Aqueous Solutions – Water as a Dissolving Medium

Chemistry Aqueous Solutions – Water as a Dissolving Medium Information: Water is one of the most important substances on earth. One of the most valuable functions of water involves its ability to dissolve many different substances. A macroscopic example of dissolving a substance would be the “disappearing” of salt or sugar when it is sprinkled into water. A solution, in which water is the dissolving medium, or solvent, is called an aqueous solution. In chemistry it is important to understand the nature of substances and how they interact with one another at a submicroscale level. At this level we cannot see things directly and instead must create models that describe what we can see at the macroscale level. In order to understand what happens when a solid dissolves in water we first need to understand the nature of water. Liquid water consists of a collection of H2O molecules. One single H2O molecule is “bent” or V-shaped, with an H-O-H angle of 104.5°. Two separate MODELS of the water molecule are provided below: Ball and Stick Model Space Filling Model The O-H bonds in the water molecule are covalent bonds formed by the sharing of electrons between the oxygen and hydrogen atoms. However, the electrons are not shared equally between the hydrogen and oxygen atoms. The oxygen atom has a much greater attraction for electrons than the hydrogen atom and therefore the shared electrons tend to spend more time closer to the oxygen atom than to either of the hydrogen atoms. This causes the oxygen atom to gain a slight excess of negative charge and causes the hydrogen atoms to become slightly positively charged. This concept is represented in the ball and stick model above where the Greek letter delta (δ) indicates a partial charge (less than one full unit of charge). Due to this unequal charge distribution, water is said to be a polar molecule - a molecule that has an uneven distribution of charge. Polar water molecules are attracted to other polar substances and to substances that are composed of electrically charged particles. Questions: 1. Provide a definition for the term solvent. 2. What kind of mixture is an aqueous solution (homogeneous or heterogeneous)? Is this always the case? Explain. 3. In the ball and stick model provided above, what does the δ indicate? 4. In the ball and stick model above 2δ- is next to oxygen and δ+ is next to each of the hydrogen atoms. Explain what this notation indicates about the water molecule. 5. Is the water molecule positively charged, negatively charged, or neutral? Support your answer with an explanation. 6. What does it mean when a substance is described as polar? Would sodium chloride be considered polar? Explain. 7. A sodium chloride crystal is a closely packed repeating sequence of the sodium chloride formula unit. The sodium chloride formula unit can be described using the following model: + - Describe what each spheres in the model represents? Why is one sphere (+) and the other sphere( -). Explain. 8. Using the model from question #7 and the model for water from the information section of this POGIL describe how water may interact with a sodium chloride crystal at a sub-microscale level. Draw a diagram using the models to illustrate your answer. 9. Based on your answer to question #8, describe why a substance may dissolve in water and “disappear”. 10. Based on your answers to questions 8 and 9 describe why a substance may not dissolve in water and remain visible in its original form.