Chapter: Substances, Mixtures, and Solubility Table of Contents Section 2: Solubility
Water—The Universal Solvent A solution in which water is the solvent is called an aqueous (A kwee us) solution. Because water can dissolve so many different solutes, chemists often call it the universal solvent. Solubility 2 2
Molecular Compounds When certain atoms form compounds, they share electrons. Sharing electrons is called covalent bonding. Compounds that contain covalent bonds are called molecular compounds, or molecules. Solubility 2 2
Molecular Compounds If a molecule has an even distribution of electrons it is called nonpolar. In a water molecule, the electrons spend more time around the oxygen atom than the hydrogen atoms. Solubility 2 2 Such a molecule is polar.
Ionic Bonds Atoms with a charge are called ions. Bonds between ions that are formed by the transfer of electrons are called ionic bonds, and the compound that is formed is called and ionic compound. Solubility 2 2 Table salt is an ionic compound that is made of sodium ions and chloride ions.
How Water Dissolves Ionic Compounds Because water molecules are polar, they attract positive and negative ions. Solubility 2 2 The more positive part of a water molecule—where the hydrogen atoms are— is attracted to negatively charged ions.
How Water Dissolves Ionic Compounds Solubility 2 2
How Water Dissolves Ionic Compounds The more negative part of a water molecule— where the oxygen atom is—attracts positive ions. Solubility 2 2 When an ionic compound is mixed with water, the different ions of the compound are pulled apart by the water molecules.
How Water Dissolves Molecular Compounds Water does dissolve molecular compounds, such as sugar, although it doesn’t break each sugar molecule apart. Solubility 2 2 Water simply moves between different molecules of sugar, separating them.
What will dissolve? When you stir a spoonful of sugar into iced tea, all of the sugar dissolves but none of the metal in the spoon does. Solubility 2 2 A substance that dissolves in another is said to be soluble in that substance. You would say that the sugar is soluble in water but the metal of the spoon is insoluble in water.
Like Dissolves Like When trying to predict which solvents can dissolve which solutes, chemists use the rule of “like dissolves like.” Solubility 2 2 Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes.
Like Dissolves Like On the other hand, if a solvent and a solute are not similar, the solute won’t dissolve. Solubility 2 2 For example, oil and water do not mix. Oil molecules are nonpolar, so polar water molecules are not attracted to them.
How much will dissolve? Solubility (sahl yuh BIH luh tee) is a measurement that describes how much solute dissolves in a given amount of solvent. Solubility 2 2 The solubility of a material has been described as the amount of the material that can dissolve in 100 g of solvent at a given temperature. When a substance has an extremely low solubility, it usually is considered insoluble.
Solubility in Liquid-Solid Solutions The solubility of many solutes changes if you change the temperature of the solvent. Solubility 2 2 For example, if you heat water, not only does the sugar dissolve at a faster rate, but more sugar can dissolve in it.
Solubility in Liquid-Solid Solutions Solubility 2 2 This graph shows how the temperature of the solvent affects the solubility of some solutes.
Solubility in Liquid-Gas Solutions Unlike liquid-solid solutions, an increase in temperature decreases the solubility of a gas in a liquid-gas solution. Solubility 2 2 You might notice this if you have ever opened a warm carbonated beverage and it bubbled up out of control while a chilled one barely fizzed. Carbon dioxide is less soluble in a warm solution.
Saturated Solutions A solution that contains all of the solute that it can hold under the given conditions is called a saturated solution. Solubility 2 2 If a solution is a liquid-solid solution, the extra solute that is added will settle to the bottom of the container. It’s possible to make solutions that have less solute than they would need to become saturated. Such solutions are unsaturated.
Saturated Solutions A hot solvent usually can hold more solute than a cool solvent can. Solubility 2 2 If a saturated solution is cooled slowly, sometimes the excess solute remains dissolved for a period of time. Such a solution is said to be supersaturated, because it contains more than the normal amount of solute.
Rate of Dissolving Some solutes dissolve quickly, but others take a long time to dissolve. Solubility 2 2 A solute dissolves faster when the solution is stirred or shaken or when the temperature of the solution is increased.
Rate of Dissolving Solubility 2 2 These methods increase the rate at which the surfaces of the solute come into contact with the solvent.
Concentration The concentration of a solution tells you how much solute is present compared to the amount of solvent. Solubility 2 2 You can give a simple description of a solution’s concentration by calling it either concentrated or dilute. A concentrated solution has more solute per given amount of solvent than a dilute solution.
Measuring Concentrations One way of giving the exact concentration is to state the percentage of the volume of the solution that is made up of solute. Solubility 2 2
Measuring Concentrations Solubility 2 2 Labels on fruit drinks show their concentration. Another way to describe the concentration of a solution is to give a percentage of the total mass that is made up of solute.
Effects of Solute Particles The effect that a solute has on the freezing or boiling point of a solvent depends on the number of solute particles. Solubility 2 2 When a solvent such as water begins to freeze, its molecules arrange themselves in a particular pattern.
Effects of Solute Particles Adding a solute such as sodium chloride to this solvent changes the way the molecules arrange themselves. Solubility 2 2 To overcome this interference of the solute, a lower temperature is needed to freeze the solvent.