Precip Essay

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Information about the cations-anion Lab Experiment The first lab is essentially a simple reaction and the the only things you report to me are the observations of the color of the solid formed and its weight. The experiment involves some compounds that are ionic compounds. As chapter 3 describes these compounds are composed of ions which are atoms or groups of atoms with a net positive or negative electrical charge due to an imbalance in the numbers of protons and electrons. Positive ions, known as cations, have more protons than electrons; hence, a net + charge. Negative ions, known as anions, have more electrons than protons; hence a net – charge. Ionic compounds in the solid state are held together by the attraction between their cations and anions. Many ionic compounds are soluble (dissolve) in water as the water has the ability to “pry” the ions apart. An example is ordinary salt, NaCl composed of Na+ and Cl- ions. We can say that when added to water, NaCl(s) → Na+(aq) + Cl-(aq) where the (s) denotes solid and the (aq) for “aqueous” denotes dissolved in water. However, there are a number of ionic compounds that cannot be “pried” apart by water and they thus do not dissolve in water (they are insoluble). In experiment 1, PbCrO4 composed of Pb2+ ions and CrO42- ions is insoluble. Now if the ions that compose an insoluble ionic compound are in the same solution from other sources they will unite to form the insoluble solid compound. The solid is called a precipitate and the formation of the solid is called precipitation. The first experiment mixes together water solutions of two soluble salts, potassium chromate,. K2CrO4, and lead(II) nitrate, Pb(NO3)2. These are respectively composed of K+ and CrO42+ ions and Pb2+ and NO3- ions. Note that ionic compound formulas must be such that the magnitudes of the total + and – charges must add up to zero. For example

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