A saturated solution is one that contains the maximum amount of solute capable of being dissolved, whereas unsaturated solutions contain less than the maximum amount of solute capable of being dissolved. Because carbonated water is saturated with carbon, it emits carbon through bubbles. Sand in water is an example of an unsaturated.
Saturated Solutions
A saturated solution is a chemical solution that contains the greatest quantity of solute contained in the solvent. The solute cannot be dissolved anymore in a saturated solution.
The saturation point of any liquid is determined by the type of the material and the temperature. A saturated solution is one in which the quantity of dissolved solute equals the saturation point of the solvent. A solvent can dissolve some particular types of solutes in it.
The maximum amount of solute that can be dissolved in a solvent at a specified temperature can be termed as a saturated solution. A solution cannot dissolve any more solute further upon reaching saturation. The undissolved substances remain at the bottom. The point at which the solute stops dissolving in the solvent is termed the saturation point.
Following are the examples of Saturated Solutions:
- Soil is a saturated mixture consisting of nitrogen. On attaining the saturation point, the excess nitrogen is emitted out into the air in the form of gas.
- Beverages, such as cold drinks are saturated solutions of dissolved carbon in water.
- Protein drinks which is a saturated solution of protein powder in milk etc.
Unsaturated Solutions
A solute must be added to a solvent in order for a solution to form. At first, the solute dissolves in a solvent and forms a homogeneous solution. A solution in which solutes dissolve is referred to as an unsaturated solution. A solution is made up of two types of particles: solutes and solvents. Water is commonly used as a solvent (which is one of the reasons why water is also called the universal solvent).
Unsaturated solutions have the ability to dissolve additional solute until they achieve saturation. Solutes will no longer dissolve in the solvent after reaching the saturation threshold, resulting in unsaturated solutions.
As a result, all solutions are considered to be largely unsaturated in nature before being transformed into saturated solutions by adding solute to them. The amount of solute that is contained in lesser amounts than the maximum value, that is before the solution reaches the saturation level is called an unsaturated solution. No remaining substances leave at the bottom, that is, all the solute is dissolved in the solvent. An unsaturated solution is basically a chemical solution that has a solute concentration lesser than its corresponding equilibrium solubility.
Following are the examples of Unsaturated Solutions:
- Salt or sugar dissolved in water below the saturation point.
- Air or mist.
- Iced coffee.
- Vinegar is the acetic acid solution in water.
Interconversion of Saturated and Unsaturated Solution
Saturated solution on heating becomes unsaturated whereas an unsaturated solution becomes saturated upon cooling. On heating the saturated solution, the solubility of that particular solute increases in the given solvent. As a result of this, more solute can be dissolved into the solvent. However, in the case of cooling a solution, the solute particles which were initially dissolved in the solvent separate out as crystals.
Solubility
The amount of the solute present in the saturated solution at the given temperature can be termed as the solubility of the solute in the solvent.
The maximum amount of a solute that can be dissolved in 100 gm of a solvent. Different solutes possess varying different solubilities. Solubility increases with an increase in temperature. In the case of saturated solutions, the solute concentration is equivalent to the equilibrium solubility. The solubility of a substance depends on the solvent. It is observed that sugar dissolves in water but not in oil.
Effect of Temperature and Pressure on Solubility
- The solubility of solids in liquids typically increases as temperature rises and decreases as temperature falls.
- The solubility of solids in liquid is unaffected by pressure variations.
- The solubility of gases in liquids typically decreases as temperature rises and rises as temperature falls.
- Gas solubility in the liquid increases with rising pressure and decreases with lowering pressure.
Concentration of Solution
The quantity of solute present in a specified quantity of the solvent can be termed as the concentration of the solution. It is measured as a fraction of the amount of solute dissolved in a given mass or volume of a solvent.
A solution in which less amount of solute is present is called a dilute solution whereas, a solution containing more solute is called a concentrated solution.
Mathematically,
- Concentration of solution = Amount of Solute / Amount of Solvent
Also,
- Concentration of solution = Amount of Solute / Amount of Solution
Sample Problems
Problem 1: A solution is formed by dissolving 20g of sodium chloride in 180 g of water as solvent. Compute the concentration of the solution.
Solution:
Given,
Mass of sodium chloride = 20 g
Mass of water = 180 g
We know,
Mass of solution = Mass of solute + Mass of solvent
= 20 g + 180 g
= 200 g
Concentration of solution is given by,
= [(Mass of solute)/ (Mass of solution)] × 100
= (20/200) × 100
= 10 %
Problem 2: How to identify whether the solution is saturated or not?
Solution:
We take a solution with a solute dissolved in the solvent. On constant stirring, if more solute can be dissolved into the solvent, then the solution is unsaturated otherwise saturated.
Problem 3: How can you create a saturated solution?
Solution:
Following are the steps to create a saturated solution:
- Adding a solute to a solvent beyond the point that solid gets dissolved in the solvent.
- Adding seed crystals to a supersaturated solution.
- Evaporating a solvent from a solution till the point the solute in the solution begins to crystallize or precipitate.
Problem 4: Give some outdoor examples of saturated solutions.
Solution:
Some of the outdoor examples of saturated solutions are:
- Seawater – A saturated solution with salt as the solute.
- Soil – A saturated solution with nitrogen.
- Air – A saturated solution with moisture.
- Freshwater – Water containing elements and metals, like potassium, dissolved till saturation.
Problem 5: Give one example to show a solution that can be unsaturated, saturated, and supersaturated at different intervals of time.
Solution:
Considering a soda bottle can show that a solution can be unsaturated, saturated, and supersaturated at different intervals of time. Before opening the soda bottle, the solution is supersaturated, On opening the bottle, the excess dissolved carbon dioxide escapes from the surface resulting in the formation of bubbles. This is a saturated solution. When left for a large amount of time, the soda water goes flat and turns into an unsaturated solution.
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There are many ways to measure the amount of solute present in a solution. Each method is useful for a different purpose in chemistry, so we're unfortunately stuck with the task of learning all of them. Without further ado, here they are:
Qualitative Concentrations
The amount of solute present in a solution can be described without numbers by one of the following terms:
- Unsaturated: "Unsaturated" refers to any solution that is still capable of dissolving more of a solute. For example, a glass of iced tea is not saturated with sugar if you've placed one tablespoon of sugar in it because it's still capable of dissolving more sugar. This term isn't very good for determining the exact quantity of solute present—for example, both a glass of water and a filled swimming pool would be said to be unsaturated salt solutions if there were one gram of salt dissolved in each.
- Saturated: These solutions have dissolved the maximum possible amount of solute. For example, if you keep adding sugar to a glass of Kool-Aid, it will eventually stop dissolving and settle to the bottom (little kids, however, refuse to believe this). This solution is said to be saturated.
- Supersaturated: These solutions are those that have dissolved more than the normal maximum possible amount of solute. These solutions are unusual and aren't very stable. For example, the addition of a small mote of dust to such a solution causes enough of a disturbance that crystals spontaneously form until the solution reaches a saturated state.
It's easy to tell if a solution is unsaturated, saturated, or supersaturated by adding a very small amount of solute. If the solution is unsaturated, the solute will dissolve. If the solution is saturated, it won't. If the solution is supersaturated, crystals will very quickly form around the solute you've added.
Molarity (M)
When reading the definitions of the following methods of determining concentration, pay close attention to whether the volume component asks for the weight or volume "of solution" or weight or volume "of solvent." When the weight or volume of the solution is specified, it means that you're interested in the amount of solution present after the solute has been added. If the weight or volume of the solvent is specified, this means that you're interested in the amount of solvent before the solute has been added.
Molarity is probably the most commonly used way of measuring concentration and is defined as the number of moles of solute per liters of solution.
Problem 1: What is the molarity of a solution with a volume of 3.0 liters that contains 120 grams of acetic acid (C2H3O2H)?
Let's say that we have made a solution by adding water to 40 grams (1.0 mole) of sodium hydroxide until the final volume of the solution is one liter (to review mole calculations, head back to The Mole). Because we have one mole of solute in one liter of solution, the molarity is equal to (1 mole)/(1 liter) = 1 M. We refer to a solution with a molarity of one as being a "one molar" solution.
Molality (m)
Molality is defined as the number of moles of solute per kilogram of solvent. For example, if we were to add two kilograms of water to 4 moles of sugar, the molality would be equal to 4 moles/2 kilograms = 2 m ("two molal"). When doing calculations with molality, note that because the density of water is 1.0 g/mL under standard conditions, the number of kilograms of water is equal to the number of liters of water.
Normality (N)
The normality of a solution is defined as the number of moles of a reactive species, usually referred to as "equivalents" per liter of solution. The use of "equivalents" will depend on the reaction being performed, so some knowledge of the specific chemical process in a reaction is necessary before computing normality. At least, that's the "normal" way of solving this problem. (I couldn't resist.)
Mole Fraction ()
Problem 2: What is the mole fraction of water in a solution made by mixing 4.5 moles of isopropanol with 15.0 moles of water?
The mole fraction is defined as the number of moles of one component in a solution divided by the total number of moles of all components in the mixture. In equation form, we can express the mole fraction of one component in a solution as being:
- A = moles of Amoles of A + moles of B + moles of C + …
where A refers to the first component, B refers to the second component, and C refers to the third component. As the "…" indicates, this calculation can be extended to include any number of components in the mixture.
Parts Per Million (ppm) and Parts Per Billion (ppb)
Both parts per million and parts per billion are units of concentration most frequently used in environmental analysis. Because the solvent used is most frequently water, the concentration of a solution in ppm can be found by dividing the number of mg (0.001 g) of solute by the number of liters of water. Parts per billion can be determined by dividing the number of g (10-6 g) of solute by the number of liters of water.
A Quick Summary of Units of Concentration
The following table includes all of the units of concentration we've mentioned in this section, as well as how to find them.
| molarity | M | moles of solute / liters of solution |
| molality | m | moles of solute / kilograms of solution |
| normality | N | "equivalents," which varies depending on the reaction being performed |
| mole fraction | moles of Amoles of A + moles of B + … | |
| parts per million | ppm | mg solute/L of water |
| parts per billion | ppb | g solute/L of water |
Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.
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