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Electrical Conductivity

There are various kinds of conductors such as metals, ionic solutions etc. Metals have tendency to lose their valence electrons therefore they convert into metal ions with positive charge. Copper, silver and gold are considered as the best metallic conductors. Some of the non-metals like graphite are also good conductors of electricity due to presence of free electrons.

If a material or object shows conductivity due to the presence of ions, it is called as ionic conductivity. Solutions or molten conductors are ionic conductors. For example, a solution of table salt is a good conductor of electricity due to presence of ions in solution. When we dissolve table salt (sodium chloride) in water, it releases sodium ions and chloride ions. These ions can flow easily that makes the solution conductor.

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What is Conductivity?

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"So the conductivity of conductors can be defined as their ability to conduct heat or electricity. It represents as k or s."

Water also contains dissolved ions and might conduct electricity. Some of the materials like Germanium, Silicon are good conductors at certain temperature. They are called as semi-conductors. They have free electrons but less than metallic conductors. Therefore they have low conductivities.  

Conductivity of Solutions

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Ionic compounds are composed of oppositely charged ions. These ions are bonded together with electrostatic force of attraction between them. 

The presence of ions makes them good conductor of electricity but only in molten or in solution. In solid state, ions are bonded with strong force of attraction between them therefore they are unable to move in any direction. In molten or solution, ions are free to move and show conductivity.

Do Bases Conduct Electricity?

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Bases are chemical substances that can give hydroxyl ions in solutions. For example sodium hydroxide releases hydroxyl ions with sodium ions.

NaOH  $\rightarrow$ Na+ + OH-

The presence of ions makes such alkaline solutions a good conductor of electricity. Bases are ionic conductor in their solutions but bad conductors in solid states due to lack of ions. The strength of bases determines the conductivity of alkaline solutions. 

A strong base will ionize quickly and releases a large of number of ions in solution. Therefore they are good conductors of electricity. On the contrary, weak bases cannot ionized completely and show low value of conductivity.  

Do Acids Conduct Electricity?

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Like bases, acids are also good conductors of electricity in their solutions. For example, the solution of hydrochloride in water results the formation of hydronium ion and chloride ions. 

The presence of ions makes such solutions a good conductor of electricity.

HCl + H2O  $\rightarrow$ H3O+ + Cl-

NaCl Conductivity

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Common table salt or sodium chloride is an electrolyte due to presence of oppositely charged ions. When we dissolve sodium chloride in water, it dissociates into sodium ions and chloride ions. These ions can flow in solution and makes the solution conductor of electricity. 

Sodium chloride cannot conduct electricity in solid state because sodium ions and chloride ions are bonded together with strong electrostatic force of attraction. As it dissolve in water to form solution, these ions become free and can move in solution.

Salinity Measurement

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We know that conductivity indicates the tendency of a solution or material to flow the electricity. For example, in aqueous solution of sodium chloride, ions pass the electricity from one to another. Or we can say that as the number of ions increases in solution, the conductivity of solution increases. Conductivity of a salt solution can be measured in terms of salinity that is density of salt in salt water. 

As the density of salt increases, salinity increases, conductivity increases.  Some of the salinity and conductivity measurements for NaCl are listed below.

 NaCl density 
 (W / V) %

 Conductivity 
 (mS / cm)

 0.1  2.0
 0.2  3.9
 0.3  5.7
 0.4  7.5
 0.5  9.2
 0.6  10.9
 0.7  12.6
 0.8  14.3
 0.9  16.0
 1.0  17.6

Ionic Conductivity Measurement

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We know that flow of electrical current occurs due to flow of electrically charged particles. The charged particles flow because of forces which act on charged particles from an applied electric field. In solid materials, flow of electric current indicates the flow of electrons.

Therefore it is known as electronic conduction. Electronic conduction exists in metallic conductors, semiconductors and many insulated materials. On the contrary, electrical conductivity depends on the number of electrons in an object. Metals are very good conductors due to large number of free electrons. 

These free electrons can be excited in empty and available energy state. Unlike metals, water and ionic materials or fluids are conductors due to net motion of charged ions that is called as ionic conduction. The mathematical expression for electrical conductivity can be shown as;

S = $\frac{J}{e}$ = $\frac{1}{r}$

Here;
•    J = Current density
•    e = Electric field intensity
•    r = Resistivity
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