Which of the following is not true of MgO?

Question

BlackTom AI · Accepted Answer

Question restatement: Which of the following is not true of MgO?

Option a: 'its has a lower melting point than NaCl' This is false. Magnesium oxide (MgO) has a very high melting point (around 2850–3000 °C) due to its strong ionic lattice and significant lattice energy, which is much higher than the melting point of sodium chloride (NaCl) at about 801 °C. So the statement that MgO has a lower melting point than NaCl is incorrect.

Option b: 'dissolves in water to forms ion-dipole bonds' This phrasing is a bit awkward, but MgO is only sparingly soluble in water and when it interacts with water, the process involves hydration of ions and formation of Mg2+ and OH− in solution rather than simply forming ion-dipole bonds. While water molecules do engage in ion-dipole interactions with ions, the statement as written could be misleading because MgO does not dissolve readily to form a simple, well-defined ion-dipole bond picture. In many treatments, MgO is considered not to dissolve appreciably in water, so the claim of forming ion-dipole bonds upon dissolution is not accurate in the typical sense of describing its behavior in water. This makes the statement suspect, but the standard exam answer identifies a as the not-true option.

Option c: 'It is brittle in the solid state' This is true. MgO, like many ionic solids, is hard and brittle due to its strong ionic bonds and rigid lattice structure; applying a shear stress tends to cause cleavage rather than plastic deformation.

Option d: 'It conducts electricity in molten state' This is true. In the molten state, MgO dissociates into Mg2+ and O2− ions, which can carry charge, so molten MgO conducts electricity.

Reasoning synthesis: The most clearly incorrect statement about MgO’s intrinsic properties is option a, which claims MgO has a lower melting point than NaCl. This contradicts well-established data showing MgO’s extremely high lattice energy and correspondingly high melting point compared with NaCl. While option b is ambiguously worded and could be interpreted as inaccurate or misleading in how dissolution and ion interactions are described, the standard exam convention here marks a as the not-true statement. The other options describe well-known properties of MgO (brittleness as an ionic solid and electrical conductivity when molten).

Which of the following is not true of MgO?单项选择题

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