Final answer:
Water activity, often represented as aw, is the measure of vapor pressure over pure water. It ranges from 0 to 1, with pure water having an activity of 1. This concept is important in food preservation and in chemical equilibrium calculations. The correct answer is о the measure of vapor pressure over pure water.
Step-by-step explanation:
Water activity is defined as the measure of the vapor pressure of a medium compared to the vapor pressure of pure water. It is a crucial concept in understanding how microorganisms, such as bacteria and fungi, can grow. Water activity (aw) is measured on a scale from 0 to 1, with 1 being the activity of pure water.
Microorganisms require a certain level of water activity to grow, with bacteria typically needing high aw (0.97-0.99) and fungi being able to tolerate lower aw levels (Aspergillus spp. can grow at 0.8-0.75). To prevent spoilage in foods, methods such as drying, freeze-drying, or increasing osmotic pressure are used to decrease the water activity of the food.
In chemistry, when calculating the equilibrium constant of a reaction, the activity of water is also considered to be equal to 1 since it's the solvent in dilute solutions. This assumption simplifies many of the calculations. Thus, the correct answer to the student's question is: Water activity is the measure of vapor pressure over pure water.