Questions A solid substance conducts electricity when melted. Is it most likely composed of molecules or ions? Explain your answer. Click here to view answer. According to the text (pages 14, 22), pure water is composed of H2O molecules. This suggests that water does not conduct electricity. Why is it dangerous to go swimming in a lightning storm? Click here to view answer. Why must all chemical equations be balanced? Click here to view answer. On page 24, the reaction of aqueous sodium chloride and aqueous lead(II) nitrate is shown. Describe, on a microscopic scale, what the ions are doing as the reaction proceeds to make lead(II) chloride, a white solid. Click here to view answer. Sodium chloride is a well-known ionic compound. Classify the following compounds as molecular or ionic and explain your answers. Sodium bromide, sodium iodide, potassium chloride, hydrogen chloride, carbon tetrachloride, calcium chloride. Click here to view answer. Hydrochloric acid, or aqueous hydrogen chloride forms an ionic solution when dissolved in water, yet is gaseous in its pure form, like a molecular compound. Explain this apparent discrepancy. Click here to view answer. Dalton's theory, while one of the cornerstones of modern chemistry, is incomplete. Suggest some of the modifications that have been necessary since its introduction in the early 19th century. Click here to view answer. What information about a chemical reaction can be obtained from a balanced chemical equation? Click here to view answer. Answers (Page 9, 15-18) The solid is most likely composed of ions. To conduct electricity, the movement or distribution of electrical charges is required. Molecules are composed of discrete neutral particles and therefore cannot conduct electricity. Ionic solids are composed of separate positively and negatively charged particles, thus providing the distribution of electrical charges required for conducting electricity. Click here to return to question. (Pages 14-23) Because it is composed entirely of neutral molecules, pure water does not conduct electricity. The water in a swimming pool, lake or the ocean is not pure; it contains dissolved ionic compounds. These dissolved ionic compounds cause the water to conduct electricity. Click here to return to question. (Pages: 7-8, 22-23) Dalton's theory states that chemical reactions are merely rearrangements of atoms. As a result, atoms cannot be created from nothing or destroyed, just re-organized. The products in a reaction must therefore have the same number and kind of atoms as the reactants. When we study stoichiometry in Chapter 3, we will realize that a balanced equation is the first step in working most quantitative chemical problems. Page 22 offers an explanation based upon models. Click here to return to question. (Page 24) First, when the aqueous solutions of sodium chloride and lead(II) nitrate were made, solids were dissolved in water. This causes the solid crystals to separate into the component ions. When the solutions are mixed, the lead(II) ions and the nitrate ions come together to form the solid lead(II) nitrate. See Sample problem 1.8 on page 24. Click here to return to question. (Pages 13-18) Sodium bromide and sodium iodide are both ionic, as they are similar in context to sodium chloride, being composed of a metal and a non-metal. Calcium chloride is also ionic, as it is composed of a metal and a non-metal. Carbon tetrachloride is molecular, as it is composed of two non-metals. Hydrogen chloride (see question 6 below) is molecular in its pure form, as hydrogen can act as a non-metal; in solution it is ionic, as hydrogen can act as a metal in this case. In chemistry, as in most fields of study in college, few answers are as straightforward as they seem on first inspection. Click here to return to question. (Page 12) The properties of hydrogen chloride arise from the special properties of hydrogen. In gaseous hydrogen chloride, which is a molecule (all common gases are molecules) hydrogen must have the properties of a non-metal. In chapter 9, when we study covalent bonding, we will explore this in more detail. In solution, the hydrogen chloride molecules dissociate to form hydrogen ions and chloride ions. Hydrogen is acting as a metal in this case. Click here to return to question. (Page 8) Consider each of the points of Dalton's theory separately. Click here to return to question. An atom is the smallest particle of an element that takes part in chemical reactions. This has stayed essentially true. While later we discovered subatomic particles, reactions involving these are not generally considered chemical reactions. The major exception is in the study of ions, most of which are atoms with one or more electrons removed. All atoms of a given element are alike. The existence of isotopes (see Chapter 2) discounts this. Most elements have several forms with the same number of protons and electrons and different number of neutrons. Compounds are combinations of atoms of more than one element. In a given compound the raltive number of each type of atom is the same. This has remained unchanged. Atoms cannot be created nor destroyed. Atoms of one element cannot be changed into atoms of another element by chemical reactions. This has also remained true, although nuclear reactions (See Chapter 20) can be used to change one element into another. While these are not chemical reactions, we will study them anyway. (Pages 22-25) A balanced chemical equation provides a wealth of information about a reaction. First, it gives the composition of the reactants and products. Since the number of atoms of each element must be the same on both sides of the equation, it also gives the relative amounts of reactants and products in the reaction. It also may give information about the reaction conditions, or special conditions, such as the need for electricity, as described on page 22, or a specific temperature require for the reaction. Starting in Chapter 3, we will use balanced chemical equations to solve both quantitative and qualitative chemical peoblems. Click here to return to question.