Solutions and Mixtures
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AP Chemistry › Solutions and Mixtures
A student mixes $10,\text{mL}$ of hexane ($\text{C}6\text{H}{14}$, nonpolar) with $10,\text{mL}$ of water (polar) in a test tube and shakes the mixture. After standing, two distinct layers are observed. Which statement is correct?
A homogeneous solution forms; water is the solvent and hexane is the solute.
A homogeneous solution forms; both liquids act as solvents since both are liquids.
No homogeneous solution forms because a polar liquid and a nonpolar liquid are immiscible.
No homogeneous solution forms because equal volumes cannot make a solution.
A homogeneous solution forms; hexane is the solvent and water is the solute.
Explanation
This question tests the concept of miscibility between liquids of different polarities and the formation of homogeneous solutions. Mixing 10 mL of nonpolar hexane with 10 mL of polar water results in two distinct layers because nonpolar and polar substances are immiscible, following the 'like dissolves like' principle where similar intermolecular forces are required for mixing. The shaking temporarily disperses the liquids, but upon standing, they separate due to the inability of water's hydrogen bonding to interact effectively with hexane's London dispersion forces. No homogeneous solution forms as the substances do not mix at a molecular level. A tempting distractor is choice A, which suggests a homogeneous solution with water as solvent, but this is incorrect due to the misconception that equal volumes guarantee miscibility regardless of polarity differences. A transferable strategy is to assess the polarity of components to predict whether they will form a single phase or separate layers.
A student adds $1.0,\text{g}$ of calcium carbonate ($\text{CaCO}_3$) to $100,\text{mL}$ of water at room temperature and stirs. A solid remains at the bottom after several minutes. Which statement is most accurate?
A homogeneous solution forms; the solvent is the substance with the higher molar mass.
No homogeneous solution forms because $\text{CaCO}_3$ is only sparingly soluble in water.
A homogeneous solution forms; $\text{CaCO}_3$ is the solvent because it is a solid.
No homogeneous solution forms because stirring cannot produce solutions.
A homogeneous solution forms; water is the solute and $\text{CaCO}_3$ is the solvent.
Explanation
This question tests knowledge of solubility limits for ionic compounds and recognition of sparingly soluble substances. Adding 1.0 g of CaCO3 to 100 mL of water results in undissolved solid remaining because CaCO3 has very low solubility in water (about 0.0013 g/100 mL at room temperature), preventing a homogeneous solution from forming despite stirring. This is due to the strong lattice energy in CaCO3 not being sufficiently overcome by hydration energy in water. The mixture remains heterogeneous with precipitate at the bottom. A tempting distractor is choice A, suggesting a homogeneous solution with reversed roles, but this stems from the misconception that all ionic solids are highly soluble in water without checking specific solubility data. A transferable strategy is to consult solubility rules or tables for ionic compounds to predict if a given amount will fully dissolve.
A student adds $3.0,\text{g}$ of potassium nitrate ($\text{KNO}_3$) to $100,\text{mL}$ of water at $60^\circ\text{C}$ and stirs until the mixture is clear. Which statement is correct?
A homogeneous solution forms; the solvent is the component present in fewer moles.
No homogeneous solution forms because ionic compounds cannot dissolve at elevated temperature.
A homogeneous solution forms; water is the solvent and $\text{KNO}_3$ is the solute.
No homogeneous solution forms because heating prevents dissolution by increasing particle motion.
A homogeneous solution forms; $\text{KNO}_3$ is the solvent and water is the solute.
Explanation
This question tests the effect of temperature on solubility of ionic compounds and identification of solution components. Adding 3.0 g of KNO3 to 100 mL of water at 60°C forms a homogeneous solution because KNO3 has high solubility in water, which increases with temperature (about 100 g/100 mL at 60°C), allowing full dissolution with stirring. Water is the solvent as the abundant liquid, and KNO3 is the solute, following standard definitions where the dissolved solid is the solute. The elevated temperature provides more kinetic energy to break ionic bonds and enhance solvation. A tempting distractor is choice B, claiming no solution due to temperature effects, but this reflects the misconception that heat always reduces solubility, whereas for most solids like KNO3, it increases it. A transferable strategy is to reference solubility curves for temperature-dependent dissolution predictions.
A student prepares a solution by dissolving 5.0 g of sucrose (C12H22O11) in 100 mL of water. Which statement correctly identifies the solute and solvent?
Sucrose is the solvent because it is the solid and water is the solute.
No solution forms because covalent compounds cannot dissolve in water.
Two layers form because sucrose is nonpolar and water is polar.
Water is the solvent and sucrose is the solute in a homogeneous solution.
Water is the solute because it is the larger volume, and sucrose is the solvent.
Explanation
This question tests identification of solute and solvent in a sugar solution. When sucrose dissolves in water, it forms a homogeneous solution with water as the solvent (the liquid medium present in greater amount) and sucrose as the solute (the solid being dissolved). Sucrose, despite being a covalent compound, dissolves well in water because it has multiple -OH groups that can hydrogen bond with water molecules. Choice D incorrectly claims covalent compounds cannot dissolve in water, when many polar covalent compounds (like sugars and alcohols) are highly water-soluble. Remember that solubility depends on intermolecular forces, not just whether a compound is ionic or covalent.
A student mixes 10 mL of hexane (C6H14) with 10 mL of water in a test tube and shakes the tube. After standing, what is the most likely result?
A homogeneous solution forms, with hexane as the solute and water as the solvent.
A homogeneous solution forms because both substances are molecular compounds.
Two layers form because hexane reacts with water to form ions.
A homogeneous solution forms because water dissolves all nonpolar liquids.
Two layers form because hexane is nonpolar and water is polar.
Explanation
This question tests understanding of miscibility based on polarity differences. Hexane (C6H14) is a nonpolar hydrocarbon while water is polar, and the principle "like dissolves like" predicts they will not mix. When combined, the two liquids form separate layers because the intermolecular forces between hexane molecules (London dispersion) and between water molecules (hydrogen bonding) are much stronger than any forces between hexane and water. Choice E incorrectly suggests hexane would dissolve in water to form a homogeneous solution, ignoring the fundamental polarity mismatch. To predict miscibility, always compare the polarities of the substances - polar dissolves polar, nonpolar dissolves nonpolar.
A student mixes 25 mL of ethanol (C2H5OH) with 25 mL of water at room temperature. Which statement best describes the mixture?
Two layers form because ethanol is denser than water.
A homogeneous solution forms because ethanol and water can hydrogen-bond.
A homogeneous solution forms, with ethanol as the solvent because it is added first.
Two layers form because ethanol is nonpolar and water is polar.
A heterogeneous mixture forms because molecular compounds cannot dissolve in water.
Explanation
This question tests understanding of miscibility between polar molecules. Ethanol (C2H5OH) and water are both polar molecules capable of hydrogen bonding - ethanol has an -OH group that can both donate and accept hydrogen bonds with water molecules. This strong intermolecular interaction allows them to mix in all proportions, forming a homogeneous solution. Choice A incorrectly claims ethanol is nonpolar, when the presence of the hydroxyl (-OH) group makes it polar. When predicting miscibility, look for similar intermolecular forces - molecules that can engage in the same types of interactions (especially hydrogen bonding) tend to be miscible.
A student attempts to dissolve $1.0,\text{g}$ of CaCO$_3$(s) in $50,\text{mL}$ of water at room temperature. After stirring, visible solid remains at the bottom. Which statement best describes the mixture?
A homogeneous solution forms, with water as the solvent and CaCO$_3$ as the solute.
A heterogeneous mixture forms because CaCO$_3$ is insoluble enough that undissolved solid remains.
A homogeneous solution forms, with CaCO$_3$ as the solvent and water as the solute.
A heterogeneous mixture forms, but it is still called a solution because it was stirred.
A homogeneous solution forms because all ionic compounds fully dissolve in water.
Explanation
This question tests understanding of solubility limits and heterogeneous mixture formation. CaCO₃ (calcium carbonate) has extremely low solubility in water at room temperature (approximately 0.001 g per 100 mL), so 1.0 g in 50 mL far exceeds its solubility limit. When a solid's solubility limit is exceeded, undissolved solid remains, creating a heterogeneous mixture with both dissolved ions in solution and solid particles that settle. The presence of visible solid after stirring confirms that the mixture is heterogeneous, not homogeneous. Choice E incorrectly assumes all ionic compounds fully dissolve in water, but many ionic compounds like CaCO₃, BaSO₄, and AgCl have very low water solubility despite being ionic. When mixing solids with liquids, check solubility data - if the amount added exceeds the solubility limit, expect a heterogeneous mixture with undissolved solid.
A student bubbles carbon dioxide gas (CO2) into water and caps the container. After shaking, no bubbles are visible and the liquid is clear. Which statement is correct?
A heterogeneous mixture forms because CO2 is nonpolar.
A homogeneous solution forms, and water is the solvent.
A homogeneous solution forms, and water is the solute.
A homogeneous solution forms, and CO2 is the solvent.
A heterogeneous mixture forms because gases cannot dissolve in liquids.
Explanation
This question tests the skill of recognizing gas solubility in liquids and identifying the solvent in gaseous solute systems. Carbon dioxide gas dissolves in water to form a clear, homogeneous solution, as CO2 molecules interact with water via dipole-induced dipole forces, and no bubbles are visible after shaking. In this solution, water is the solvent as it is the liquid medium containing the dissolved gas. The capped container and shaking enhance dissolution by increasing pressure and contact. A tempting distractor is choice B, which identifies CO2 as the solvent, arising from the misconception that the gaseous component acts as the solvent. To determine solution types with gases, consider the liquid as the solvent and check for uniformity and absence of phases to confirm homogeneity.
A student adds a small amount of iodine (I2, nonpolar molecular solid) to a beaker containing hexane (C6H14, nonpolar) and swirls. The mixture turns uniformly purple. Which statement is correct?
A homogeneous solution forms, and iodine is the solvent.
A heterogeneous mixture forms because solids cannot dissolve in liquids.
A homogeneous solution forms, and iodine is the solvent because it provides the color.
A heterogeneous mixture forms because nonpolar solutes only dissolve in polar solvents.
A homogeneous solution forms, and hexane is the solvent.
Explanation
This question tests the skill of applying the 'like dissolves like' rule to predict solubility and identify solvents in nonpolar systems. Iodine, a nonpolar molecular solid, dissolves in hexane, a nonpolar solvent, forming a uniformly purple mixture because their similar nonpolar natures allow for dispersion forces to facilitate dissolution. In this homogeneous solution, hexane is the solvent as it is the liquid medium in larger amount. The uniform color without visible solids confirms complete dissolution. A tempting distractor is choice B, which identifies iodine as the solvent, based on the misconception that the colored or solid component dictates the solvent role. When dealing with nonpolar solutes and solvents, use the 'like dissolves like' principle and identify the solvent as the predominant liquid component to classify the mixture accurately.
A student adds $2.0,\text{g}$ of sodium chloride (NaCl) to $50,\text{mL}$ of liquid water at room temperature and stirs for 2 minutes. Which statement best describes the mixture?
A homogeneous solution forms; both NaCl and water are solvents because both are present.
No homogeneous solution forms because water and NaCl are both polar substances.
A homogeneous solution forms; water is the solvent and NaCl is the solute.
A homogeneous solution forms; NaCl is the solvent and water is the solute.
No homogeneous solution forms because ionic compounds cannot dissolve in polar solvents.
Explanation
This question tests the understanding of solubility principles, specifically how ionic compounds interact with polar solvents and the identification of solvent versus solute. When 2.0 g of NaCl, an ionic compound, is added to 50 mL of water, a polar solvent, it dissolves completely due to the 'like dissolves like' rule, where the polar water molecules can effectively solvate the Na+ and Cl- ions, forming a homogeneous solution. Water is identified as the solvent because it is present in a much larger quantity compared to NaCl, which is the solute as the dissolved substance. The stirring helps in dispersing the ions evenly throughout the solution. A tempting distractor is choice B, which incorrectly identifies NaCl as the solvent and water as the solute, stemming from the misconception that the solid must be the solvent due to its state of matter rather than considering the relative amounts. A transferable strategy is to always evaluate the polarity of substances and the relative quantities to determine solubility and roles in a solution.