Question

The elements whose oxides can turn litmus solution blue are?
Options-
A.Carbon and Sulphur
B.Sodium and Carbon
C.Potassium and Magnesium
D.Magnesium and Sulphur

Answer 1

When different elements react with oxygen to form a new compound which contains at least one oxygen atom are known as oxides. Litmus paper is used to identify acid and base. Colour of litmus paper turns red when exposed to acidic solution while red litmus turns blue with basic solution.

Complete answer:
Carbon reacts with oxygen or air to form its oxide. Nature of oxides varies when we move down the group, for example monoxide of carbon is neutral but has slight acidic properties while dioxide of carbon is highly acidic in nature and forms carbonic acid when treated with water.
$C{O_{2\left( {aq} \right)}} + {H_2}O \to {H_2}C{O_{3\left( {aq} \right)}}$
Due to the acidic nature of carbon oxides it turns blue litmus to red.
Sulphur forms two types of oxides, monoxide of sulphur is weakly acidic because it generates weak acid sulfurous acid while trioxide of sulphur generates sulphuric acid which is highly acidic in nature.
$S{O_{3\left( {aq} \right)}} + {H_2}O \to {H_2}S{O_{4\left( {aq} \right)}}$
Due to the acidic nature of sulphur oxides it turns blue litmus to red.
Alkali metals like Potassium, rubidium have a tendency to exist as oxide, peroxide as well as superoxide. Oxides of potassium are basic in nature because when potassium oxide is dissolved in water it forms potassium hydroxide.
${K_2}{O_{\left( {aq} \right)}} + {H_2}O \to 2KO{H_{\left( {aq} \right)}}$
Due to basic in nature oxide of potassium turns red litmus to blue.
Alkaline earth metal like Magnesium shows reactivity towards oxygen to form oxides with general formula $\left( {MO} \right)$ where $M$ is alkaline earth metal and $O$ is oxygen. All the oxides of alkaline earth metal are basic in nature because when they are treated with water they form hydroxide of metals.
$Mg{O_{aq}} + {H_2}O \to Mg{\left( {OH} \right)_{2\left( {aq} \right)}}$
Due to basic in nature magnesium oxide turns red litmus to blue.
$\Rightarrow$ Potassium and Magnesium oxides turn litmus solution blue.

Therefore, option $\left( C \right)$ is the correct option.

Note:
Oxidation number of oxygen atoms vary in case of superoxide and peroxides. In general oxygen in superoxide has $\left( { - \dfrac{1}{2}} \right)$ oxidation number and $\left( { - 1} \right)$ in case of peroxide. All the oxides of alkaline earth metal are basic in nature except beryllium oxide which can react with both acid as well as base and hence amphoteric in nature.