3.3 Saturated Solutions

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 many 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 3.03. The quantities of water (H2O) and halite (NaCl) used to make the mixtures are listed in Table 3.01. Notice that in some of the mixtures, there are undissolved white halite crystals remaining on the bottom of the glass vials. 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 phase such as a mineral or a gas is a property that magmas share with brines. If a liquid is saturated with a particular phase, adding more of that phase (e.g. more halite to the brine in a beaker or more olivine to the basalt magma) will not change the composition of the liquid.

The proportions of water and salt used for the mixtures in Figure 3.03 are shown by the red circles on a graph of chemical composition vs. temperature in Figure 3.t04. The horizontal axis is in units of weight percent NaCl. Weight percent NaCl is grams of NaCl per 100 grams of mixture. For these H2O-NaCl mixtures, weight percent NaCl is calculated with the formula:


In Figure 3.04, pure H2O is shown on the left (0 weight percent NaCl) and pure NaCl is shown on the right (100 weight percent NaCl). A dashed line in Figure 3.04 separates the unsaturated brines (those that have no halite remaining) from the halite-saturated brines (those that have undissolved halite on the bottom of the vials in Figure 3.03). 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 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.

The dashed line in Figure 3.04 marks the weight percent NaCl in a halite saturated solution at 21°C. Click on Figure 3.04 to see an enlarged version, and on that page 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.