Two Liquids, One Beaker
A student mixes 40 mL of a clear, colourless liquid A with 40 mL of a clear, colourless liquid B in a beaker at room temperature. The mixture warms noticeably, and after two minutes a white solid appears. The student filters off 1.8 g of solid, then tests the filtrate. The filtrate conducts electricity. Liquid A alone does not conduct; liquid B alone does not conduct. When the student adds a few drops of universal indicator to the filtrate, it turns red-orange, indicating pH around 3–4. The student says a 'physical change' occurred because both starting liquids were colourless and no gas was seen.
1. Give two observations that argue against a purely physical change, and explain the reasoning for each.
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First, the mixture warms: a release of heat on mixing indicates a chemical reaction, because simple mixing of two liquids usually produces little or no temperature change unless bonds are formed or broken. Second, the filtrate conducts electricity although neither starting liquid did. Ions must have been produced, and neutral molecules do not conduct. A third point is the appearance of a new solid that was not present in either liquid. Together these show new substances with new properties, which is the hallmark of a chemical change, not a physical one.
2. The filtrate is acidic. Propose a consistent picture of what happened, naming the likely ion types, and explain why the solid formed.
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A consistent picture is a precipitation reaction between two soluble salts, one of which releases hydrogen ions. For example, a solution containing a metal ion plus a solution containing a hydrogen carbonate or hydrogen sulfate could react to give an insoluble salt plus an acidic solution. The warming fits an exothermic precipitation or acid-base step. The filtrate conducts because it contains mobile ions such as K⁺ or Na⁺ balanced by an anion, and its pH of 3–4 means hydrogen ions are present in excess. The white solid is the insoluble product that exceeded its solubility and came out of solution; 1.8 g is the recovered portion, not necessarily all that formed.
3. The student wants to know whether the reaction went to completion. Describe a test using the filtrate and state what result would show excess of one reactant.
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Add a few drops of a solution containing the cation of liquid A to a small portion of the filtrate. If more white solid forms, liquid B's anion was still present in excess and the reaction had not gone to completion with respect to that ion. Repeat with a solution containing the anion of liquid B on a fresh portion; further precipitate means liquid A's cation was in excess. If neither addition produces more solid, both ions were largely consumed and the reaction went essentially to completion. Using small separate portions avoids contaminating the main filtrate.