Covalently bonded molecules have low melting and boiling points because:
- A.Intermolecular forces are medium
- B.Intermolecular forces are strong
- C.Intermolecular forces are very strong
- D.Intermolecular forces are weak
Correct answer
Explanation
The correct answer is D, Intermolecular forces are weak. Melting and boiling only have to break the weak forces between the molecules, not the strong bonds inside them, so little heat is needed.
A covalent bond is formed by the sharing of electrons and is itself strong, but it acts only within the molecule. Separate molecules are held to one another by weak van der Waals forces, and these give way at low temperature, which is why water, ammonia, carbon dioxide and most organic compounds melt and boil easily. Ionic compounds such as sodium chloride behave in the opposite way: a strong electrostatic force runs through the whole crystal, so their melting points are very high. Giant covalent solids like diamond and silicon carbide are the exception, since their bonds form a continuous network.
Options A, B and C are wrong because they all claim the forces between molecules are medium or strong, which would raise the melting and boiling points instead of lowering them.
Exam tip: strong bond inside the molecule, weak force between molecules, so covalent compounds melt at low temperature.
Practice Questions
View allThe fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by
- 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.
Read the full explanation →Which one of the following statements is correct?
- A.Urea is produced in liver.
- B.Urea is produced in blood.
- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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Liver cells turn the toxic ammonia released from the breakdown of amino acids into urea through the ornithine cycle, also called the urea cycle.
Read the full explanation →Which one of the following is the correct sequence of passage of light in a compound microscope?
- 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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Light passes through the condenser to the specimen, is picked up by the objective lens, travels up the body tube and reaches the eye through the eyepiece.
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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It binds oxygen at its iron-containing haem groups and carries some carbon dioxide on its protein chains. Each haemoglobin molecule has four polypeptide chains and four haem groups, and each haem iron can hold one oxygen molecule, forming oxyhaemoglobin in the lungs.
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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They are the proteins around which DNA winds to form chromatin in the nucleus of eukaryotic cells. Histones are small basic proteins, rich in the positively charged amino acids lysine and arginine, which attract the negatively charged phosphate groups of DNA.
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