Question

Which of the following gives white precipitate with a solution of $AgN{O_3}$ , a white precipitate with dilute ${H_2}S{O_4}$ and a green flame test?
A) Copper chloride
B) Copper nitrate
C) Lead nitrate
D) Barium chloride

Answer 1

Chemical reactions are of four types. One of them is double displacement reaction, in which the reactant ions exchange places to form a new product. Usually a double displacement reaction results in precipitate formation.

Complete answer:
In general, chemical reactions are of four types. Combination reaction, decomposition reaction,, displacement reaction, and double displacement reaction sometimes also known as precipitation reaction.

i) Combination reaction in which two or more reactants combines to form a single product-
$S + {O_2} \to S{O_2}$
ii) Decomposition reaction: a single reactant is decomposed or broke down into two or more products-
$CaC{O_3} \to CaO + C{O_2}$
iii) Displacement reaction: a single free element replaces one element from the other compound-
$Zn + 2HCl \to ZnC{l_2} + {H_2}$
iv) Double displacement reaction: for an ionic compound, cation in the first compound combines with anion of another compound, and anion of the first compound then combines with cation of the second. We can say exchange of cation and anion is occurring-
$CaC{O_3} + 2HCl \to CaC{l_2} + {H_2}C{O_3}$

Here in the above question, one compound when reacts with$AgN{O_3}$, it will give a white ppt.
$BaC{l_2} + AgN{O_3} \to AgCl \downarrow + BaN{O_3}$
Here in this double displacement reaction silver displaces barium form its place and silver chloride form as a product, which then precipitate. The color of silver chloride is white. Hence white precipitate will appear.

When the same compound reaction with dilute ${H_2}S{O_4}$ . The reaction will be
$BaC{l_2} + {H_2}S{O_4} \to BaS{O_4} + 2HCl$
Here also a double displacement reaction took place and a white precipitate of barium sulphate appeared. Also the flame color of Barium chloride is green.

Therefore the correct answer is option D.

Note: Flame test is an analytical procedure used to detect the presence of metal ions based on their characteristic emission spectrum. The color of flames in general depends on the temperature. It is a quick and relatively simple procedure for the detection of metal ions in qualitative salt analysis.