Solutions

Nature of Solutions

What happens when sugar is poured into a cup of hot water? Gradually, the sugar seems to disappear. The sugar dissolves, or breaks apart, in the water. The molecules of sugar separate and mix with the water molecules. In time, they become uniformly mixed into a solution of water and sugar.

A solution is a mixture in which one substance is completely and evenly dissolved in another. In contrast to heterogeneous mixtures, where substances do not combine, homogeneous solutions have substances that are evenly distributed so every drop or granule of the mixture is exactly the same as the others. Solutions do not settle into layers, as water and sand might, and they are usually transparent or evenly colored. They often look like single, pure substances, a characteristic that makes them difficult to recognize. However, if you tasted a solution of water and sugar, or water and salt, you would notice at once that it was a solution because of the sweet or salty taste.

Although many common solutions are liquids solutions can exist as all three states of matter: solids, liquids, and gases, or some combination thereof. A teardrop is a solution of water, salt, and other compounds. A steel bridge is a solution of iron, carbon, and other elements. Air is a gaseous solution made up of 78 percent nitrogen, 21 percent oxygen, and I percent other trace gases. Brass is a type of solid solution called an alloy that is made from a mixture of copper, zinc, and other materials. An alloy is a solid solution in which the atoms of two or more metals are uniformly mixed.

The Parts of a Solution

Every solution consists of two parts-a solute and a solvent. The solute is the substance that is dissolved in a solution, such as sugar or salt. The solvent is the substance that does the dissolving in a solution, such as water or another liquid. The solvent makes up the larger amount of a solution.

When sugar is dissolved in water, sugar is the solute and water is the solvent. In a solution of air nitrogen is the solvent and oxygen is the solute.

Solutions form as a result of the interactions between the particles of a solvent and the particles of a solute. The solvent breaks apart the particles of the solute to make the solution. In order for a solution to form, solute particles must separate from one another, and the solute and solvent particles must mix.

Aqueous Solutions

When the solvent in a solution is water, the solution is called an aqueous solution. The word aqueous means “related to water.” Fruit punch, tea, contact lens solution, liquid soap, and vinegar are examples of aqueous solutions. Water is the solvent in many solutions. Because many different kinds of substances can dissolve in water, it is known as the “universal solvent.”

Think about Science

Directions: Answer the following questions

  1. In sweet tea, which of these are the solutes? A. Natural flavors B. Water and sugar C. Particles from tea leaves and sugar D. Water and particles from tea leaves

The Solution Process and Solubility

The solution process involves the dissolving of a solute in a solvent. Water will form a solution with two different types of compounds: ionic compounds or polar covalent compounds. Ionic compounds, such as salt, are made of a positive and a negative ion held together. Polar covalent compounds, such as sugar, consist of atoms held together by the sharing of electrons. Because of the way the electrons are shared in polar covalent compounds, the molecules have two sides with opposite charges, positive and negative.

Dissolving Ionic Compounds

Water itself is made up of polar molecules that attract charged particles. In a solution of saltwater, the positively charged ions of salt (Na+)are attracted to the partially negative oxygen atoms of the water molecules. The negatively charged ions of salt (Cl) are attracted to the partially positive hydrogen atoms of the water molecules. As the ions are pulled toward the oppositely charged ends of the water molecules, the force that holds them weakens. The compound dissociates, or splits, into its individual ions that mix uniformly with the water, making saltwater.

Dissolving Covalent Compounds

When water dissolves polar covalent compounds, the compounds generally do not dissociate, as do ionic compounds. They become separated from one another. For example, when polar molecules such as sugar dissolve in water, the molecules do not split. The molecules are pulled from one another, surrounded by water molecules, and then evenly distributed in the water.

Rate of Dissolving

Different solutes dissolve in different solvents at different rates. Some solutes dissolve quickly and others slowly. Rate of dissolving is a measurement that describes how quickly a solute dissolves in a given solvent. The rate at which a solute dissolves is affected by several factors: the type of solvent involved, stirring, surface area, and temperature.

Type of Solvent

Polar solutes and ionic solutes dissolve in polar solvents. Nonpolar solutes dissolve in nonpolar solvents. This general rule can be summarized as “like dissolves like.” Recall that salt and sugar can dissolve in water because they are “like” water- they are polar. However, oil, which is nonpolar, will not be able to dissolve in water, a polar solvent. These are “unlike.”

Stirring

Shaking or stirring a solution helps the solute dissolve more quickly and increases the rate of dissolving. Moving the molecules around helps bring fresh solvent in contact with a solute. That is why stirring iced tea after you put sugar into it will sweeten the tea faster.

Surface Area

Breaking a solid into smaller pieces will increase the rate of dissolving in the same way that it increases its rate of reaction. Dissolving occurs at the surface of a solid. Breaking up a solid increases the solid’s surface area, which increases the amount of solute that comes into contact with the solvent. This causes the solute to dissolve more quickly. For example, if you drop a sugar cube into iced tea, it will take a while for it to dissolve. You can sweeten the tea more quickly by using an equal amount of granular sugar. The pieces are much smaller, so a much greater number of sugar molecules will be in direct contact with the water molecules of the tea.

Surface Area

Temperature

Increasing the temperature of the solvent will increase the rate of dissolving. The increased temperature causes both the solute and solvent particles to come in contact with one another more quickly and more frequently, The particles move around and interact with one another at a faster rate, helping bring fresh solvent in contact with the solute.

Solubility

Some substances will not form a solution no matter how much they are stirred, shaken, or heated. Such a substance is insoluble in that solvent. Vinegar is insoluble in oil, and oil is insoluble in water. When a substance can dissolve in a solvent, the substance is soluble. Sugar, salt, some vitamins, and minerals are soluble in water.

Solubility is the maximum amount of a solute that can dissolve in a given amount of a solvent under a given set of conditions, like temperature or pressure. If a solute has a high solubility, a large amount of the solute can dissolve under given conditions. If a solute has a low solubility, only a small amount of the solute can dissolve under the conditions. When a solute has an extremely low solubility, it is considered insoluble. Solubility is usually expressed in grams of solute per 100 g of solvent as shown in the table.

Solubility of Substances in Water at 20° C

Solid Substances Solubility in g/100 g of Water
salt ( sodium chloride ) 35.9
baking soda ( sodium bicarbonate ) 9.6
sugar ( sucrose ) 203.9
Gaseous Substances * Solubility in g/100 g of Water
oxygen 0.0005
carbon dioxide 0.16

* at normal atmospheric pressure

Rules of Solubility

Conditions such as temperature and pressure can affect the solubility of a substance by either increasing or decreasing its solubility. For example, the solubility of liquids and solids increases as temperature increases. In contrast, the solubility of gases in liquid solvents decreases as temperature increases. Although pressure has no effect on liquid and solid solutes, it can affect the solubility of gaseous solutes in liquid solvents. Increasing pressure causes the solubility of gases in liquid solvents to increase, and decreasing pressure causes the solubility to decrease. For example, carbonated beverages are bottled under pressure to keep carbon-dioxide gas in solution. When a bottle is opened, the pressure is released and the gas bubbles out of the solution.

Describing Solutions

A solution can contain different proportions of solute and solvent. For example, a glass of lemonade may be sour or sweet. For this reason, scientists use concentration and solubility to describe solutions more precisely.

Using Concentration

Concentration is the amount of solute that is dissolved in a quantity of solvent. If there is a little solute in a solution, the solution is dilute. If there is a lot of solute in a solution, the solution is said to be concentrated.

Concentration can be described more precisely by providing the proportions of the substances in the solution. A solution of cranberry juice, for example, might have 100 percent juice or only 10 percent juice.

Using Solubility

Scientists use solubility to describe how much solute is in a solution. Saturation is when a solution contains the maximum amount of dissolved solute it can hold at a given temperature. If more solute is added to a saturated solution, the solute will not dissolve. For example, when adding sugar to tea, there will come a time when the sugar starts to accumulate at the bottom of the glass rather than dissolve. The tea is saturated.

When a solution contains less than the maximum amount of solute it can hold at a given temperature, the solution is unsaturated. More solute will dissolve if it is added to the solution under the existing conditions. When a solution contains more dissolved solute than a saturated solution under the same conditions it is supersaturated, or is “more than saturated.”

Think about Science

Directions: Fill in the blank.

  1. [ blank ] and [ blank ] are two factors that can affect solubility.
  2. Scientists describe solutions using either [ blank ] or [ blank ].
  3. When a solution contains more dissolved solute than a saturated solution, the solution is called [ blank ].

Acids and Bases

There are different definitions of acids and bases, but in all cases, acidic and basic solutes have specific results when they form solutions in water. In aqueous solutions, acids increase the hydrogen (H+) concentration, and bases increase the hydroxide (OH-) concentration.

Acids

An acid is a compound that, when dissolved in water, will produce positively charged hydrogen ions (H+). Acids taste sour, are highly reactive, and can corrode metals. Vinegar, orange juice, lemon juice, batteries, and the body’s digestive fluids all contain acids. The strength of an acid is determined by the degree to which the acid dissociates in solution. An acid is considered strong if nearly all the molecules are converted into ions in water. An acid is considered weak if only a small fraction of the molecules dissociate in water.

Bases

When dissolved in water, a base forms hydroxide ions (OH-), a negatively charged compound made of one oxygen atom and one hydrogen atom. Bases are able to take a proton from an acid or to give up an unshared pair of electrons to an acid. Bases are described as alkaline and they dissolve in water and have a slippery feel. Many hydroxides are bases. Household cleaning agents such as ammonia, borax, lye, and detergents are common examples of bases. When an acid combines with a base, a salt forms, and water is released because the metal found in the base replaces the hydrogen contained in the acid. Inorganic acids, bases, and inorganic salts can conduct electricity when dissolved in water.

Chemists apply a litmus test to a substance to determine whether it is an acid or a base. An acid turns blue litmus paper red, and a base turns red litmus paper blue. When you combine acids and bases, they neutralize each other.

pH Level

Acids and bases vary in strength, as do solutions containing acids and bases. Some solutions might have a high concentration of hydrogen ions and thus would be very acidic, whereas others might have only a small concentration of hydrogen ions and thus would be much less acidic. To determine how acidic or alkaline (basic) a solution is, scientists use a measurement called pH. The pH of a solution is a measure of the concentration of hydrogen ions in the solution. The greater the hydrogen-ion concentration, the more acidic the solution, and the lower the pH level. The lower the hydrogen-ion concentration, the more alkaline the solution and the higher the pH level. To indicate the pH level, scientists use a pH scale that ranges from Oto 14. If a solution has a pH less than 7, it is considered acidic, and if a solution has a pH greater than 7, it is considered alkaline. If a solution has a pH of exactly 7, it is considered neutral, which means it is neither acidic nor alkaline because it has an equal number of hydrogen and hydroxide ions. Pure water is neutral.

Acid-Base Reactions

Acids and bases can react with each other. When an acid is added to a base solution, their hydroxide ions and hydrogen ions combine to form molecules of water. The other ions combine to form a salt. A salt is an ionic compound formed from the negative ions of an acid and the positive ions of a base. The reaction of an acid with a base is called a neutralization reaction. This is because the products formed are neutral- they are neither acids nor bases. The general formula for an acid-base reaction is:

Here is an example.

This example shows the formation of table salt. Although people often think only of table salt when they hear the term salt, there are hundreds of different salts that can form when an acid reacts with a base.

Think about Science

Directions: Answer the following questions.

  1. What could be the pH level of an alkaline solution? A. 2 B. 5 C. 7 D. 13
  2. What is a solution that has excess hydrogen ions? A. acid B. base C. neutral D. saturated