Formation of SolutionsChapter 8

Section One

Science Journal Entry

• Explain the difference between a solute and a

solvent.

Dissolving into Solution• Any state of matter can become part of a

solution. For a solution to form, one substance must dissolve in another.

• A solute is a substance whose particles are dissolved in a solution. The substance that dissolves a solute is called a solvent. In seawater, salt is the solute and water is the solvent. The air we breathe consist of the solvent nitrogen (78%) with 21% oxygen (O2) being one of the solutes.

The Bends• A diver’s air tank holds the same proportions

of 78% nitrogen and 21% oxygen.• Because of the pressure when descending,

nitrogen gas will dissolve in the blood and tissues of the body of a diver as he or she descends. If the diver comes up too quickly the nitrogen bubbles come out of the solution and are trapped in joints causing great pain. This condition is known as the bends.

Dissolving in Water• Substances can dissolve in water in one of

three ways: dissociation, dispersion, and ionization.

• Attractions that hold a solute together and the attractions that hold a solvent together must be overcome in order for the solute and solvent to be attracted to each other.

• Water being a polar molecule is attracted to the ions in a solute. The water molecules surround and then pull the ions of the solute apart. This is called dissociation.

Sugar and Water Dissolving

• Sugar dissolves in water by dispersion, or breaking into small pieces that spread throughout the water.• Both sugar and water are polar

molecules so they attract each other. Water molecules that are constantly moving collide frequently with the surface of the sugar crystals.

Dispersion of Molecular Compounds

• Attractions form between water and the exposed sugar molecules. When enough of these water molecules have surrounded the sugar molecule, the sugar molecule breaks free and is pulled into solution by the water molecules. • Then another layer of sugar molecules is

exposed and the process is repeated until the sugar is evenly dispersed throughout the water.

Dispersion of Sugar in Water

Ionization of Molecular Compounds• When HCl dissolves in water, the hydrogen

proton from each HCl gas molecule is transferred to a water molecule making each H2O become a hydronium ion H3O+ and a Cl- ion.

• The HCl and water form a solution, two molecular compounds react to form two ions. The process in which neutral molecules gain or lose electrons is known as ionization. This is the only process that is a chemical change.

Properties of Liquid Solutions• Three physical properties of a solution that can

differ from those of its solute and solvent are conductivity, freezing point, and boiling point.

• Although solid sodium chloride is a poor conductor of electric current, when it is in water it conducts electricity well since the ions are able to move more freely. Hydrogen chloride as a gas does not conduct electricity but when it ionizes in water it does conduct electricity.

Salting Roads with MgCl2

• Magnesium chloride, MgCl2 is applied to roads to lower the freezing point of water.• When magnesium chloride

dissolves in melting ice and snow, it dissociates into magnesium (Mg2+) ions and chloride (Cl-) ions.

How Does It Lower Freezing Point

• Ice forms when water molecules are able to arrange themselves in a rigid, hexagonal structure. The presence of magnesium and chloride ions interferes with the freezing process, disrupting the formation of ice crystals. This solution can have a freezing point as low as -15oC.

Boiling Point• Ethylene glycol, C2H6O2 added to

water is used as a coolant in most car radiators. • It prevents the engine from

overheating by raising the boiling point. • It also lowers the freezing point of

water so that the coolant does not freeze during spells of cold weather.

Heat of Solution• When a solution forms, attractions among

solute particles and the attractions between solvent particles must be broken. This requires energy so it is an endothermic process.

• As the solute dissolves, new attractions form between solute and solvent particles releasing energy so it is an exothermic process. The difference between these energies is called its heat of solution.

An Exothermic Solution Process• During the formation of a solution, energy is

either released or absorbed.• When sodium hydroxide, NaOH, dissolves in

water, the solution becomes warmer. In fact, one mole of NaOH in water releases 44.5 kj (kilojoules) of heat when new attractions are formed between NaOH and water (H2O). This is 44.5 kj more energy than is required to break the attractions among NaOH crystals and water molecules (H2O), its heat of solution.

Cold Packs• A cold pack has two sealed bags. The inner

thin-walled bag contains water while the outer bag contains solid ammonium nitrate (NH4NO3) powder.

• Squeezing the bag bursts the inner bag releasing the water to dissolve the ammonium nitrate. This powder absorbs energy from the water causing the temperature of the solution to drop rapidly.

Why Use a Cold Pack?

• A cold pack removes heat from the inflammation around an injury and decreases the size of capillaries (small blood vessels) in the injured area.• This reduces swelling and bruising.

Factors Affecting Rates of Dissolving• Factors that affect the rate of dissolving

include surface area, stirring and temperature.

• In much the same way as a chemical reaction, the rate of dissolving depends upon the collisions of particles within the solution. You can increase the surface area of a solid by dividing it into smaller pieces exposing more particles increasing the rate of dissolving.

Stirring and Increasing Temperature• Stirring moves dissolved particles away from

the surface of the solid, and allows for more collisions between solute and solvent particles.

• Increasing the temperature of a solvent causes its particles to move faster, on average.

• Both the number of collisions and the energy of these collisions with solute increases.