Salt, NaCl, is dissolved in water, H2O, at 25oC so that no solid remains. This solution is then heated in a flask until it boils. The vapor produced has a temperature of 120 oC and rises through a condensing tube to be condensed and collected in a separate container. Solid white crystals of salt remain in the flask after all of the liquid has boiled away. The boiling point of the solution was 105 oC instead of the normal 100 oC for pure water. This result made a student wonder how adding salt to water changes the boiling point (temperature) of water. To determine this, the class decided to dissolve 5 different amounts of salt into 100 mL of water and then measure the temperature at which each of the solutions boils.
1. Salts are ionic compounds made up of positive and negative ions.
a. Identify which element in sodium chloride, NaCl, is the positive ion and which is the negative ion.
b. In each case, explain what happens to the atoms of each element to form ions.
c. How does this relate to the number of valence electrons each element has?
2. Another salt is calcium chloride, CaCl2.
a. What is the charge on each ion in this compound?
b. When one molecule of this compound dissolves in water, how many total ions will be formed by dissociation?
c. How would you write the equation for this process?
d. How would the equation in c differ from the equation for the decomposition of CaCl2 into its elements (ex: why is the coefficient for and form of Cl different)?
e. Identify which of these processes is chemical verses which one is physical. (Write the balanced equation for each before trying to answer this.)
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