Substances, Mixtures, and Solubility

Chapter 8

Substances

• A substance is matter that has the same fixed composition and properties.

• A substance cannot be broken down into simpler parts by ordinary physical processes (boiling, grinding, filtering).

• Only a chemical process can change a substance into one or more new substances.

• An element is an example of a pure substance.

Compounds

• A compound is made of two or more elements that are chemically combined.

• Compounds have a fixed composition.• The ratio of atoms in a compound is always

the same.

Compounds

• Water is a substance because it stays the same chemically when frozen or boiled.

• Water is also a compound.

Mixtures

• Mixtures are combinations of substances that are not bonded together.

• The substances can be separated by physical processes.

• Mixtures do not always contain the same proportions of the substances.

Types of mixtures

• In a heterogeneous mixture the substances are not mixed evenly.

• Different areas of the mixture have different compositions.

Types of mixtures

• A homogeneous mixture contains two or more substances that are evenly mixed on a molecular level but the substances are not bonded together.

• A homogeneous mixtures can also be called a solution.

How do solutions form?

• The formation of a solution requires that a solute be dissolved in a solvent.

• A solute is the substance that disappears when added to another substance.

• The solvent is the substance that dissolves the solute.

How can a solid form from a solution?

• Sometimes a solid can come back out of its solution and form a solid.

• Crystallization occurs when the solvent evaporates, leaving a solid. This is the results of a physical change.

Forming solids from a solution

• Sometimes a solid is formed due to a chemical reaction that occurs when solutions are mixed together.

• The solid is called a precipitate.

• A precipitate is the results of a chemical change.

Types of solutions

• Solutions can be made up of different combinations of solids, liquids, and gases.

• Examples (see table 2, pg. 221)

Examples of common solutionsSolvent / State Solute / State State of Solution

Earth’s atmosphere Nitrogen / gas Oxygen / gasCarbon dioxide / gasArgon / gas

Gas

Ocean water Water / liquid Salt / solidOxygen / gasCarbon dioxide / gas

Liquid

Carbonated beverage

Water / liquid Carbon dioxide / gas

Liquid

Brass Copper / solid Zinc / solid Solid

Liquid solutions

• Liquid solutions can be either liquid, solid, or even a gas.

• Liquid-gas solutions– A gaseous solution and a liquid solvent– Carbonated beverages

• Liquid-liquid solutions– Both the solute and solvent are liquid– Vinegar : water (solvent) and acetic acid (solute)

Gaseous solutions

• Both the solvent and solute are gases.• The air we breath– 78 % nitrogen– 21% oxygen– 1% other gases

Solid solutions

• The solvent is a solid.• The solute can be a solid, liquid, or gas.• The most common solid solutions are solid-

solid solutions• Solid-solid solutions made from two or more

metals are called alloys– Steel (iron and carbon atoms)– Brass (zinc and copper atoms)

Solubility

• Water is often the solvent for many solutions.

• A solution in which water is the solvent is called an aqueous solution.

• Water is often referred to as the universal solvent.

Molecular compounds

• The sharing of electrons is called covalent bonding.

• Compounds that have covalent bonds are called molecular compounds (molecules).

Non-polar covalent compounds

• There is an even distribution of electrons in a non-polar covalent molecule.

• The atoms share electrons.

• The bonds between atoms are called non-polar covalent bonds.

Polar covalent compounds

• There is an uneven distribution of electrons in a polar molecule

• Atoms share electrons.• The bonds between

atoms are called polar covalent bonds.

• Water is a molecule with polar covalent bonds.

Ionic bonds

• In some atoms the electrons are not shared.• The atoms will either gain or lose electrons.• The number of protons and electrons in the

atoms becomes unequal.• The atom takes on a positive or negative

charge.• Atoms with a charge are called ions.

Ionic bonds

• Bonds between ions that are formed by the transfer of electrons are called ionic bonds.

How does water dissolve ionic compounds?

• Remember, water has positive and negative charges parts.

• Positive part will attract negative ions.• Negative part will attract positive ions.• When an ionic compound is mixed with water,

the different ions are pulled apart by the water molecule.

How does water dissolve molecular compounds?

• Water can dissolve molecular compounds.• However, the water does not break the

molecule apart, but moves between the molecules.

• The water separates the molecules of the compound.

Likes dissolve likes

• This means that polar solvents dissolve polar solutes and non-polar solvents dissolve non-polar solutes.– Sugar and water: both are made up of polar molecules– Salt and water: Na and Cl ion pair is similar to the water

molecule, both having + and - charges• If a solvent and solute are not similar, the solute

will not dissolve . Non-polar molecules will not dissolve polar molecules.– Oil and water

How much will dissolve?

• Solubility is a measurement that describes how much solute dissolves in a given amount of solvent at a given temperature.

• Substances that have extremely low solubility are usually considered insoluble.

Solubility of liquid-solid solutions

• Temperature is included as an explanation for how much solute will dissolve in a given amount of solvent.

• The solubility of many solutes changes as temperature changes.

• Increasing the temperature of the solute will often times increase the amount of solute that will dissolve.

Solubility in liquid-gas solutions

• In these solutions, an increase in temperature decreases the solubility of the gas.

• Gases are often less soluble in a warm solution.

Saturated solutions

• A solution that contains all the solute that it can hold under the given conditions is called a saturated solution.

• In a liquid-solid solution, any extra solute that is added will only settle to the bottom of the container.

• Some solutions can become supersaturated, meaning it contains more than the normal amount of solute.

Rate of dissolving

• Several factors, including time, motion, temperature, and contact area, affect the rate of dissolving.– Temperature– Stirring or shaking

Concentration

• The concentration of a solution is how much solute is present compared to the amount of solvent.

• Concentrated solutions have more solute per given amount of solvent than dilute solutions.

• Concentrations can be stated as a percentage of the volume of solution that is solute.

Acid solutions

• Acids are substances that dissolve in water and release positively charged hydrogen ions.

• When an acid mixes with water, the acid dissolves, releasing the hydrogen ion.

• The hydrogen ion combines with water molecules to form positively charged hydronium ions (H3O+).

Properties of Acid solutions

• Acids taste sour, conduct electricity, are corrosive, and reactive with certain metals.

• Acids have a pH between 0 and 7.

What does pH mean?

• pH is a measure of how acidic or basic a solution is and relates to the concentration of hydronium ions and hydroxide ions.

What is the pH scale?

• The pH scale ranges from 0 to 14. • Acidic solutions have pH values below 7. – A solution with a pH of 0 is very acidic .

• Hydrochloric acid has a pH of 0.

• A solution with a pH of 7 is neutral, meaning it is neither acidic or basic. – Pure water is neutral.

• Basic solutions have pH values above 7. – A solution with a pH of 14 is very basic.

• Sodium hydroxide can have a pH of 14.

What does a change in pH mean?

• A change in pH represents a 10 fold change in the acidity of the solution.

• Example - if one solution has a pH of 1 and a second solution has a pH of 2, the first solution is not twice as acidic as the second, but 10 times more acidic than the second solution.

Calculating change in pH

• Use the following calculation to determine the difference in pH strength - 10n, where n = the difference between pHs.

• Example - pH3 - pH1 = 2, 102 = 100 time more acidic.

How is the strength of an acid determined?

• The strength of an acid is related to how easily the acid separates into ions, or how easily a hydrogen ion is released when the acid dissolves in water.

• More hydronium ions means the strong-acid solution has a lower pH than the weak-acid solution.

• The pH of a solution is more acidic when greater amounts of hydronium ions are present.

How us is the strength of a base determined?

• The strength of a base is related to how easily the base separates into ions, or how easily a hydroxide ion is released when the base dissolves in water.

Strength of acids and bases

• An acid containing more hydrogen atoms is not necessarily stronger than an acid containing fewer hydrogen atoms.

• An acid’s strength is related to how easily a hydrogen ion separates, not how many hydrogen atoms it has.

• Example - nitric acid (HNO3) is stronger than carbonic acid (H2CO3)

Examples of acid solutions

• Vinegar (made from acetic acid) is used in salad dressings.• Vitamin C (ascorbic acid) comes from lemons, limes, and

oranges.• Ants inject formic acid into their victims.• Sulfuric acid is used in fertilizers, steel, paints, and plastics.• Hydrochloric acid (commercially called muriatic acid) is

used in pickling (process that removes impurities from the surface of metals).

• Nitric acid is used in the production of fertilizers, dyes, and plastics.

Acid solutions

• Acids are found in foods such as lemons and oranges that contain ascorbic acid and are used in many products such as batteries and plastics.– Sulfuric acids is used in fertilizers, steel, paints,

plastics.– Hydrochloric acid (muriatic acid) used in process

called pickling.– Nitric acid is used in the production of fertilizers,

dyes, and plastics.

Acids

• Stalactites and stalagmites are formed when carbon dioxide in soil dissolves in water, helps form caves, stalagmites, and stalactites.

Base solutions

• When bases dissolve in water, some hydrogen atoms from the water molecule are attracted to the base.

• A hydrogen atom in the water molecule leaves behind the other hydrogen atom and oxygen atom.

• This pair of atoms is a negatively charged ion called a hydroxide ion.

• A hydroxide ion has a formula of OH-. • Most bases contain a hydroxide ion which is released

when the base dissolves in water.

Bases

• Bases are substances that accept hydrogen ions; when dissolved in water, a hydroxide ion forms.

• Bases taste bitter, feel slippery, are corrosive, and conduct electricity.

• Soaps, cleaning products and blood in the human body are basic.

What is an indicator?

• Indicators are compounds that react with acidic and basic solutions and produce certain colors, depending on the solution’s pH.

• Because there are different colors at different pHs, indicators can help determine the pH of a solution.

• Litmus is a indicator that is soaked onto paper. • When litmus paper is placed in an acidic solution, it

turns red. • When the litmus paper is placed in a basic solution,

it turns blue.

What is neutralization?

• Neutralization is the reaction of an acid with a base. • When acidic and basic solutions react, hydronium

and hydroxide ions react to form water. • Through neutralization the properties of both the

acid and base are diminished, or neutralized. • When one hydronium ion reacts with one hydroxide

ion, the product is two water molecules.