The quantity of heat needed to change unit mass of a substance from liquid to vapour without changing the temperature, is called
- A.specific heat
- B.specific latent heat
- C.thermal capacity
- D.heat energy
Correct answer
Explanation
The correct answer is B, specific latent heat. The heat needed to turn unit mass of a liquid into vapour at constant temperature is its specific latent heat of vaporisation.
During a change of state the heat supplied does not raise the temperature; it is used to break the attractions between the particles, which is why it is called latent or hidden heat. For water the latent heat of vaporisation is about 2,260 kJ per kg (22.6 × 10⁵ J/kg), and the latent heat of fusion of ice is about 334 kJ per kg. This is why a burn from steam at 100°C is worse than one from boiling water at 100°C: steam gives up its extra latent heat on the skin.
A, specific heat, is the heat needed to raise the temperature of unit mass by one degree, so it involves a change of temperature.
C, thermal capacity, is the heat needed to raise the temperature of a whole body by one degree.
D, heat energy, is the general name for the energy, not this particular quantity.
Exam tip: latent heat changes the state, not the temperature; water, vaporisation 2,260 kJ/kg and fusion 334 kJ/kg.
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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