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Solutes and Solubility

Introduction

Chemical Reactions: Introduction

In this CELL, students are introduced to the physical property of solubility, the difference in solubility of different compounds, and the effect of varying conditions on solubility.

In its simplest form, matter is composed of elements. Elements cannot be broken down into smaller components through chemical reactions. However, atoms of the elements can combine with one another to form compounds. Compounds, like all matter, have both physical and chemical properties. Scientists use these properties to identify and classify compounds. A chemical formula is a chemist’s shorthand for designating and naming each chemical compound since the formula of each compound never varies. If it did, the formula would designate a different compound. The atoms composing each compound are always the same and always present in the same ratio. These ratios are indicated in the chemical formula. For example, the chemical formula for water is H2O. This means that there are two atoms of hydrogen for every atom of oxygen.

Adding more atoms to a reaction does not change the basic fact that the exact same number of each atom in the reactant side of a chemical reaction must also be present on the products side. Take the following chemical reaction, for example. This is a reaction between the base (high pH) ammonium hydroxide (NH4OH) and the acid (low ph) hydrochloric acid (HCl).

The reactants of the above chemical equation are ammonium hydroxide and hydrochloric acid. The products are water and ammonium chloride. Notice how we have colored each hydrogen (H) atom on both “sides” red. This may help you follow what happens to the hydrogen atoms during the reaction. Notice that there are 6 H atoms in the reactants and 6 H atoms in the products. This is consistent with the Law of Conservation of Matter (sometimes called the Law of Conservation of Mass). This law states that matter can neither be destroyed nor created, but it can change forms. Therefore, while there are 6 H atoms on each side of the above equation, the H atoms are in different forms. Let’s look at the other atoms in this reaction and see if the Law of Conservation of Matter hold true for them:

The stick models of the molecules of the reactants and products are shown at the top of this illustration. As can be seen, the Law of Conservation of Matter holds true for every atom in the reaction.  The reactants have 6 Hs, 1 N (nitrogen), 1 O (oxygen), and 1 CL (chloride). The exact same count is found in the products. Therefore, each atom has been conserved.

Students likely possess little background knowledge regarding atoms, subatomic particles, and the association of atoms and ions through ionic and covalent bonds. For this reason, the composition of matter and compounds in this CELL is described as that consisting of different types of particles. Chemical formulas are only referenced as a means to compare the different atoms and their numbers in the compounds used.

 

Different Compounds and Molecules May Contain the same Atoms

Different compounds often contain the same atoms and elements. However, the ratios of the elements in each compound are different. For example, table sugar (sucrose) and rubbing (isopropyl) alcohol are each composed of carbon, hydrogen, and oxygen. However, the chemical formula for sugar is C12H22O11, while the chemical formula for isopropyl alcohol is C3H8O. The three elements are present in different ratios in each compound, and this difference in ratios results in the distinctive differences between the two compounds.

 

Solubility

Solubility is a property that is specific to each chemical compound. It is dependent not only on the different atoms that compose an element but is also dependent on the arrangement of the atoms in the compound. The chemical formula of a compound represents the different atoms that compose a compound. The formula stipulates how many of each different type of element is present but really does not describe the arrangement of the atoms. Students will be asked to recognize that different compounds possess different formulas. What is not obvious to students is that the solubility of a compound cannot be assessed based simply on the chemical formula of a compound.

Solubility is the ability of a solute to dissolve in a solvent. This may include solids dissolving in solids or liquids, and liquids dissolving in gases, and gases dissolving on other gases. However, during this CELL, students will limit their exploration of solubility to that of solids dissolving in liquids. A liquid that contains a dissolved solid is called a solution (see the NaCl solution in the illustration below). A solute is the component in the solution which dissolves or is present in the smallest amount (NaCl in this example). A solvent is the component of a solution that does not change state or is present in the greatest amount (water in this example).

 

This CELL is limited to solutions in which the solvent is water. Water is unique among compounds because its structure includes both a negatively and a positively charged part (pole). The two hydrogen atoms of each molecule are grouped to one side of the oxygen atom (upper left of the illustration on the right above) resulting in a positive pole, while the negatively charged oxygen atom results in a negative pole.

The two oppositely charged poles provide the water molecule with unique properties as a solvent. Water molecules can interact with one another through weak bonds termed hydrogen bonds, in which a negative pole of one molecule interacts with the positive pole of a second molecule. The interaction of many molecules in this way results in water’s relatively high boiling point and its cohesiveness as a liquid.

Water can also interact in this way with solute molecules. The solubility of a solute results from the combination of breaking the interactions among solute molecules and the breaking of the interactions among solvent molecules. Replacing these interactions are new interactions between solute and solvent molecules. Water is able to interact with a variety of different solute molecules due to its polarity and the formation of hydrogen bonds and other weaker interactions.

 

Limits of Solubility: Saturation

The degree to which a solute is soluble in water is dependent on how effectively the solute-solute and the solvent-solvent interactions are disrupted and replaced by the solute-solvent interactions. If the disruption is limiting or the formation of the new interactions is limiting, solubility is low. Differences in these interactions determine the differences in solubility of each different solute in different solvents. Compounds may be soluble in one solvent and insoluble in another. Some solutes are not soluble at all in a particular solvent. These differences are directly related to the interactions described above.

A particular solid solute that is soluble in a particular liquid solvent such as water is not infinitely soluble, however. Usually, a solid solute reaches a limiting concentration beyond which the solvent molecules are not able to interact with that amount of solute molecules. At amounts higher than this limiting concentration, the solid solute will not dissolve. This limiting concentration is termed the saturation point.

Factors that Influence Solubility

The saturation point and the rate at which a solute dissolves can be affected by increasing the dispersion of the solute and solvent molecules by physical means such as stirring or by heating the mixture. In each case, the solute and solvent molecules are forced to move with greater kinetic energy resulting in a greater potential for interaction.

The ratio of solute to solvent is expressed in terms of concentration and is often represented as grams per milliliter. When concentration is represented as a percentage, such as a 10% salt solution, for example, the concentration is interpreted as grams per 100 ml. Therefore, a 10% salt solution has 10 g of salt for every 100 ml of solution. 

Investigation One introduces students to the quantitative property of solubility. Quantitative properties are those properties that can be measured or quantified. In Investigation One students quantitate the solubility of different chemical compounds by determining how many grams of each are soluble in a set volume of water.

Effective dispersal of the molecules or ions of a solute in a solvent aids the solubility and increases the rate of solubility of the solute. In Investigation Two, students will investigate the effect of stirring on the rate of solubility of a solute. In Investigation Three, students will investigate the effect of increasing temperature on the rate of solubility of a solute. Increasing the temperature increases the kinetic energy or movement of both the solute and solvent molecules. Increasing the movement of the molecules makes it more likely that a solute and a solvent molecule will interact with one another facilitating the solubilization of the solute.

Finally, the Performance Assessment allows students to utilize the procedural skills learned and the concepts studied in the previous Investigations to solve an open-ended problem. The problem is a scenario in which students are pharmacists who must maximize the concentration of both a children’s and a larger adult’s dose of a particular medication. Students must determine the best means to accomplish the task and discuss their reasoning for choosing their particular solution.

CONTENT

  • Fun Facts
  • Learn the Lingo
  • Get Focused

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FUN FACTS

Everyday Solubility

Can you think of times when solubility is involved in daily activities? How about adding sugar to tea? What is the first thing you do when you add a teaspoon of sugar to either a glass of iced tea or a cup of hot tea? You stir it, of course. By stirring the mix, you are increasing the kinetic energy by adding energy from your own hand and arm muscles.

Next, have you ever notice the difference in how fast sugar dissolves in tea depending upon whether it is hot or cold? In which case is it easier to get the sugar to dissolve, hot or cold tea? Hot, of course. This is because in addition to the kinetic energy you supply by stirring, temperature also greatly increases the amount of kinetic energy in the solution. Increased kinetic energy generally always increases both the rate and degree of solubility.

 

FUN FACTS

Conservation of Matter

The Law of Conservation of Matter or sometimes referred to as the Law of Conservation of Mass, was formulated by the brilliant eighteenth-century chemist Antoine Lavoisier.

The Law states that matter (or mass) cannot be created or destroyed. Matter, however, can change form. This is exactly what happens in chemical reactions. This is also the reason that, for every chemical reaction, we must account for each and every atom in both the reactants and products. This is one of the most important laws of chemistry and physics. 

Unfortunately, during the French Revolution (1789-1799), Lavoisier was arrested for his previous affiliation with the Royal government and beheaded by guillotine in Paris (1794) at the age of 50.

 

LEARN THE LABLEARNER LINGO

The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.

 

Investigation 1:
  • Solution: A mixture of two or more substances that is the same throughout the mixture
  • Solute: The substance that changes its state when a solution is created or that is in the smallest amount in a solution
  • Solvent: The substance in a solution that does not change its state when a solution is created or that is present in the largest amount
  • Dissolve: To completely mix with and become part of another substance
  • Soluble: The ability to be dissolved in another substance
  • Concentration: The amount of one substance in a specific volume of another substance; it is usually described as grams per milliliter
  • Solubility: The property of mixing and dissolving in another substance
  • Saturation point: A concentration above which the solute will no longer dissolve in the solvent
Investigation 2:
  • Properties: Characteristics of a substance that can be seen or measured
  • Rate: How fast something occurs per unit of time. An example is speed which is distance in meters traveled per minute.
Investigation 3:
  • There are no new Key Terms introduced in Investigation 3.

GET FOCUSED

The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, students should be able to answer the following questions:

Investigation 1:
  • What affects the solubility of a solute? 
  • Is there a relationship between the concentration of a solution and the saturation point of a solute? 
Investigation 2:
  • What affects the solubility of a solute? 
  • Can the rate of solubility of a solute be changed? 
Investigation 3:
  • What affects the solubility of a solute?

 

  • Can you change the rate of solubility of a solute?