Please explain why molten magnesium chloride conducts electricity whereas solid magnesium chloride does not.

 

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In order for a substance to conduct electricity charge must be able to move through it. An ionic solid has a rigid structure in which ions and their electrons remain in fixed positions so charges are unable to move through the substance. This also explains the brittleness of ionic solids....

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They tend to shatter when a force is applied because of the rigid structure. Particles within the crystal lattice are unable to move in response to the force while maintaining the structure of the lattice.

An ionic solid begins to melt when heated because the increased vibration of the ions overcomes the electrostatic force holding them in the crystal lattice and they begin to move around. An ionic substance conducts electricity in the liquid or molten state because these mobile ions are able to carry charge throughout the material.

The same thing happens when ionic solids dissolve in water. The hydrated ions are free to move and the solution conducts electricity.

Metals, unlike ionic solids, conduct electricity in the solid state because their shared valence electrons are free to move around and carry charge.

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Discuss why magnesium chloride conducts electricity when it is dissolved in water but not in the solid state.

Magnesium chloride, like other ionic compounds, is a strong electrolyte which means it completely dissociates into its ions in aqeuous solution (i.e. when dissolved in water). In order for something to conduct electricity, there have to be charge carriers to move the charge from one location to another. This can happen in solutions that have ions present or in metals where the electrons are not "tied" to a single atom but are usually described as being in a "sea of electrons" shared by all atoms. While solid magnesium chloride does not have the ability to conduct electricity; molten ionic salts will conduct electricity.

Substances such as glucose or sucrose are non-electrolytes and do not break up in water.  Therefore they are not conductors of electricity.

If we need ions in water, then why do we have to worry about water and electricity?  Pure water breaks up a very small amount in water which wouldn't carry much charge but the reality is that most water is not very pure and there is a sufficient concentration of particles available in the water to carry a charge.

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