Ionic compounds have high melting and boiling points. This is due to :
- A.the presence of strong electrostatic forces between ions in the crystal lattice.
- B.the weak intermolecular van der Waals forces.
- C.the presence of a 'sea of delocalized electrons'.
- D.lower lattice energy.
Correct answer
Explanation
The correct answer is A, the presence of strong electrostatic forces between ions in the crystal lattice.. Oppositely charged ions in an ionic crystal pull on each other strongly from every side, so a great deal of heat is needed to break the lattice apart.
In common salt, every sodium ion is surrounded by chloride ions and every chloride ion by sodium ions. This three-dimensional network is held by electrostatic attraction, and the energy needed to pull it apart is the lattice energy. Because that energy is large, sodium chloride melts only at about 801 °C. The same strong attraction makes ionic solids hard and brittle.
B is wrong because weak van der Waals forces hold molecular solids such as wax and iodine, which melt easily.
C is wrong because a sea of delocalised electrons describes metallic bonding, not ionic bonding.
D is wrong because a lower lattice energy would mean a lower melting point, the opposite of what is seen.
Exam tip: ionic solids have high melting points, are hard and brittle, and conduct electricity only when molten or dissolved in water.
Practice Questions
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- A.Galileo
- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
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- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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- A.Condenser-Objective lens-Eyepiece-Body tube
- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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Read the full explanation →Which one of the following statements regarding haemoglobin is correct?
- A.Haemoglobin present in RBC can carry only oxygen but not carbon dioxide.
- B.Haemoglobin of RBC can carry both oxygen and carbon dioxide.
- C.Haemoglobin of RBC can carry only carbon dioxide.
- D.Haemoglobin is only used for blood clotting and not for carrying gases.
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Read the full explanation →Which one of the following statements regarding histone proteins is correct?
- A.Histones are proteins that are present in mitochondrial membrane.
- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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