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IGCSE · Cambridge · Chemistry

1.1 Solids, liquids and gases

Particle structure in each state

The particle model describes how the arrangement, separation and motion of particles differ in solids, liquids and gases.

Solid
Liquid
Gas

Exam cue Draw gas particles much further apart than liquid particles; a common error is drawing them too close together.

Flashcard 3 of 12

Distinguish between boiling and evaporation.

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Answer

Boiling happens at a fixed temperature throughout the liquid; evaporation happens only at the surface, at any temperature below the boiling point.

MCQ 1 of 12Correct

Which property distinguishes a liquid from both a solid and a gas?

  1. AFixed shape and fixed volume
  2. BNo fixed shape and no fixed volume
  3. CNo fixed shape, but a fixed volume
  4. DFixed shape, but no fixed volume

Every mark, accounted for.

Written answers are marked against the mark scheme, point by point. Every mark awarded or withheld, with the reason.

Your answerQuestion 14 · 4 marks

Describe how a triglyceride is synthesised from glycerol and fatty acids.

“A triglyceride is made when one glycerol molecule joins to three fatty acids1. Each fatty acid is attached by its own condensation reaction2, so three molecules of water are released3. The fatty acids are held to the glycerol by glycosidic bonds4.”

A Level · Edexcel · Biology · 1.1.1 Water, carbohydrates and lipids

Mark scheme evaluationPoint by point
  1. One glycerol reacts with three fatty acidsNames both reactants and the one-to-three ratio. [1]
  2. Three condensation reactions, one per fatty acidGives each fatty acid its own condensation. [1]
  3. Three water molecules are releasedStates the by-product and how many. [1]
  4. Three ester bonds formNot awarded: glycosidic bonds join sugars. Glycerol and fatty acids are joined by ester bonds. [0]
3 / 4 marks A strong answer. One word cost the fourth mark: these are ester bonds, not glycosidic.

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A Level · Edexcel · Biology 1.1.1 Water as a solvent in transport Water acts as a vital biological solvent because its dipole nature allows it to dissolve charged or polar substances and transport them around organisms. The unequal sharing of electrons between oxygen and hydrogen makes water a polar molecule. Oxygen pulls the shared electrons more strongly, becoming slightly negative. When ionic or polar solutes enter water, the dipoles attract them and pull them apart, dissolving them. Read the note
2 PracticeTopic questions and MCQs.
Multiple choice · Question 1 Which feature of a water molecule allows it to dissolve ionic solutes such as sodium chloride? AThe presence of hydrogen bonds between adjacent water molecules BThe unequal sharing of electrons between oxygen and hydrogen, creating a dipole CThe bent V-shape of the molecule reducing its size DThe covalent bonds between oxygen and hydrogen being non-polar

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