The change of density accompanying the melting can be explained in terms of the kind of dominant defects, and these, in turn, depend on the solid structure. In particular, materials characterized by almost isotropic interactions between the molecules crystallize into compact three-dimensional structures, like the face-centered cubic fcc structure. In such a case, the dominant effect of the exponential growth of defects at the melting transition is a sudden decrease of density going from the solid to the liquid.
The situation is reversed in materials like water and elements like Bismuth or Silicon, where the molecular or atomic interactions are highly anisotropic. In the case of water, the shape of the molecule, and the important role of the hydrogen bond favor an open structure of the crystal in the same way as the anisotropic interaction between Silicon atoms favors the open diamond structure.
At the melting point, the most frequent defects in such open structures induce a local and global collapse of the crystal structure, creating the conditions for a liquid phase at a higher density than the coexisting solid. In summary, the reason for the increase of density at the melting point of some materials like water can be traced back to the presence of dominant anisotropic interactions favoring open crystalline structure in the solid phase.
You may try to find similarities between water and bismuth, both expand when solidify. Most other materials contract when solidify. It mostly depends on crystalline structure. Water in form of ice happens to have crystalline structure that takes more space than liquid water.
The reason is attributed to the hydrogen bonds that determine the structure of ice. The molecules of water in ice are arranged in a cage-like fashion, with rather hollow spaces in between them, thanks to the hydrogen bonds shown as dotted lines:.
When ice melts, the energy supplied helps in breaking these hydrogen bonds, and the molecules of water come closer, hence increasing the density of water, or 'contracting it'. This process goes on till the water reaches about 4 degrees Celsius, at which water has the highest density.
After this, water expands like any other liquid. This property has an interesting, but equally important application in nature. As ice is less dense than water, it floats up to the surface of the water bodies and acts as a thermal insulator to the underwater world.
This is a boon to aquatic animals and fish during harsh winters. It also explains other properties of water like the unusually high latent heat of fusion and specific heat. The structure of ice comes from hydrogen bonds. These occur because the electrons are more strongly attracted to the oxygen atom, so this atom is slightly negative, whereas the hydrogen atoms become slightly positive. Each child will need enough water to fill both cups.
If you are going to undertake The Tip of the Iceberg activity on another day, plan to return the cups with ice to the freezer to keep them frozen. Activity Invite the children to think about what they have learned about ice and water. What state of water is ice? A solid. What has to happen to turn water into ice? It has to freeze. At what temperature does water turn into ice? Can they think of any examples in their lives when they make ice from water?
Some children may share that they have made ice cubes or popsicles in their freezers. What else do they know from their experiences about ice and water? Some children may observe that the ice "grows" or expands in the ice cube tray. Water is an exception. Ice melts to water causes contraction and water freezes to ice causes expansion.
This is why ice floats on water. Cause for constant temperature during melting: The absorbed heat energy during melting is used to weaken the attractive forces between particles and not the kinetic energy of the particles. When impurities are mixed with a pure substance, the melting point is affected. This change in the melting point has its usefulness:. Even though the examples above are metals, the change in melting point due to application of pressure occurs for non-metals as well.
An example will be ice. With the addition of pressure, the melting point of ice will be lowered. He began writing online in , offering information in scientific, cultural and practical topics.
His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts. TL;DR Too Long; Didn't Read Ice expands at a fixed rate, liquid water expands at an accelerating rate with increasing temperature and steam again expands at a fixed rate. Volume Vs. Weight of Water. What Is the Density of CO2? Ice Cubes Melting Process. How to Calculate Thermal Expansion of a Cylinder. Homemade Ice Keeper Science Project.
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