Question

Name two metals which react violently with cold water. Write any three observations you would make when such a metal is dropped into water. How would you identify the gas evolved if any during the reaction?

Answer 1

Hint: Metals when reacted with cold water produces metal hydroxide. If the metal is reacted with steam, like magnesium, metal oxide is produced as a result of metal hydroxides splitting upon heating. There are some metals which do not react with water at all such as gold, lead, silver and copper.

Complete step by step solution:
Potassium and sodium are metals which react violently with cold water. Potassium and sodium react with cold water because being an alkali metal it reacts vigorously and produces hydrogen gas. The violence and the speed of reaction both increases down the group. When sodium is added to water, the ball of sodium is formed on melting around the surface. It fizzes rapidly and the hydrogen gets burned before sodium disappears with the orange flame. The observations made when sodium is reacted with cold water are that the reaction is explosive and it burns with a yellow flame. The reaction is also vigorous and exothermic in nature. The reaction for sodium with cold water is-
\[2Na+2{{H}_{2}}O\to 2NaOH+{{H}_{2}}\].
When potassium is added to cold water the metal starts melting and floating. The movement of the metal is very quick on the surface of water. The ignition of hydrogen is instant. The metal is set on fire with a lilac flame and a spark. Sometimes a small explosion occurs at the end of reaction. The observation is that it forms a colourless basic solution with hydrogen gas. The reaction is exothermic in nature. The reaction is so explosive that the hydrogen which is evolving catches the fire immediately. The reaction for potassium with water is \[2K+2{{H}_{2}}O\to 2KOH+{{H}_{2}}\]
The hydrogen gas is evolved while reacting potassium and sodium respectively with water. The gas evolved can be identified by bringing the burning candle towards it. If the candle burns with a pop sound then the gas evolved is identified as hydrogen gas.

Note: Potassium is a soft metal with a silver luster and has a low melting point. It is a good conductor of electricity and heat. It imparts lavender colour flame and green colour water vapor. Sodium is silver white color metal with malleable and ductile property. It is the metal that can easily be cut with a knife. It is a good conductor of heat and electricity.