Which of the statements given below is incorrect?

1. $\mathrm{C l_{2} O_{7  }}$is an anhydride of perchloric acid
2. $\mathrm{O_{3}}$ molecule is bent
3. ONF is isoelectronic with $\mathrm{N O_{2}^{-}}$ 
4. $\mathrm{O F_{2}}$ is an oxide of fluorine

The variation of the boiling point of the hydrogen halides is in the order HF > HI > HBr > HCl. 
The higher boiling point of hydrogen fluoride is explained as:

The strong reducing behaviour of H₃PO₂ is due to:

1. Presence of one -OH group and two P-H bonds.

2. High electron gain enthalpy of phosphorus.

3. High oxidation state of phosphorus.

4. Presence of two -OH groups and one P-H bond.

The acidity of diprotic acids in aqueous solution increases in the order:

1. H₂S < H₂Se < H₂Te

2. H₂Se < H₂S < H₂Te

3. H₂Te < H₂S < H₂Se

4. H₂Se < H₂Te < H₂S

Which of the following compounds does not release oxygen when heated?

1. Zn(ClO₃)₂
2. K₂Cr₂O₇
3. (NH₄)₂Cr₂O₇
4. KClO₃

The incorrect statement regarding oxoacids of phosphorus is:

Sulphur trioxide can be obtained by which of the following reaction:

1. $CaS{O}_4+C\stackrel{∆}{\to }$

2. $F{e}_2(S{O}_4{)}_3\stackrel{∆}{\to }$

3. $S+H_{2}S{O}_4\stackrel{∆}{\to }$

4. $H_{2}S{O}_4+PC{l}_5\stackrel{∆}{\to }$

The given sequence among the following arrangements that is not strictly according to the property indicated against it.  

Which of the following compounds has the lowest melting point?

1.  ${\mathrm{CaBr}}_2$

2.  ${\mathrm{Cal}}_2$

3.  ${\mathrm{CaF}}_2$

4.  ${\mathrm{CaCl}}_2$