Acids, Bases, and Solutions

Understanding Solutions A solution is a mixture with its particles uniformly spread throughout a solvent A solvent is the part of the solution that dissolves other substances The solute is a substance that is dissolved by the solvent

A solution has the same properties throughout It contains solute particles (molecules) that are too small to see and are suspended in the solvent Sugar dissolved in coffee is an example of a solution The coffee is the solvent and the sugar is the solute Solvent is water Solutes are sugar and flavoring

Salutes in a solution seem to disappear when they are mixed in a solvent When a solution forms, particles of the solvent surround and separate the particles of the solute

Solutes and solvents have different physical properties such as boiling point, melting point, color, size, density, and others Physical changes do not change the chemical properties of a substances Physical changes can often be undone to recover the original materials

Dissolving a solute into a solvent is an example of a physical change Physical properties can be used to recover the solute from the solvent Suppose you dissolve salt into water Because water has a lower boiling point than salt, the water can be boiled off leaving the salt behind

Solutes have various affects on solvents The addition of solutes to a solvent can lower its freezing point or raise its boiling point Ordinarily, the freezing point of water is 0°C By adding salt to water, the freezing point can be lowered to -4oC (That’s why salt is spread on roads to lower the freezing point in the winter…Oh! That’s right…California doesn’t have icy roads!

When liquid water freezes with the solute particles (salt) in it, the solute particles make it harder for the water molecules to form ice crystals The temperature must drop lower than 0°C for The solution to freeze Therefore, the presence of a solute in water lowers the freezing point of the water

The same is true for increasing the boiling point Solutes like salt raise the boiling point of water In a water solution, some of the molecules are water and others are particles of the solute The water molecules in a solution now need more energy to reach the boiling point By adding salt to the water decreases cooking time for food because the water is hotter

Car manufacturers make use of solutes to protect engines from heat and cold The coolant in a car’s radiator is a solution of water and another liquid called anti-freeze The mixture of the two liquids has a higher boiling point and lower freezing point then water alone The solution can reduce the risk of damage to the car from freezing and overheating

In many common solutions, the solvent is water Water dissolves so many substances that it is called The "universal solvent" For example, water is the solvent for sodas Food coloring, carbon dioxide, sugar, and flavoring are the solutes

Life depends on water solutions Nutrients used by plants and animals are dissolved in water within the cells Water is the solvent in blood, saliva, and tears

Many solutions are made with solvents other than water A solution may be a combination of gases, liquids, or solids For example Gasoline is the solution of several different liquid fuels Alloys are solutions of different metals Air is a solution of different gases

Colloids and Suspensions Not all mixtures are solutions Colloids and suspensions are mixtures that have different properties than solutions A colloid is a mixture that contains small, undissolved particles too small to be easily seen , but big enough to scatter light trying to pass through it Because the particles scatter the light, it is impossible to see through the mixture Milk is a colloid

A suspension is a mixture in which particles can be seen suspended (held up) in a liquid or gas The particles can be easily separated by settling or filtration The particles are visible and larger than the particles in solutions or colloids Snow globes and salad dressings are examples of suspensions

Ionic and Molecular Compounds in Solutions When an ionic compound mixes with water, the positive and negative ions are attracted to the polar water molecules Water molecules surround each ion as the ions leave the surface of the compound As each layer of the compound is exposed, more ions can dissolve Ions - - - - - - + + + + + + O + O + - - Water molecule - - - - -

Not every substance breaks into ions when dissolved in water A molecular compound, such as sugar, breaks up into individual neutral molecules The polar water molecules attract the slightly taller sugar molecules This causes the sugar molecules to move away from each other The covalent bonds within the molecules remain unbroken

Solutes and Conductivity Supposed to have a water solution, but you don't know if the solute is salt or sugar A solution of ionic compounds (salt) conducts electricity A solution molecular compounds (sugar) does not If the solution does not conduct electricity, no ions are present which means you have a sugar solution Salt or Sugar?

Concentration and Solubility Concentration is the amount of solute in a solvent The more solute, the greater the concentration It takes approximately 43 gallons of maple sap to make 1 gallon of maple syrup In order to get the sap sweet enough for pancake syrup, the water (solvent) in the sap must be removed to make a sweeter solution

Fruit juices are sometimes packaged as concentrates To make the concentrate, water (solvent) is removed from the natural juice When you prepare the juice to drink, you dilute the concentrate by adding back the water

Measuring Concentration To measure concentration, you compare the amount of solute to the total amount of the solution Concentration is measured as a percent of solute in the solution by volume or mass The mass of a solute or solvent is measured in grams The volume of a solute or solvent is measured in milliliters or liters

Solubility Solubility is the measure of how much solute can dissolve in a solvent at a given temperature There is a maximum amount of solute that can be dissolved in a solvent before the solvent becomes saturated and no more solute can dissolve

Working with Solubility You can identify a substance by its solubility Solubility is a property of matter If you had a white powder and could not tell if it was salt or sugar, you could identify it by its solubility Salt dissolves faster because salt molecules are smaller than sugar molecules Sugar

Factors Affecting Solubility The solubility of solutes change when conditions change The factors that affect solubility of a substance include PRESSURE, the TYPE of SOLVENT, and TEMPERATURE Example: Sugar will dissolve faster in hot tea than ice tea A sugar solution will become saturated (no more sugar will dissolve) in ice tea faster than hot tea Hot tea will dissolve much more sugar than ice tea before becoming saturated

Pressure Increasing the pressure increases the solubility of gases Soda water contains dissolved carbon dioxide gas To increase the carbon dioxide concentration in the solution, the gas is added under high-pressure Opening the bottle releases the pressure and makes the hissing sound you hear

Scuba divers must be aware of the effects of pressure on gases When divers breath compressed air, nitrogen from the air dissolves in their blood More dissolves as they go deeper If divers return to the surface too quickly, nitrogen bubbles form in the blood and block blood flow and divers double over in pain, which is why this condition is sometimes called “The Bends"

Solvents Some solvents and solutes are not compatible Your vinegar and oil salad dressing has to be shaken because vinegar and oil do not mix When you stop shaking the dressing, it quickly separates into layers of vinegar and oil

For liquid solutions, the solvent affects how well a solute dissolves Polar compounds are molecules that have a positive charge on one side and a negative charge on the other side Polar compounds dissolve in polar solvents Nonpolar compounds do not dissolve in polar solvents Example— Polar water-based paints clean up with soap and polar water Nonpolar oil-based paints require a nonpolar solvents such as turpentine

Waterand oil do not mix because water is a polar compound and oil is nonpolar Polar compounds a nonpolar compounds do not mix

Temperature Solubility increases as the temperature increases Example— At room temperature, not much sugar can dissolve in water The solubility of sugar in 100g of water changes from 180g at 0°C, to 231g at 25°C, to 487g at 100°C

Unlike most solids, gas in a liquid becomes less soluble when the temperature of the liquid increases Example— More carbon dioxide will dissolve in cold water than hot water Warm soda taste "flat“ because warm soda contains less carbon dioxide Fish love cold water because cold water can hold more oxygen

Describing Acids and Bases Properties of Acids Acids are compounds whose properties include the kinds of reactions they undergo An acid tastes sour, reacts with metals, carbonates, and turns blue litmus paper red

Reactions with Indicators Litmus paper is called an indicator It is made by coating strips of paper with Litmus which is made from plants Acids turn blue litmus paper red Other color litmus papers are used to indicate other substances

Some common acids are hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, and acetic acid

Sour taste Citrus fruits- lemons, grapefruit, oranges, and limes all contain citric acid Other fruits (cherries, tomatoes, apples) and many other types of foods contain acids, too

Scientists never taste chemicals in order to identify them You should never taste a substance unless you know it is safe to eat

Reactions with Metals Acids react with certain metals, such as magnesium, zinc, and iron, to produce hydrogen gas When they react, the metals seem to disappear in the acid solution This observation is one reason acids are described as corrosive, meaning they "wear away" other materials

Reactions with Carbonates A product of an acid's reaction with a carbonate is carbon dioxide Geologists use this property of acids to identify rocks containing limestone Limestone is a compound that contains the carbonate ion If you pour hydrochloric acid on limestone, bubbles of carbon dioxide appear on the rock's surface

Properties of Bases Bases are another group of compounds that can be identified by their properties A base tastes bitter, feels slippery, and turn red litmus paper blue Common bases include sodium hydroxide, calcium hydroxide and ammonia Red = Acid Blue = Base

Bitter Taste Bases are bitter The slight bitterness of soda is caused by the base quinine Soaps, shampoos and detergents are bitter too, but they are NOT safe to taste

Slippery Feel The slippery feel is a characteristic of bases Remember, don’t touch anything that you don’t know what it is Strong bases can irritate or burn your skin A safer way to identify bases is by using an indicator like litmus paper

Reactions with Inhibitors Since litmus paper can be used to test acids it can also be used to test bases Bases turn red litmus paper blue Litmus paper gives a reliable, safe test

Uses of Acids and Bases Acids and bases are almost everywhere Manufacturers, farmers, and builders are only some people who depend on acids and bases in their work Some acids are vitamins, including vitamin C and folic acid

Vitamins are essential in small amounts to normal growth and functioning of the body Many cell processes also produce acids as waste products for example— Lactic acid builds up in your muscles when you make them work too hard

Acids and Bases in Solution Acids in solution A hydrogen ion (H+) is an atom of a hydrogen that has lost its electron Hydrogen ions are the key to the reaction of acids

Acids in a water solution produce positive hydrogen ions These hydrogen ions form the properties of acids for example— When acid interacts with certain metal atoms, hydrogen gas (H2) is produced Hydrogen ions also react with blue litmus paper, turning it red

Bases in solution Not all bases contain hydroxide ions for example— the gas ammonia does not, but in solution, ammonia will react with water to form hydroxide ions A base produces hydroxide ions in water Hydroxide ions are responsible for the bitter taste and slippery feel of bases, and turn red litmus paper blue

Strength of acids and bases The strength of acids and bases refers to how well an acid or a base produces ions in water With a weak acid, very few ions form in solution At the same concentration, a strong acid produces more hydrogen ions Examples of strong acids include hydrochloric acid, sulfuric acid, and nitric acid Most other acids, such as acetic acids are weak

Measuring pH Knowing the concentration of hydrogen ions is the key to knowing how acidic or basic a solution is To describe the concentration of ions, chemist use a numeric scale called the pH scale The pH scale is a range of values from 0 to 14

Most acidic items are at the low-end of the pH scale A pH lower than 7 is acidic The most basic items are at the high-end of the scale A pH higher than 7 is basic If the pH is 7, the solution is neutral That means it's neither an acid nor a base Pure water has a pH of 7

A low pH indicates that the concentration of hydrogen ions is big In contrast, a high pH indicates that the concentration of hydrogen ions is low If you keep these ideas in mind, you can make sense of how the scale works

The pH of a solution can be determined by use of pH paper pH paper turns a different color for each pH value Matching the color the paper with the color of the test scale indicates how acidic or basic the solution is

Some indicator solutions will change color over the entire pH scale other indicators solutions only change color within a range of approximately two pH units Knowing the pH range over which this color change occurs gives you a rough estimate of pH Most chemistry laboratories contain a pH meter that makes rapid pH measurements

Using acids and bases safely Even a dilute solution of hydrochloric acid can eat a hole in your clothing In order to handle acids and bases safely, you need to know both their strengths and their concentration

Acid-base reactions After neutralization, an acid or base solution is less acidic or basic than either of the starting solutions The pH depends on the type, the volume, and the concentration of the reactants A solution with a small amount of strong base reacting with a much larger amount of strong acid will remain acidic When mixing a solution of strong acid containing hydrogen ions and a solution of strong base containing an equal amount of hydroxide ions a neutral solution will be the result

"Salt" may be a familiar name of the stuff you sprinkle on food But to a chemist a salt is any compound made from the neutralization of an acid with a base A salt is made from the positive ions of a base and the negative ions of an acid One product of a reaction of nitric acid with potassium hydroxide (base) is neutral water

The other products is potassium nitrate, and salt Potassium nitrate is soluble in water Some salts are insoluble and form precipitates