Question

Heavy water is obtained by: (a) Prolonged electrolysis of \(\mathrm{H}_{2} \mathrm{O}\) (b) Heating \(\mathrm{H}_{2} \mathrm{O}\) (c) Boiling water (d) Fractional distillation of \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

Heavy water is obtained by fractional distillation of water.

Step-by-step solution

Understanding Heavy Water

Heavy water, known as deuterium oxide (D_2O), is a form of water where the usual hydrogen atoms (protium) are replaced with deuterium, a heavier isotope. It occurs naturally but is much less common than ordinary water.

Analyzing the Methods

Examine the provided options to determine which methods realistically separate or produce substances based on differing weights (molecular properties), which is how heavy water is typically obtained.

Option (a): Prolonged Electrolysis of \(\mathrm{H}_{2} \mathrm{O}\)

This process involves using electricity to break down water into hydrogen and oxygen gas over time. Heavier isotopes like deuterium accumulate but are not the primary method for acquiring bulk heavy water.

Option (b): Heating \(\mathrm{H}_{2} \mathrm{O}\)

Heating normal water does not separate or concentrate isotopes; it merely changes the state of water by converting it to steam.

Option (c): Boiling water

Boiling simply involves changing the phase without differentiating between isotopes, similar to simple heating.

Option (d): Fractional Distillation of \(\mathrm{H}_{2} \mathrm{O}\)

Fractional distillation separates substances based on boiling points. Heavy water has a slightly higher boiling point than regular water, making fractional distillation an effective method for separating it.

Concluding the Correct Method

From the examined options, fractional distillation is the common industrial method used to obtain heavy water by taking advantage of its distinct boiling point.

Key concepts

Deuterium Oxide

Deuterium oxide, commonly known as heavy water, is a unique form of water where the typical hydrogen atoms in water are replaced with deuterium. Deuterium is an isotope of hydrogen that contains an additional neutron, making it heavier than the common hydrogen isotope, protium. This additional neutron is what gives deuterium oxide its characteristic higher mass and slightly different physical properties compared to normal water. For example, it has a higher boiling and melting point and a greater density.

• Heavy water is often used in nuclear reactors as a neutron moderator.
• It is found naturally in water, but in very small amounts, roughly 1 part in 20 million.
• Its extraction is generally more complex due to its rarity.

Understanding deuterium's role as a heavier isotope of hydrogen is key to comprehending why heavy water behaves differently than regular water.

Fractional Distillation

Fractional distillation is a method used to separate components in a mixture based on their differing boiling points. In the context of heavy water, this process is particularly useful because deuterium oxide (heavy water) boils at a slightly higher temperature than ordinary water. When you heat a mixture of regular and heavy water, the regular water vaporizes first, allowing for the collection and subsequent separation of heavy water from the mixture.

• This makes fractional distillation an effective industrial method for obtaining heavy water.
• It involves multiple rounds of distillation to increase purity.
• The slight temperature difference (about 1.5°C higher for heavy water) is enough for significant separation over many cycles.

As a result, fractional distillation is an essential technique in the chemical industry, particularly for substances like heavy water where even slight differences in physical properties are exploited for separation.

Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but differ in their number of neutrons. This difference in neutron count gives isotopes distinct atomic masses while retaining similar chemical behavior. In the case of water, the most common isotope is protium ( ^1H), while deuterium ( ^2H) is a heavier isotope with one neutron.

• Isotopes like deuterium alter physical properties such as melting and boiling points.
• In chemical reactions, isotopes typically behave similarly due to identical electronic structures.
• Despite their common chemistry, physical separations can be achieved due to weights.

Isotopic differences are subtle, yet crucially important in fields like nuclear science, medicine, and environmental science, as well as in the production of heavy water.

Electrolysis of Water

Electrolysis of water is a chemical process that uses electricity to break down water into hydrogen and oxygen gases. By applying an electrical current through water, the molecule splits into its constituent elements. In the case of deuterium, heavier water molecules (containing deuterium) can result from prolonged electrolysis because lighter molecules (like protium) escape or react first due to their lower mass.

• This is not the primary method for acquiring heavy water but helps increase concentration.
• Electrolysis is more commonly used in industries to produce hydrogen fuel.
• The setup requires electrodes, a power source, and sometimes an electrolyte to enhance conductivity.

While not as practical for obtaining large quantities of heavy water due to its inefficiency, electrolysis remains an essential industrial process for generating pure hydrogen gas and can indirectly aid in isolating deuterium-enriched water.