When $$\alpha {\text{ - }}D{\text{ - }}$$  glucose and $$\beta {\text{ - }}D{\text{ - }}$$  glucose are dissolved in water in two separate beakers I and II respectively and allowed to stand, then –

Solution :
$$\alpha {\text{ - }}D{\text{ - }}$$  glucose or $$\beta {\text{ - }}D{\text{ - }}$$  glucose when dissolved in water and allowed to stand, following equilibrium is established.
$$\mathop {\alpha {\text{ - }}D{\text{ - glucose}}}\limits_{\left( { + {{111}^ \circ }} \right)} \rightleftharpoons {\text{Open chain form}} \rightleftharpoons \mathop {\beta {\text{ - }}D{\text{ - glucose}}}\limits_{\left( { + {{19}^ \circ }} \right)} $$
Specific rotation of $$\alpha {\text{ - }}$$ form falls, while that of $$\beta {\text{ - }}$$ form increases until a constant value of $$ + {52.5^ \circ }$$  is reached at equilibrium. This phenomenon is known as mutarotation.