NEET Practice Questions (MCQs) with Answers & Solutions

In the borax bead test, borax ( ext{Na}_2 ext{B}_4 ext{O}_7 ullet 10 ext{H}_2 ext{O}) is heated in a flame to form a glassy bead of sodium metaborate ( ext{NaBO}_2). This bead can react with certain metal oxides to form characteristic colors. Hence, the compound formed is Meta borate.

Lead (Pb) does not give the borax bead test. The borax bead test is typically used for detecting transition metals which form colored beads due to the formation of metal borates. Lead is not a transition metal and does not form the colored beads characteristic of this test.

$CuF_2$ (Copper(II) fluoride) is a colored compound. Copper compounds often display colors due to the d-d electronic transitions in the copper ions. In contrast, compounds like CuI, NaCl, and $MgCl_2$ are typically white or colorless.

When concentrated $H_2SO_4$ is added to dry $KNO_3$, brown fumes of nitrogen dioxide ($NO_2$) are evolved. The reaction can be represented as: $$2KNO_3 + H_2SO_4 \rightarrow 2HNO_3 + K_2SO_4$$ $$2HNO_3 \rightarrow 2NO_2 + H_2O + O_2$$ The $NO_2$ gas is brown in color.

Barium salts give a green-colored flame when tested by Pt wire. This flame test is a qualitative analysis method used to detect the presence of barium ions. The characteristic green flame is due to the excitation of electrons in barium ions.

When sodium sulphite (Na2SO3) is heated with dilute hydrochloric acid (HCl), sulfur dioxide (SO2) gas is liberated. This gas turns acidified potassium dichromate paper green due to the reduction of orange potassium dichromate (K2Cr2O7) to green chromium sulfate (Cr2(SO4)3) in acidic conditions.