A conductivity apparatus tests whether a solution has ionic or covalent substances based on the ability of the solution to conduct electricity. Here's how it works:
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Ions complete the flow of electrons to form a circuit: This is the correct mechanism behind the conductivity test. Ionic substances, when dissolved in water, dissociate into ions. These ions are charged particles that can move freely in the solution, allowing them to carry an electric current. When the conductivity apparatus is immersed in an ionic solution, the presence of these ions enables the flow of electricity, which lights up the bulb.
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It fails to light up except in homogeneous solutions: This statement is somewhat misleading. The light bulb may not light up if there are no conductive ions present in the solution (as is the case with pure water or covalent compounds that do not ionize) but this is not exclusively about the homogeneity of the solution.
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It lights up when the leads touch each other: This simply indicates that a complete circuit is formed when the leads are in direct contact; it does not relate to the conductivity of the liquid being tested.
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The covalent substances bond to the leads to prevent the light bulb from lighting: This is not accurate. Covalent substances do not typically bond to the leads in a way that prevents conductivity; rather, they simply do not dissociate into ions and hence do not conduct electricity.
In summary, the critical factor is that ions complete the flow of electrons to form a circuit, which allows the light bulb to light up if the solution contains ionic substances. Covalent compounds, being non-ionic, would not facilitate this electron flow, resulting in the light bulb remaining off.