Chemistry Keypoints; Solubility; Solubility is the ability of a substance to dissolve in a particular solvent. It is an important concept in chemistry as it helps to determine the amount of a substance that can be dissolved in a given solvent under specific conditions. The following topics will be covered in this class:
Chemistry Keypoints; Solubility
(a) Unsaturated, saturated, and supersaturated solutions:
An unsaturated solution is one in which more solute can be dissolved in the solvent. A saturated solution is one in which no more solute can be dissolved in the solvent at a given temperature and pressure. A supersaturated solution is one in which more solute is dissolved than would normally be possible in a saturated solution under the same conditions.
Solubility curves and simple deductions from them:
A solubility curve is a graphical representation of the solubility of a substance as a function of temperature. The curve shows the maximum amount of solute that can be dissolved in a given amount of solvent at different temperatures.
Simple deductions from solubility curves include determining the solubility of a substance at a specific temperature and predicting the amount of a substance that will precipitate out of a solution upon cooling or heating.
Solubility is defined in terms of a mole per dm^3 and simple calculations:
Solubility is often defined in terms of the number of moles of solute that can be dissolved in a given volume of solvent (usually measured in dm^3 or liters). For example, the solubility of sodium chloride (NaCl) in water at 25°C is approximately 36 grams per 100 ml or 0.36 moles per liter.
Simple calculations involving solubility include determining the amount of solute that can be dissolved in a given volume of solvent, and calculating the concentration of a solution.
(b) Solvents for fats, oil, and paints and the use of such solvents for the removal of stains:
Solvents such as acetone, alcohol, and mineral spirits are commonly used to dissolve fats, oils, and paints. These solvents are also useful for removing stains from fabrics and surfaces. For example, acetone is often used to remove nail polish, while mineral spirits are used to clean brushes after painting with oil-based paints.
(c) False solutions (Suspensions and colloids):
False solutions, also known as heterogeneous mixtures, are mixtures in which the particles are not evenly distributed throughout the mixture. Suspensions and colloids are two types of false solutions.
Suspensions are mixtures in which the particles are larger and can be seen with the naked eye. Examples include muddy water, sand in water, and blood. Colloids are mixtures in which the particles are smaller and cannot be seen with the naked eye. Examples include fog, milk, aerosol spray, and rubber solution.
Understanding the properties and examples of suspensions and colloids is important as they have many practical applications, such as in the production of paints and coatings, cosmetics, and food products. For example, emulsion paints are made by suspending pigment particles in a liquid, while mayonnaise is a colloid made by suspending oil droplets in an aqueous solution.
Chemistry Keypoints; Solubility
(i) Distinguish between the different types of solutions:
There are three types of solutions: unsaturated, saturated, and supersaturated. An unsaturated solution is one in which the solvent can dissolve more solute at a given temperature, while a saturated solution is one in which the solvent has already dissolved as much solute as possible at that temperature.
A supersaturated solution is one in which the solvent has dissolved more solute than it should be able to at that temperature, and the excess solute will eventually precipitate out of the solution.
(ii) Interpret solubility curves:
A solubility curve is a graph that shows the solubility of a substance at different temperatures. The curve can be used to determine the amount of solute that can dissolve in a given amount of solvent at a specific temperature.
For example, if the solubility curve shows that 50g of solute can dissolve in 100g of solvent at a certain temperature, then that is the maximum amount of solute that can dissolve in that amount of solvent at that temperature.
(iii) Calculate the amount of solute that can dissolve in a given amount of solvent at a given temperature:
The amount of solute that can dissolve in a given amount of solvent at a given temperature can be determined by using the solubility curve.
For example, if the solubility of a substance is 20g per 100g of solvent at a certain temperature, then a solution containing 40g of solute and 200g of solvent would be saturated.
(iv) Deduce that solubility is temperature-dependent:
The solubility of most solids in liquids increases as the temperature increases, while the solubility of most gases in liquids decreases as the temperature increases. This is because as the temperature increases, the particles of the solvent have more kinetic energy, and therefore move around more, which makes it easier for the solute particles to dissolve.
(v) Relate the nature of solvents to their uses:
Different solvents have different properties, such as polarity, boiling point, and viscosity. These properties make certain solvents better suited for dissolving certain substances. For example, water is a polar solvent that is good for dissolving salts and other polar compounds, while nonpolar solvents like hexane are better for dissolving nonpolar compounds like oils and fats.
(vi) Differentiate among true solutions, suspensions, and colloids:
In a true solution, the solute particles are evenly distributed throughout the solvent, and the mixture is transparent and homogeneous. In a suspension, the solute particles are not evenly distributed throughout the solvent, and the mixture appears cloudy or opaque. In a colloid, the solute particles are intermediate in size between those in a solution and those in a suspension, and the mixture appears translucent.
(vii) Compare the properties of a true solution and a ‘false’ solution:
A true solution is transparent, homogeneous, and stable over time, while a ‘false’ solution, such as a suspension or a colloid, is not transparent or homogeneous, and the solute particles will eventually settle out of the mixture over time.
(viii) Provide typical examples of suspensions and colloids:
Examples of suspensions include muddy water, blood, and sand in water. Examples of colloids include milk, aerosol spray, and fog. Suspensions are typically separated by filtration, while colloids can be separated by methods such as centrifugation.
