cations (QA)

• Qualitative analysis in chemistry involves finding out what substances are present in a solution or mixture
• There are two main types of analysis in chemistry: qualitative and quantitative
• Quantitative analysis aims to determine the amount of a particular substance present
• Qualitative analysis aims to identify what substances are present in a mixture
• For qualitative analysis, we look for cations, anions, and gases in a solution
• In qualitative analysis, we use specific chemical tests to identify the presence of cations, anions, and gases in a solution
• To identify cations in a solution, we can add either aqueous sodium hydroxide or aqueous ammonia
• When adding reagents for cation identification, we start by adding drop-wise and then in excess
• If a precipitate forms when adding drop-wise, note the color of the precipitate
• When adding sodium hydroxide or ammonia drop-wise, look for the formation of a precipitate
• After adding the reagent in excess, observe if the precipitate dissolves
• If the precipitate dissolves in excess reagent, note the color of the resulting solution
• Specific outcomes for cation identification tests include the behavior of the precipitate with sodium hydroxide and ammonia
• For example, calcium ions form an insoluble white precipitate with sodium hydroxide
• Zinc ions form a soluble white precipitate with sodium hydroxide, resulting in a colorless solution
• Remember the outcomes for cations like calcium, aluminum, zinc, lead, copper, iron2, iron3, and ammonium when using sodium hydroxide and ammonia
• By observing the reactions during cation identification tests, we can predict the presence of specific ions in a solution
• For example, a light blue precipitate that dissolves to a dark blue solution indicates the presence of copper two ions
• Special tests like the one for ammonium ions involve detecting ammonia gas production by warming the mixture
• Ammonia gas is pungent and turns moist red litmus paper blue
• Tests for lead and aluminum cations may require additional tests like adding sodium chloride to differentiate between the two ions
• Adding potassium iodide can help distinguish between lead and aluminum ions
• When testing for cations, the precipitates formed are metal hydroxides
• Adding sodium hydroxide or aqueous ammonia to a cation results in the formation of a solid metal hydroxide
• For example, when testing for iron(II) ions and adding sodium hydroxide, a dirty green precipitate of iron(II) hydroxide is formed
• When adding aqueous ammonia to iron(II) ions, a dirty green precipitate of iron(II) hydroxide is also formed
• Ammonia in aqueous ammonia ionizes to produce ammonium ions and hydroxide ions, which react with the cation to form the precipitate
• In writing ionic equations for precipitation reactions, first write the formula of the precipitate in solid state, followed by the ions that make up the precipitate in aqueous state
• The shortcut for writing ionic equations for precipitation reactions is to write the formula of the precipitate first, followed by the ions that make up the precipitate in aqueous state
• When adding aqueous ammonia to different cations, zinc ions are the only ones that will form a precipitate that dissolves in excess aqueous ammonia
• Zinc ions will form a white precipitate that is soluble in excess aqueous ammonia
• When testing for ammonium ions, adding sodium hydroxide and heating will produce ammonia gas, turning moist red litmus paper blue
• Zinc ions will form a white precipitate that dissolves in excess sodium hydroxide and aqueous ammonia
• When adding sodium hydroxide and aqueous ammonia to zinc ions, the correct observation is that a white precipitate forms and dissolves in excess
• In the test for cations, observations are made by adding sodium hydroxide or aqueous ammonia drop by drop to observe precipitate formation and its solubility in excess
• The color of the precipitate and the solution after adding excess reagent helps determine the cation present in the unknown solution