Liquid water is denser than ice due to
- A.higher surface tension
- B.hydrogen bonding
- C.van der Waals forces
- D.covalent bonding
Correct answer
Explanation
The correct answer is B, hydrogen bonding. Hydrogen bonds lock the molecules of ice into an open cage, so ice takes up more space than the same mass of liquid water.
Each water molecule can form hydrogen bonds with four neighbours. When water freezes, these bonds hold the molecules in a fixed hexagonal lattice with a lot of empty space inside it. When ice melts, some of the bonds break, the lattice collapses and the molecules pack more closely, so the liquid is denser. Water is densest at about 4 degrees Celsius. This is why ice floats and why lakes freeze from the top down, leaving water below in which fish survive the winter.
A is wrong because surface tension acts at the surface and does not set the density of the bulk liquid. C is wrong because van der Waals forces are much weaker and do not build the open lattice of ice. D is wrong because the covalent O–H bonds inside each molecule are the same in ice and water.
Exam tip: ice floats because hydrogen bonds give it an open lattice; water is densest at 4 °C.
Practice Questions
View allWhich one of the following regions in plants is responsible for the growth of its roots and shoots?
- A.Collenchyma
- B.Endodermis
- C.Cortex
- D.Apical meristem
Show answer
Apical meristems sit at the tips of roots and shoots, and their dividing cells make the plant grow in length.
Read the full explanation →Out of different types of tissues found in plants, which one of the following is a complex tissue?
- A.Parenchyma
- B.Sclerenchyma
- C.Xylem
- D.Collenchyma
Show answer
Xylem is made of more than one type of cell working together, which is what makes a tissue complex. Plant permanent tissues are of two kinds.
Read the full explanation →Transport of Na+ and Ca2+ across the plasma membrane is an example of
- A.antiport
- B.symport
- C.uniport
- D.aquaporins
Show answer
In the sodium-calcium exchanger, Na+ ions enter the cell while Ca2+ ions are pushed out, so the two ions cross the membrane in opposite directions.
Read the full explanation →The oxygen evolved during photosynthesis comes from splitting of
- A.water
- B.carbon dioxide
- C.oxygen
- D.light
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In photosynthesis, the oxygen released into the air comes from water molecules that are split using light energy. Photosynthesis takes place in the chloroplasts of green plants.
Read the full explanation →Which one of the following could be the melting point of iron?
- A.25°C
- B.37°C
- C.500°C
- D.1500°C
Show answer
Pure iron melts at about 1,538°C, so 1,500°C is the only option close to its melting point. Iron is a metal with strong metallic bonds, so a great deal of heat is needed to break up its solid structure.
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