Chapter 20

The Dissolving Process

Part 2

Rate of Solution• I want to

dissolve a block of sugar as quickly as possible.

• What should I do?

Rate of Solution

The rate at which a material dissolves can be affected by:

1. Surface area

Rate of Solution

The rate at which a material dissolves can be affected by:

2. Agitation (stirring)

Rate of Solution

The rate at which a material dissolves can be affected by:

3. Temperature

• Temperature not only affects how quickly we can dissolve a solute but also how much solute can be dissolved.

Factors Affecting SolubilityFactors Affecting SolubilityTemperature EffectsTemperature Effects

Temperature and Solubility of Solids

Temperature Solubility (g/100 g H2O)

KCl(s) NaNO3(s)

0° 27.6 74

20°C 34.0 88

50°C 42.6 114

100°C 57.6 182

Temperature and Solubility of Solids

Temperature Solubility (g/100 g H2O)

KCl(s) NaNO3(s)

0° 27.6 74

20°C 34.0 88

50°C 42.6 114

100°C 57.6 182

The solubility of most solids increases with an increase in the temperature.

Temperature and solubility

Factors Affecting SolubilityFactors Affecting SolubilityTemperature EffectsTemperature Effects

Temperature and Solubility of Gases

Temperature Solubility (g/100 g H2O)

CO2(g) O2(g)

0°C 0.34 0.0070

20°C 0.17 0.0043

50°C 0.076 0.0026

Temperature and Solubility of Gases

Temperature Solubility (g/100 g H2O)

CO2(g) O2(g)

0°C 0.34 0.0070

20°C 0.17 0.0043

50°C 0.076 0.0026

The solubility of gases decreases with an increase in temperature.

Thermal Pollution

Thermal Pollution

• Thermal pollution can occur when heated water is discharged into cooler streams or rivers. This heated water can increase biological activities and result in the reduction in oxygen content of the water.

Unsaturated Solutions

• Contains less than the maximum amount of solute.

• No undissolved solute present.

• Can dissolve more solute.

Dissolved solute

Saturated Solutions

• Contains the maximum amount of solute that can dissolve.

• Tend to have undissolved solute at the bottom of the container.

• Solution is in equilibrium with undissolved solute.

What is the solubility of NaCl at room temperature (25°C)?

Solubility ≈ 36g NaCl/100g of water

How could I dissolve the extra 4.0g of NaCl without adding more water?

Heat up the water to ≈ 90°C

Solubility ≈ 40g NaCl/100g of water

If we would cool the water back to room temperature we may create a supersaturated solution.

Supersaturated• A supersaturated solution contains more than the maximum

amount of solute and is unstable. It returns to a saturated solution if disturbed in some way (i.e.: a seed crystal)

Pressure and Gas Solubility

• More gas can dissolve at higher pressure.

Nitrogen Narcosis

Nitrogen Narcosis

• This is a condition that can result from diving too deep, bringing on disorientation, euphoria, errors in judgment and even hallucinations or unconsciousness. Any of those can be fatal.

The Bends

The Bends• A sometimes fatal condition resulting from

the formation of nitrogen bubbles in the blood and tissues, because of too rapid ascent, seen especially in deep-sea divers ascending rapidly from a dive.

• August 21, 1986

• Lake Nyos – Cameroon,

Africa

• 1800 people, thousands of cattle, and many more birds and animals are found dead.

Lake Nyos

Homework

• Worksheet: The Dissolving Process