Section 4.2: Saturated Solutions (continued)

Halite crystals are very soluble in water and will dissolve quickly if the glass of water is stirred. The water becomes "salty" (a brine) and some of its physical properties change. Properties that change include taste, density, refractive index, and electrical conductivity. The values of these properties depend on how much salt is added to the water. Mixtures of various proportions of halite and water (after stirring and settling) are shown in Figure 4.03.
Salt Solutions

Figure 4.03. Salt solutions (brines) made by mixing various proportions of halite (NaCl) and water. The masses in grams of NaCl and H2O for each solution are listed. Notice that undissolved halite crystals remain on the bottom of the beakers for some solutions.

The quantities of water (H2O) and halite (NaCl) used to make the mixtures are listed in Table 4.01. As you can see, saltier brines are less transparent than less salty brines. Notice that in some of the mixtures, there are undissolved white halite crystals remaining on the bottom of the beakers. Brines that have so much NaCl in solution that no more halite will dissolve are said to be "saturated with halite". Possible saturation with a crystal is a property that brines share with magmas.

The proportions of water and salt used for the mixtures in Figure 4.03 are indicated by the red circles on a graph of chemical composition vs. temperature in Figure 4.04. The horizontal axis is in units of weight percent NaCl. Weight percent NaCl is calculated with the formula:


Figure 4.04. Chemical compositions of mixtures of water and halite (NaCl) in Figure 4.03 shown on a graph of weight percent NaCl vs. temperature. The solutions were all made at 21°C. The dashed line separates the mixtures with brines that are saturated with halite from the mixtures with brines that are not saturated with halite.

Pure H2O is shown on the left (0 weight percent NaCl) and pure NaCl (halite) is shown on the right (100 weight percent NaCl). A dashed line in Figure 4.04 separates the unsaturated brines (those that have no halite remaining) from the saturated brines (those that have undissolved halite on the bottom of the vials in Figure 4.03). For the solutions to the left of the line, the weight percent NaCl for the mixture gives the chemical composition of the brine because all of the NaCl is in the liquid. For the solutions to the right of the line, the the weight percent NaCl for the mixture does not give the chemical composition of the brine because some of the NaCl is in the halite crystals that did not dissolve. Based on the experiments, the maximum possible weight percent NaCl for unsaturated brines is given approximately by the position of the dashed line. A more precise determination of the dashed line location could be made by doing experiments with more closely spaced compositions.

You can read the weight percent NaCl in a halite saturated solution at 21°C from Figure 4.04. Examine the enlarged version of Figure 4.04 and click on the Coordinates button to activate crosshairs when you move your mouse over the figure. Use the crosshairs to get an exact answer to the following question:

The weight percent NaCl in a halite-saturated solution at 21°C is (number only):

Press "Enter" after you type in the number.

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