Question

Sodium reacts with cold water to form a compound that burns with a flame. Guess the colour of flame.
(A) Lilac
(B) Golden yellow
(C) Brick red
(D) White

Answer 1

Hint: When we drop sodium metal it reacts rapidly with water. The reaction of sodium with water forms a colourless solution of sodium hydroxide (NaOH) and hydrogen gas (\[{{H}_{2}}\]). The resulting solution is basic because of the dissolved hydroxide. The reaction is exothermic.

Complete step by step answer:
> So, let us first discuss sodium. We should know that sodium is a metallic element placed in the first group of periodic tables. We can easily cut sodium with a knife. We will observe that the resulting surface is a shiny surface but this soon dulls because of the action of air and moisture.
> So, let us discuss the reaction of sodium with air. If we take small piece of sodium, and try to burn it in air, it will produce white sodium peroxide,\[N{{a}_{2}}{{O}_{2}}\]together with some sodium oxide, \[N{{a}_{2}}O\], which is also white.
\[\begin{array}{*{35}{l}}
   2Na\left( s \right)\text{ }+\text{ }{{O}_{2}}\left( g \right)\text{ }\to \text{ }2N{{a}_{2}}{{O}_{2}}\left( s \right) \\
   4Na\left( s \right)\text{ }+\text{ }{{O}_{2}}\left( g \right)\text{ }\to \text{ }2N{{a}_{2}}O\left( s \right) \\
\end{array}\]
> Now, we will discuss about reaction of sodium with water:
- When we drop a piece of sodium metal it reacts vigorously with water. We will notice during the reaction that the sodium metal become so hot that it catches fire and burns with a characteristic orange colour.\[\begin{align}
  & 2Na\left( s \right)+2{{H}_{2}}O\to 2NaOH\left( aq \right)+{{H}_{2}}\left( g \right) \\
 & \begin{array}{*{35}{l}}
   2Na\left( s \right)\text{ }+\text{ }{{O}_{2}}\left( g \right)\text{ }\to \text{ }2N{{a}_{2}}{{O}_{2}}\left( s \right) \\
   4Na\left( s \right)\text{ }+\text{ }{{O}_{2}}\left( g \right)\text{ }\to \text{ }2N{{a}_{2}}O\left( s \right) \\
\end{array} \\
\end{align}\]
\[2Na\left( s \right)+2{{H}_{2}}O\to 2NaOH\left( aq \right)+{{H}_{2}}\left( g \right)\]
- By reaction of sodium with water, there will be a formation of a colourless solution of sodium hydroxide (NaOH) and hydrogen gas (\[{{H}_{2}}\]).
So, from the above discussion we can say that option B is the correct answer. As sodium metal is dropped in water and as the reaction proceeds the sodium metal in water becomes so hot that it will catch a fire. And that fire is of characteristic golden yellow colour
Note:
We should know that sodium reacts vigorously with all the halogens to form sodium halides. So, it reacts with fluorine, chlorine, bromine, and iodine, to form respectively sodium(I) fluoride NaF, sodium(I) chloride, NaCl, sodium(I) bromide, NaBr, and sodium(I) iodide, NaI