Experiment: Optical Activity

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Objective: Investigate the rotation of the plane of polarisation by sugar solutions

Sugar solutions are optically active, in other words they rotate the plane of polarisation of any linearly polarised light that is passed through them. The direction of rotation depends on the molecular properties of the sugar. Thus, solutions of glucose and saccharose (sucrose) rotate the plane of polarisation to the right (clockwise), whereas fructose solutions rotate it to the left (counter-clockwise), as found when the angle of rotation is measured with a polarimeter. The experiment also includes measuring the angle of rotation to study the behaviour of a saccharose solution when hydrochloric acid is added. This causes a gradual reversal (“inversion”) of the direction of rotation from clockwise to counter-clockwise, as the double-ring structure of the saccharose molecule is split into two, giving an equimolar mixture of glucose and fructose. The angle of rotation of the mixture is the sum of the angles of rotation of the dextro-rotatory glucose and the more strongly laevo-rotatory fructose.

Additionally recommended: Fruit Sugar (Fructose), 500 g, Grape Sugar (Glucose), 500 g, Cane Sugar (Sucrose), 500 g