Downs Cell Definition in Inorganic Chemistry

In more detail

The cell uses simple chemical table salt, which is known as sodium chloride. Solid salt does not conduct electricity at all. Therefore, the salt must be heated until it melts into a flowing liquid.

Pure salt melts at a very high 801 degrees Celsius. To save money and energy, factories mix in some calcium chloride. This extra chemical safely lowers the melting point down to about 600 degrees Celsius.

Inside the cell, a large carbon block acts as the positive side, called the anode. An iron ring surrounds it to act as the negative side, called the cathode. When the power turns on, positively charged sodium ions rush to the iron ring.

They gain electrons and turn into pure liquid sodium metal. Liquid sodium is very light, so it floats to the top for collection. Meanwhile, negative chloride ions rush to the carbon block.

They lose electrons and bubble up as toxic chlorine gas. A metal screen carefully keeps the floating sodium away from the rising chlorine. If they touched, they would violently explode and turn right back into salt.

Key facts

Field

Inorganic Chemistry

Process type

Electrolysis

Electrolyte used

Molten NaCl and CaCl₂ mixture

Operating temperature

Approximately 600°C

Cathode product

Liquid sodium metal

Anode product

Chlorine gas

Example

Inside a working Downs cell, the electricity forces a complete breakdown of the melted salt. At the cathode, the reaction is Na+ plus one electron makes liquid Na. At the anode, two Cl- ions lose electrons to form one molecule of Cl₂ gas. Combining these gives the overall cell reaction where two NaCl molecules turn into two Na atoms and one Cl₂ molecule. The cell constantly pumps out pure liquid sodium from one pipe and yellow chlorine gas from another.

Frequently asked questions

Why do factories add calcium chloride to the Downs cell?

Calcium chloride drops the melting point of the salt from 801°C to 600°C. This massive drop saves the factory a huge amount of heating energy.

Why can't the Downs cell just use cheap salt water instead of melted salt?

In water, electricity would turn the water into hydrogen gas instead of making sodium metal. Any sodium that did form would instantly explode upon touching the water.

How does the cell keep the sodium and chlorine from mixing back together?

An iron wire screen sits directly between the anode and cathode. It lets electricity flow but physically blocks the rising gas and floating liquid from touching.