Question

A substance which gives a brick red flame and breaks down on heating giving oxygen and a brown gas is:
(A) Calcium carbonate
(B) Magnesium carbonate
(C) Magnesium nitrate
(D) Calcium nitrate

Answer 1

Hint: Mostly chemists have to identify the composition of the unknown salts. There are various experiments that can be performed to identify the cation and anion present in the salt and this is known as salt analysis. Calcium when heated gives a colored flame whereas magnesium doesn't. Nitrate on heating gives out colored fumes or gas whereas carbonate on heating does not give any gas rather gives a precipitate.
Complete answer step by step:
The basic tests that are performed on a salt are:
1. The appearance of the salt: How does the salt look like?
2. Heating effect: Does the salt give out any gas, smell or colored fumes?
3. Flame test: What is the color of the flame when you heat the salt?
4. Solubility in water: Is the salt water-soluble or not?
5. The reaction of salt with various compounds.

Now, if we look at the question, it is given that substance gives a brick red flame. Now looking at the options, we know that magnesium has a colorless flame, therefore it has to be a calcium compound. And this eliminates the options (b) and (c). Now, it is also given in the question that the substance breaks down on heating giving oxygen and a brown gas. This brown gas shows that it is an oxide of nitrogen that is released and not carbonate. Therefore, this inference eliminates the option (a). Hence, option (d) is the correct answer. The reaction involved in this process is given below:
$2{ Ca }{ (N{ O }_{ 3 }) }_{ 2 } \longrightarrow 2CaO + 4N{ O }_{ 2 } + { O }_{ 2 }$

Note: Some cations do not form salts with certain anions.

• ${Ba}^{2+}, {Sr}^{2+}, {Pb}^{2+}, and {Ca}^{2+}$ do not form salts with sulphate anion $(S{O}_{4}^{2-})$.
• Phosphate anion $(P{O}_{4}^{2-})$ only form salts with cations of group 0, group 1 and group 2.