Animated Solution for Chemistry - s and p-Block Elements: Very pure hydrogen (99.9) can be made by which of the following processes?

The quest for ultra-pure hydrogen is a classic challenge in industrial chemistry. When a problem asks for hydrogen with a purity of $99.9\%$, we must carefully evaluate the byproducts of each chemical process. Let's embark on a journey to understand why some methods fall short and why one specific reaction reigns supreme.

Let's look at the options provided. The reaction of methane with steam is a well-known industrial process. However, it produces "syngas," which is a mixture of carbon monoxide ($CO$) and hydrogen ($H_2$). Separating these two gases perfectly is a monumental task, meaning the resulting hydrogen will always carry trace impurities.

Similarly, mixing natural hydrocarbons doesn't directly yield pure hydrogen gas. What about the electrolysis of water? While it is a fantastic method, standard electrolysis typically yields hydrogen that is about $99.5\%$ pure due to dissolved gases like oxygen.

This brings us to the magic of ionic hydrides. Saline hydrides, such as calcium hydride ($Ca{H}_2$) or sodium hydride ($NaH$), are essentially solid, portable forms of hydrogen. When they come into contact with water, a vigorous and highly specific reaction occurs.

Let's write down the chemical equation for calcium hydride reacting with water:

Why does this reaction guarantee such high purity? The secret lies in the physical states of the products.

In this reaction, hydrogen ($H_2$) is the only gas evolved. The other product, calcium hydroxide ($Ca(OH{)}_2$), remains behind in the aqueous solution or as a solid precipitate. Because there are no other gaseous byproducts to mix with the hydrogen, we don't need any complex separation techniques. The hydrogen bubbles up, ready to be collected at an exceptional purity level.

Therefore, the reaction of salts like hydrides with water is the definitive answer for producing very pure hydrogen instantly.

As a bonus fact for your JEE preparation, keep in mind that there is another method for obtaining ultra-pure hydrogen ($>99.95\%$). This is achieved through the electrolysis of warm aqueous barium hydroxide ($Ba(OH{)}_2$) solution using nickel electrodes. Always read the options carefully, as both methods are favorites among examiners!