Which one of the following processes involves chemical reaction?
- A.Evaporation of petrol
- B.Dissolution of salt in water
- C.Storing of oxygen gas under pressure in a gas cylinder
- D.Burning of magnesium ribbon in air
Correct answer
Explanation
The correct answer is D, Burning of magnesium ribbon in air. Magnesium combines with oxygen to form a new substance, magnesium oxide, so a chemical reaction takes place.
When a magnesium ribbon is heated, it burns with a dazzling white flame and leaves a white powder: 2Mg + O₂ → 2MgO. The product has properties quite different from the metal, the change cannot be reversed by simple means, and energy is given out as heat and light. These are the signs of a chemical change. In a physical change, by contrast, the substance only changes its state, shape or form and keeps its chemical identity.
A is wrong: evaporating petrol only turns liquid into vapour; the petrol molecules stay the same.
B is wrong: salt dissolved in water can be recovered unchanged by evaporating the water.
C is wrong: compressing oxygen into a cylinder only packs its molecules closer together.
Exam tip: new substance plus heat or light means a chemical change; change of state or dissolving is physical.
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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