Factors Affecting the Rate of Dissolving and Solubility

Recall, solubility is the amount of solute that dissolves in a given volume of solvent at a certain temperature. Solubility is determined by the intermolecular attractions between solvent and solute particles.

Factors that Affect the Rate of Dissolving You may have observed that a solute, such as sugar or salt, dissolves faster in hot water than in cold water. In fact, for most solid solutes, the rate of dissolving is greater at higher temperatures. At higher temperatures, the solvent molecules have greater kinetic energy so they collide with the undisolved solid molecules more frequently and this increases their rate of dissolving.

Think about having a cup of coffee and adding a spoonful of sugar Think about having a cup of coffee and adding a spoonful of sugar. What do you do once you pour the sugar into the hot coffee? You stir! Agitating a mixture by stirring or shaking the container increases the rate of dissolving. Agitation brings fresh solvent in contact with undissolved solute. Another way to increase the rate of dissolving is to use smaller bits of solute. Decreasing the size of the particles, increases the rate by increasing the surface area of the solute exposed to the solvent. Think of a sugar cube versus loose sugar. The loose sugar dissolves faster.

Predicting Solubility You can predict the solubility of a binary compound by comparing the electronegativity of each element in the compound. If there is a large difference in the two electronegativities, the bond is ionic, or maybe polar. This type of compound probably dissolves in water. If there is only a small difference in the two electronegativities, the bond is not polar or ionic, and probably does not dissolve in water.

The Solubility of Covalent Compounds Many covalent compounds do not have negative or positive charges to attract water molecules so they are not soluble in water. There are some exceptions though. Methanol, ethanol, and sugars are covalent compounds that dissolve in water because they contain some polar bonds, which are able to make hydrogen bonds with water.

Sucrose molecules have a number of sites that can form a hydrogen bond with water to replace the attraction between the sucrose molecules. Sucrose molecules separate and become hydrated. The molecules remain neutral so they do not conduct electricity. These are non-electrolytes. A sucrose molecule contains several O-H atom connections. The O-H bond is highly polar, with the H atom being positive. The negative charges on the water molecules form hydrogen bonds with a sucrose molecule.

Insoluble Covalent Compounds The covalent compounds that are found in oil and grease are insoluble in water. They have no ions or highly polar bonds so they cannot form hydrogen bonds with water. Non-polar compounds tend to be soluble in non-polar solvents like benzene or kerosene. The forces between the solute molecules are replaced by the forces between solute and solvent molecules.

In general… Ionic solutes and polar covalent solutes dissolve in polar solvents. Non-polar solutes dissolve in non-polar solvents. The phrase, Like Dissolve Like, summarizes these observations. If a compound has polar and non-polar components, it may dissolve in both polar and non-polar solvents.

Factors that Affect Solubility Molecule size Temperature Pressure

Molecule Size and Solubility Small molecules are often more soluble than larger molecules. Methanol (CH3OH) and Ethanol (CH3CH2OH) are both completely miscible in water. These compounds have an OH group that can form hydrogen bonds with water. Larger groups with the same OH group, like pentanol (CH3CH2 CH2 CH2CH2OH) but with more carbon atoms, are far less soluble.

Temperature and Solubility You know that temperature affects the rate of dissolving (higher temperature, faster dissolving). Solubility is usually given as the number of grams of solute per 100 mL of solvent at a specific temperature - pentanol is soluble 3 g per 100 mL at 25 oC. Specifying temperature is essential since the solubility of a solute differs at different temperatures.

Factors affecting Solubility-Temperature In general, as the temperature of the solvent increases, the solubility of the solid solute also increases.

Exceptions being gases. They tend to be more soluble at low temperatures & high pressures

When a solid dissolves in a liquid, energy is needed to break the strong bonds between particles in the solid. At higher temperatures, more energy is present so the solubility of most solids increases with temperture.

The bonds between particles in a liquid are not as strong as the bonds between particles in a solid. When a liquid dissolves in a liquid, additional energy is not needed so the solubility of most liquids is not greatly affected by temperature.

Gas particles move quickly and have a great deal of kinetic energy. When a gas dissolves in a liquid, it loses some of its energy. At higher temperatures, the dissolved gas gains energy again and as a result, comes out of solution and is less soluble. Solubility of gases decreases with higher temperatures.

A Side Note: For most solids and almost all ionic substances, solubility increases as temperature of the solution increases. Gases on the other hand, always become less soluble as temperature increases. (This is why a refrigerated soft drink taste fizzier than the same drink at room temperatures. The warmer drink contains less dissolved carbon dioxide than the cooler drink).

This property of gases makes heat pollution a serious problem. Many industries and power plants use water to cool down overheated machinery. The resulting hot water is returned to lakes and rivers. Adding warm water to lakes and rivers may not seem like actual pollution, but as the temperature of the water increases, the dissolve oxygen in the water decreases and aquatic wildlife and plants may not have enough oxygen to breath.

Pressure and Solubility The final factor that affects solubility is pressure. Changes in pressure have hardly any effect on solid or liquid solutions. Such changes do affect the solubility of a gas in a liquid solvent. The solubility of the gas is directly proportional to the pressure of the gas above the liquid.

When you open a carbonated drink, you can observe the effect of the pressure on solubility. Inside a soft drink bottle, the pressure of the carbon dioxide gas is very high. The carbon dioxide then stays in the solution. When you open the soft drink bottle, you can hear the carbon dioxide gas escaping to the atmosphere, where the amount of CO2 is lower. The solubility of CO2 in the liquid soft drink decreases, bubbles begin to rise as the gas comes out of solution, and you only have a short time to enjoy the fizz before the drink becomes flat.