The Amazing World of Solutes, Solvents, and Solutions · Lesson 7 of 7
Summary & Practice
“Connect the whole chapter through a recap table, worked reasoning, and mixed practice.”
• Connect solutions, concentration, saturation, solubility, and density. • Select appropriate methods and units for mass, volume, and density. • Compare the effects of temperature on solubility and density. • Apply chapter ideas to numerical and observational questions. • Explain conclusions using evidence and identify common misconceptions.
The Whole Chapter at a Glance
The chapter began with substances mixing and ended with objects floating, measurements, and changes in density. These are connected ideas. Dissolving changes a liquid’s composition; temperature can change a dissolving limit or an occupied volume; careful measurements help us explain the resulting observations. Use this table to recall what each lesson added before attempting the mixed questions.
| Lesson | What we learned | A useful check or connection |
|---|---|---|
| Solutions, Concentration, and Saturation | A solution is uniform. Solutes dissolve in solvents. Concentration describes composition; saturation refers to a temperature-dependent limit. | Undissolved solid is separate from the dissolved solute; concentrated does not automatically mean saturated. |
| Solubility, Temperature, and Solvent Choice | Most solid solubilities rise with temperature; gas solubilities generally fall. Suitable solvents extract substances; evaporation recovers salt. | Specify solvent quantity and temperature. Dissolving speed is different from the maximum amount that can dissolve. |
| Density and Relative Density | Density = mass ÷ volume. Relative density compares with water at the same temperature. | Density has units; relative density has none. More total mass does not automatically mean greater density. |
| Measuring Mass and Liquid Volume | Zero and tare balances. Choose cylinders by capacity and scale. Read the water meniscus at eye level. | Exclude container mass, count scale intervals, and use consistent units. |
| Finding the Volume and Density of Solids | Cuboid volume uses three dimensions; irregular-solid volume uses displacement. Combine mass and volume to find density. | Use full immersion and final minus initial volume. A hollow object’s outside volume is not its material volume. |
| Floating, Sinking, and Changes in Density | Compare average density with the liquid. Expansion generally lowers density; compression raises it. Water behaves unusually near freezing. | Salt water can support an egg; ice floats; gas compression and air warming give different volume changes. |
Keep Related Ideas Distinct
Many mistakes arise because two familiar words seem to describe the same thing. Before calculating, ask which quantity a question actually concerns. A solution can be concentrated without being saturated, and a large object can have high mass without high density.
| Ideas to distinguish | How to tell them apart |
|---|---|
| Concentration and solubility | Concentration is the actual dissolved amount relative to a quantity; solubility is a maximum at specified conditions. |
| Saturated and unsaturated | Compare the dissolved amount with the limit at the same temperature and solvent quantity. |
| Mass and weight | Mass is matter quantity in g or kg; weight is gravitational force in N. |
| Volume and density | Volume is occupied space; density is mass per unit of that space. |
| Density and relative density | Density includes units; a ratio to water density at the same temperature has no unit. |
| Dissolved gas and visible bubbles | Dissolved gas is distributed in the liquid; bubbles are separate gas pockets. |
Formulas and Units to Reuse
Choose the relationship only after identifying what has been given and what is required. In a displacement question, first find the object’s volume; in a density question, then divide its mass by that volume. Keep units visible through the steps so that the answer has a physical meaning.
| Relationship | Use |
|---|---|
| 1 L = 1000 mL = 1 dm³ | Convert liquid volumes. |
| 1 mL = 1 cm³ | Convert a displacement reading to solid volume. |
| 1 g/cm³ = 1000 kg/m³ | Convert density units. |
| Temperature rises: most solid solubilities rise; gas solubilities generally fall | Compare dissolving limits under comparable conditions. |
| Constant mass: volume rises → density falls; volume falls → density rises | Explain expansion and compression. |
Connect an Observation to a Calculation
An answer is stronger when it includes both a numerical result and a reasoned interpretation. Decide what the calculation says about the material or situation. The following examples connect density, a change of shape, and floating behaviour rather than treating them as unrelated facts.
Problem
A compact solid sculpture has mass 225 g and volume 90 cm³. Find its density and predict whether it sinks in water of density 1 g/cm³.
- 1.Calculate density: 225 g ÷ 90 cm³ = 2.5 g/cm³.
- 2.Compare: 2.5 g/cm³ is greater than 1 g/cm³.
- 3.The compact sculpture is denser than water and tends to sink when freely placed in it. The prediction uses the stated solid volume and excludes hollow enclosed air or separate support.
Problem
A 120 g piece of clay occupies 60 cm³. It is flattened into a sheet without losing material, trapping new air, or changing its actual volume. Does its density change?
- 1.Initially, density = 120 ÷ 60 = 2 g/cm³.
- 2.Reshaping changes the outline, but the given mass and material volume remain 120 g and 60 cm³.
- 3.Density remains 2 g/cm³. A changed outline alone is different from thermal expansion or compression.
Problem
An unpeeled orange floats, but the same orange sinks after peeling. How can removing some mass make it sink?
- 1.The peel also occupies volume and includes air-containing spaces. Removing it reduces both mass and total volume.
- 2.Density depends on their ratio. If volume decreases proportionally more than mass, the remaining orange’s average density increases.
- 3.The unpeeled orange can have average density below water, while the peeled orange is above water. Less total mass therefore does not guarantee floating.
Mixed Chapter Check
These questions move between definitions, observations, numerical work, and methods. Read each question’s conditions carefully. For example, a statement about an unsaturated solution needs a specified temperature, while a density prediction needs a clear choice of object volume and surrounding liquid.
Quiz
Two solutions use equal water quantities at 25°C. P is saturated with the same solute; Q is unsaturated. Which must be true?
A tared sample has mass 48 g. It raises water from 20 mL to 36 mL when fully immersed. What is its density?
Which comparison correctly combines the chapter’s temperature trends?
An object has density 0.8 g/cm³. Using water density 1.0 g/cm³ at the same temperature, what is its relative density?
Which change alone guarantees lower density for a fixed sample?
Practice Problems
- Correct each false statement and explain each true one: (a) Oxygen is more soluble in hot water than cold water under comparable conditions. (b) Sand and water form a solution. (c) Occupied space is called mass. (d) An unsaturated solution must contain more dissolved solute than a saturated solution of the same solute at the same temperature with equal solvent quantity. (e) A well-mixed combination of atmospheric gases is uniform.
- Complete and explain: (a) For displacement, the solid must be ___ immersed and the ___ in water level measured. (b) The maximum dissolved amount in a specified solvent quantity at a specified temperature is ___. (c) Expansion at constant mass generally ___ density. (d) A glucose solution that can dissolve no more glucose at that temperature is ___.
- Oil forms a separate layer above water. Explain what this suggests about their densities and why it does not mean that every quantity of oil has less mass than every quantity of water.
- A compact sculpture has mass 225 g and volume 90 cm³. Calculate density and predict its behaviour in water of density 1 g/cm³.
- Choose the correct description of a saturated solution at a fixed temperature: (a) It can dissolve unlimited solute. (b) It must be unsaturated. (c) It cannot dissolve additional amounts of that solute at that temperature. (d) It forms only at high temperature. Explain why each rejected description fails.
- A bottle has capacity 2 L and contains 500 mL water. How much more water can it hold? State the answer in mL and L.
- An object has mass 400 g and volume 40 cm³. Find its density and interpret the value.
- An unpeeled orange floats but the peeled orange sinks. Explain using mass, volume, trapped air, and average density.
- Object A has mass 200 g and volume 40 cm³. Object B has mass 240 g and volume 60 cm³. Calculate both densities and decide which is denser. Explain why comparing masses alone gives the wrong conclusion.
- A 120 g clay piece occupies 60 cm³. It is flattened without material loss or actual volume change. Calculate its density before and after reshaping.
- An iron block has mass 600 g and density 7.9 g/cm³. Find its volume. Start by rearranging the density relationship in words.
- Contained water is warmed using a hot-water bath near 70°C. Its level rises in a narrow glass tube. Explain what happens to volume and density if no water is lost. Sketch the before-and-after arrangement.
- A balance shows 125 g for a beaker with liquid and 45 g for the empty beaker. Find the liquid mass. If 100 mL of that liquid was measured, calculate its density and state what tare would change in the weighing procedure.
- A cylinder has eight equal intervals between 20 and 40 mL. Find one division. Explain why eye-level meniscus reading and full immersion matter in a density measurement.
- Compare heating a solid solution, heating oxygen-containing water, and compressing a fixed gas sample at constant temperature. For each, state what quantity changes and explain why the three trends need not match.
Sculpture: 225 ÷ 90 = 2.5 g/cm³, so it tends to sink. Bottle: 2000 − 500 = 1500 mL = 1.5 L remaining capacity. The 400 g object: 400 ÷ 40 = 10 g/cm³. A: 200 ÷ 40 = 5 g/cm³; B: 240 ÷ 60 = 4 g/cm³; A is denser despite having less mass. Clay: 120 ÷ 60 = 2 g/cm³ before and after. Iron: V = m ÷ density = 600 ÷ 7.9 ≈ 75.95 cm³, about 76 cm³. Liquid: 125 − 45 = 80 g; density = 80 ÷ 100 = 0.8 g/mL. Cylinder division: (40 − 20) ÷ 8 = 2.5 mL.
Investigate, Discuss, and Ask Your Own Questions
The final projects invite you to use observations and evidence rather than repeat slogans. A good investigation changes one intended factor while keeping other relevant conditions similar. A good explanation also distinguishes what you observed from what you still need to research.
- Investigate salt in equal quantities of water, vinegar, and cooking oil at similar temperatures. Add equal small portions, stir comparably, and record whether each portion dissolves. Explain the limits of a spoon-based qualitative comparison.
- Discuss the claim that water is a versatile solvent. Support it with examples of substances that dissolve and substances that do not. Explain why versatile does not mean able to dissolve everything.
- Research the Dead Sea and another very salty water body. Investigate the connection between salt content and floating. Distinguish the absence of fish from the false claim that no life of any kind exists; some microorganisms can live in highly saline environments.
- Connect Ningel salt production with concentration, saturation, and evaporation. Compare the purpose of allowing evaporation there with limiting evaporation in a temperature-solubility experiment.
- Write one question connecting solutions with density and another connecting temperature with aquatic life. Exchange questions with a classmate, answer using chapter ideas, and identify what extra evidence would be needed for claims beyond the chapter.
Key Takeaways
• Uniform solutions contain solutes dissolved in solvents; concentration, saturation, and solubility describe different aspects of them. • Solubility comparisons require specified conditions; most solids and gases show different temperature trends. • Density is mass per volume, while relative density is a unit-free comparison with water at the same temperature. • Reliable calculations begin with taring, suitable instruments, consistent units, and correct meniscus or displacement readings. • Floating depends on density relative to the liquid and on whole-object structure, including enclosed air. • Expansion generally lowers density at fixed mass; compression raises it, especially for gases. • Water’s maximum density near 4°C and less dense floating ice have consequences for aquatic life. • Use calculations, observations, and careful comparisons together to explain unfamiliar situations.
Previous · Lesson 6
Floating, Sinking, and Changes in Density
Next
End of chapter