Question at position 20 Analysis of an unknown substance showed that it has a high boiling point and is brittle. It does not conduct electricity as a solid but conducts electricity when melted or dissolved in water. Which of the following substances would have those characteristics? KBr HCl I2 SiF4 Cu单项选择题
A
KBr
B
HCl
C
I2
D
SiF4
E
Cu

Question

Question at position 20 Analysis of an unknown substance showed that it has a high boiling point and is brittle. It does not conduct electricity as a solid but conducts electricity when melted or dissolved in water. Which of the following substances would have those characteristics? KBr HCl I2 SiF4 Cu单项选择题

A

KBr

B

HCl

C

I2

D

SiF4

E

Cu

BlackTom AI · Accepted Answer

Consider the properties described: a high boiling point, brittle solid, and lack of electrical conductivity as a solid but conductivity when melted or dissolved.

Option 1: KBr. This is an ionic compound composed of K+ and Br- ions. Ionic solids are typically hard and brittle and have high boiling points due to strong electrostatic attractions in the lattice. They do not conduct electricity as solids because ions are fixed in place, but they do conduct when melted (molten) or dissolved in water because the ions are free to move. This matches the given clues precisely.

Option 2: HCl. Hydrogen chloride is a diatomic molecule in the gas phase and as a solid would be a molecular solid, not an ionic one. While HCl can conduct electricity in aqueous solution, many molecular solids do not exhibit a high boiling point and brittleness like ionic solids, and the statement that it forms a solid with high boiling point is inconsistent with HCl’s typical phase behavior at common temperatures.

Option 3: I2. Iodine is a covalent molecular solid characterized by relatively low melting/boiling points compared with ionic salts and does not form a structure where ions are free to move. It does not conduct electricity in solid form and only under unusual conditions might conduct poorly when molten, so it does not fit the “conducts when melted or dissolved” criterion as a typical behavior.

Option 4: SiF4. Silicon tetrafluoride is a covalent molecular compound (though with somewhat polar bonds). It does not form a classic ionic lattice, so its solid would not be expected to be highly brittle in the same way as ionic solids, and its electrical behavior aligns poorly with the statement that it conducts when melted or dissolved. Its boiling point is not characteristically high for an ionic solid.

Option 5: Cu. Copper is a metal, so its solid is malleable rather than brittle, and it conducts electricity in the solid state due to delocalized electrons. The given properties (brittle solid and no solid-state conduction) do not match copper at all.

In summary, only KBr aligns with all the described characteristics: high boiling point, brittle solid, nonconductive as a solid but conductive when melted or dissolved. The other options fail to match at least one aspect of the description, whether due to molecular rather than ionic character, different conductivity behavior, or malleability.

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