Solutions

Section 15.1

Solutions

Solutions are homogeneous mixtures containing two or more substances called the solute and the solvent. The solvent is the largest part of the solution and the solute is the smallest part of the solution

Solutions

Even though most solutions are liquid, solutions do exist as solids, liquids and gases.

Solid Alloys

Gases: Air = Oxygen + Nitrogen

Liquids

Solubility

Some substances readily combine to form solutions, others do not. A substance that dissolves in a solvent is said to be soluble. For example, sugar is soluble in water.

A substance that will not dissolve in a solvent is said to be insoluble, like oil and water.

Oil and Water Insoluble

Sugar and Water Soluble

Solubility

If two liquids are soluble in each other they are said to be miscible. If two liquids will not dissolve, they are immiscible.

Water and alcohol: Miscible

Oil and Water: Immiscible

Solvation

Solvation is the process of the solute particles breaking apart the solute crystal into particles the size of ions. If water is the solvent, this is accomplished by the polarity of the water molecule. One of the characteristics of a solution is that the particles of solute do not settle to the bottom of the container. This occurs because the particles (atoms and ions) are so small that they are unaffected by gravity.

The Tyndall Effect and Colloids

Another characteristic of solutions is that they do not exhibit the Tyndall Effect. Because the solute is broken down into ion sized particles, light waves are able to pass through a solution without being dispersed.

In a colloid, the breakdown of solute stops at the molecular size, and the larger particles disperse light and give colloids a characteristic cloudy appearance.

SolutionColloid

Aqueous Solutions of Ionic Compounds

When an ionic substance such as salt is solvated by water, the sodium and chloride ions are suspended in the water as charged particles, and the solution is said to be an electrolyte because it will conduct electricity.

One of the more commonly recognized electrolytes is Gatorade because it restores sodium and potassium to the human body and allows our nervous system to work efficiently.

Aqueous Solutions of Molecular Compounds

Molecular compounds such as sugar, that are covalently bonded, do not break apart into ions. Instead, the positive end of the hydrogen bond of the water molecules combine with the negative end of the hydroxide (OH) sites and pull apart individual molecules from the molecular crystal. This creates a neutral particle, and as a result, molecular compounds do not form electrolytes like ionic compounds do.

Factors that Affect the Rate of Solvation

Solvation can be increased by increasing the number of times atoms or molecules touch each other, and this is done by:

1. Agitating (mixing, stirring, shaking) the mixture

2. Increasing the surface area of the mixture

3. Increasing the temperature of the mixture

Solubility

Solubility refers to the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure. As a substance is being solvated, eventually a point will be reached where the particles of solute will begin to collide and reform the crystal. At this point, the solute is said to be in equilibrium, or saturated.

This solution is saturated because:

Solvation = Crystal Formation

Solubility Curves

Each substance has its own unique solubility, and chemical solubility is depicted on a solubility curve. A saturated solution contains the maximum amount of dissolved solute for a given amount of solvent at a given temperature and pressure.

Supersaturated

Saturated

Unsaturated

Solubility Curve Practice

1. Which salt is least soluble in water at 20°C?

2. How many grams of potassium Chloride can be dissolved in 200 g of water at 80°C? 3. At 40°C, how much potassium nitrate can be dissolved in 300 g of water?

Solubility Curve Practice

4. Which salt show s the least change in solubility from 0° to 100°C?

5. At 30°C, 90 g of sodium nitrate is dissolved in 100 g of water. Is the solution saturated, unsaturated, or supersaturated?

6. A saturated solution of potassium chlorate is formed from 100 g of water. If the solution is cooled from 80°C to 50°C, how many grams of precipitate are formed?

Solubility Curve Practice

7. What compound shows a decrease in solubility form 0°C to 100°C?

8. Which salt is most soluble at 10°C?

9. Which salt is least soluble at 50°C?

10. Which salt is least soluble at 90°C?

Supersaturated Solutions

A supersaturated solution is one that is holding more solute than a saturated solution at the same temperature and pressure. The diet coke and Mentos explosion is a good example of this. The CO2 that is present in a carbonated drink is much greater than the normal saturation level for CO2. When nucleated sites (Mentos) are placed into the carbonated drink, suddenly the CO2 “jumps” out of solution and the famous Mentos explosion occurs.

Temperature and Solubility

As temperature is increased, the solubility of a solid solute will generally increase.

As the temperature is increased, the solubility of a gas will decrease.

Pressure and Solubility

The solubility of any gas increases as the external pressure is increased; this phenomena is described by Henry’s Law. All carbonated beverages are based on this law. Once you “pop the top” on a Coke, the external pressure has been reduced, and the solubility of the CO2 goes down. This is why a Coke “goes flat” in a very short time.

Henry’s Law is described by the following equation:

S1 = S2

P1 P2

Henry’s Law Problems

1. If 0.55 g of a gas dissolves in 1.0 L of water at 2 atm of pressure, how much will dissolve at 4.5 atm of pressure?

2. A gas has a solubility of 0.66 g/L at 10 atm of pressure. What is the pressure on a 1.0 L sample that contains 1.5 g of gas?

Henry’s Law Practice Problems

3. If .68 g of a gas at 5 atm of pressure dissolves in 1.0 L of water at 25°C, how much will dissolve in 1.0 L of water at 8 atm of pressure and the same temperature?

4. A gas has a solubility of 1.46 g/L at 8 atm of pressure. What is the pressure of a 1.0 L sample that contains 2.7 g/L?

Solubility Curve