Which one of the following statements is NOT correct ?
- A.The interconversion of states of matter is a physical change
- B.During burning of a candle, both chemical and physical changes take place
- C.During a physical change, the chemical composition of molecules changes
- D.Rusting of iron is a chemical change
Correct answer
Explanation
The correct answer is C, the statement that the chemical composition of molecules changes during a physical change. That is false: in a physical change the molecules stay the same, and only their arrangement, state or shape alters. A change in composition, forming new substances, is what defines a chemical change. Dissolving sugar, breaking glass and boiling water are physical changes; cooking food, burning and digestion are chemical ones. Option A is wrong to pick because it is true: melting, freezing, boiling and condensation only change the state, so they are physical changes. Option B is wrong to pick because it is true: the wax melts, a physical change, and the wax vapour burns to form carbon dioxide and water, a chemical change. Option D is wrong to pick because it is true: iron reacts with oxygen and moisture to form a new substance, hydrated iron oxide, so rusting is chemical. Exam tip: candle burning = physical (melting) + chemical (burning); composition changes only in chemical change.
Practice Questions
View allEqual volumes of all gases, under the same conditions of temperature and pressure, contain an equal number of molecules. This statement is known as
- A.Avogadro's law
- B.Boyle's law
- C.Charles's law
- D.Dalton's law of partial pressures
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It is the reason one mole of every gas occupies the same 22.4 litres at standard temperature and pressure, and it allowed chemists to settle the formulae of gases such as hydrogen and oxygen as having two atoms each.
Read the full explanation →Carbon forms two oxides, carbon monoxide and carbon dioxide, in which twelve grams of carbon combines with sixteen grams and thirty two grams of oxygen respectively. This illustrates
- A.The law of conservation of mass
- B.The law of constant proportions
- C.The law of multiple proportions
- D.Avogadro's law
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When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other bear a simple whole number ratio, and here the oxygen masses of sixteen and thirty two grams for the same twelve grams of carbon are in the ratio…
Read the full explanation →How many moles are present in 36 grams of water?
- A.One mole
- B.Two moles
- C.Three moles
- D.Half a mole
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The molecular mass of water is eighteen unified atomic mass units, two hydrogen atoms of 1 u each and one oxygen atom of 16 u, so one mole of water weighs eighteen grams.
Read the full explanation →One unified atomic mass unit, written as u, is defined as
- A.The mass of one hydrogen atom
- B.One twelfth of the mass of one atom of carbon twelve
- C.One sixteenth of the mass of one atom of oxygen
- D.The mass of one electron
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This standard was adopted so that the atomic masses of all elements could be compared on one relative scale, and on it hydrogen comes out as about 1 u, oxygen as 16 u and chlorine as 35.5 u. Option A is wrong because the hydrogen atom was the standard in…
Read the full explanation →At standard temperature and pressure, the volume occupied by one mole of any gas is
- A.11.2 litres
- B.22.4 litres
- C.24.4 litres
- D.1 litre
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It follows from Avogadro's law: since equal volumes of all gases at the same temperature and pressure contain an equal number of molecules, one mole of hydrogen, of oxygen or of carbon dioxide all take up the same space at standard temperature and pressure, even though their masses are two…
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