable prediction for the melting point of bronze?

Review

  1. Define melting.
  2. What happens to particles of matter when it changes from a solid to a liquid?
  3. What is the melting point of a substance? What is the melting point of ice?

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