When 1.O M solution of NaCl at 25 degrees C, is heated to 55 degrees C the:

A) density increase and the molarity increases
B) density increases and the molarity decreases
C) density decreases and the molarity decreases

The answer is C.

density decreases and the molarity decreases

To determine the change in density and molarity of a NaCl solution when it is heated from 25 degrees C to 55 degrees C, we need to consider the effect of temperature on the properties of the solution.

First, let's understand the concept of density. Density is defined as the mass of a substance (in this case, the solution) divided by its volume. Mathematically, density (D) is expressed as D = mass/volume.

Now, when a solution is heated, its volume generally increases due to thermal expansion. As a result, the density of the solution decreases. So, the correct option should indicate a decrease in density.

Next, let's consider the effect of temperature on the molarity of a solution. Molarity (M) is defined as the number of moles of solute (in this case, NaCl) present in one liter of solution. Mathematically, molarity is expressed as M = moles of solute/volume of solution (in liters).

When we heat a solution, it does not affect the number of moles of solute present. Therefore, the molarity remains constant. So, the correct option should indicate no change in molarity.

Based on the above understanding, we can conclude that the correct answer is:

C) density decreases and the molarity remains constant.