English:Concentration and Solubility
Concentration and Solubility
Introduction
Concentration and solubility help you describe what happens when one substance mixes evenly with another. You meet these ideas when sugar dissolves in a drink, salt mixes with water, crystals form as a solution cools, or a colored solution becomes paler after water is added.
This aiMOOC is designed for Grades 7–8. You will learn how to identify the solute and solvent in a solution, compare dilute and concentrated solutions, calculate simple mass concentrations, explain solubility and saturation, interpret solubility curves, and plan fair investigations.
Learning goals: By the end of this course, you should be able to explain the difference between concentration and solubility, use evidence from observations and graphs, carry out simple calculations, and apply the ideas to laboratory and everyday situations.
Solutions: solute, solvent, and solution
A solution is a homogeneous mixture. This means that the substances are mixed evenly on the scale you can observe. In a simple solution, the substance that is dissolved is called the solute, and the substance that does the dissolving is called the solvent.
For example, when a small amount of table salt dissolves in water, salt is the solute and water is the solvent. The salt does not disappear. Its particles become separated and spread throughout the water. If the water is later evaporated, the salt can be recovered.
The particle model helps you explain this process. Water molecules attract the ions in a sodium chloride crystal, separate them from the crystal, and surround them. The dissolved particles remain present even though you cannot see individual particles with your eyes.
A solution does not have to contain a solid dissolved in a liquid. Gases can dissolve in liquids, and liquids can mix with other liquids. In this course, however, most examples use solids dissolved in water because they are easy to investigate safely in a school laboratory.
Soluble, insoluble, and miscible
A substance is described as soluble in a particular solvent if a useful amount of it can dissolve under the stated conditions. A substance that dissolves only to a very small extent may be described as insoluble for a school-level investigation.
Whether a substance dissolves depends on both substances involved. Sugar dissolves well in water, while cooking oil does not form one uniform solution with water. Therefore, you should not say that a substance is simply "soluble" without thinking about the solvent.
Two liquids that mix in all proportions to form one phase are called miscible. Liquids that separate into layers are called immiscible.
Concentration
Concentration describes how much of a substance is present in a given amount of mixture or solution. In Grades 7–8, a useful way to describe concentration is the mass of solute in a certain volume of solution.
A solution with relatively little solute per unit volume is called dilute. A solution with relatively more solute per unit volume is called concentrated. These words are comparisons, so it is best to say what two solutions you are comparing.
Color can sometimes provide a visual clue. For example, if the same colored solute is used in two solutions, the darker solution may be more concentrated. However, color alone is not a universal measurement of concentration.
Calculating mass concentration
For a simple mass-per-volume concentration, use:
concentration = mass of solute ÷ volume of solution
A common unit is grams per litre, written as g/L. You may also see g/dm³ because 1 litre equals 1 cubic decimetre.
Suppose 12 g of a solute is present in 0.60 L of solution. The concentration is:
12 g ÷ 0.60 L = 20 g/L
Always check that the volume refers to the final solution, not just the amount of solvent that was used before dissolving the solute.
You also need useful volume conversions:
- Litre: 1 L = 1000 mL
- Cubic decimetre: 1 dm³ = 1 L
- Mass: 1000 mg = 1 g
Comparing amount and concentration
Amount of solute and concentration are not the same idea. A large container can hold more total solute than a small container while still having a lower concentration.
Imagine Solution A contains 10 g of sugar in 100 mL of solution, while Solution B contains 20 g of sugar in 500 mL of solution. Solution B contains more sugar in total, but Solution A is more concentrated because it contains more sugar per unit volume.
This distinction is important when you compare drinks, laboratory solutions, environmental samples, and industrial mixtures.
Dilution and concentration changes
Dilution lowers concentration by adding more solvent while keeping the amount of dissolved solute the same. If you add water to a colored solution and mix it thoroughly, the same solute particles are spread through a larger volume.
A solution can become more concentrated if some solvent is removed, for example by evaporation, provided the solute does not also evaporate. A solution can also become more concentrated if extra solute dissolves.
A useful particle-model question is: Has the number of solute particles changed, or has only the space available to them changed? During simple dilution, the number of solute particles stays the same while the volume increases.
Solubility
Solubility tells you the maximum amount of a particular solute that can dissolve in a particular solvent under specified conditions. Temperature must usually be stated because solubility can change with temperature.
For solids dissolved in water, solubility is often reported as grams of solute that can dissolve in 100 g of water at a stated temperature. This is different from concentration, which tells you how much solute is actually present in a certain amount of solution.
A solution can have a concentration lower than its solubility limit. As more solute is added, the concentration increases until the solution reaches saturation.
Unsaturated, saturated, and supersaturated solutions
An unsaturated solution contains less dissolved solute than the maximum possible amount under the current conditions. More solute can still dissolve.
A saturated solution contains the maximum amount of dissolved solute that can remain in solution under the stated conditions. If extra solute is added, some may remain undissolved.
A supersaturated solution temporarily contains more dissolved solute than would normally remain dissolved at that temperature. It is unstable. Crystallization can begin when the solution is disturbed or when a suitable crystal surface is introduced.
Solubility and temperature
For many solid solutes in water, solubility increases as temperature rises, but the size and direction of the change depend on the substance. You should therefore use experimental data instead of assuming that every solid behaves in exactly the same way.
For gases dissolved in liquids, increasing temperature often decreases solubility. This is one reason warm water can hold less dissolved gas than cooler water under otherwise similar conditions.
A solubility curve shows how solubility changes with temperature. The horizontal axis usually shows temperature, and the vertical axis shows a solubility value.
When you read a solubility curve:
- Locate the temperature: Find the required temperature on the horizontal axis.
- Read the curve: Move to the line for the chosen substance.
- Read the value: Move across to the vertical axis to estimate the solubility.
- Compare: Decide whether a stated amount is below, at, or above the solubility limit.
A point below the curve usually represents an unsaturated amount. A point on the curve represents saturation. If an amount lies above the curve, not all of it can normally remain dissolved at equilibrium; if it is temporarily all dissolved, the solution is supersaturated.
Solubility is not the same as dissolving rate
A very common mistake is to confuse how much can dissolve with how fast dissolving happens.
Solubility is about the maximum amount that can dissolve under specified conditions.
Rate of dissolving is about how quickly a solute enters the solution.
Stirring usually makes a solid dissolve faster because fresh solvent is brought into contact with the solid. Crushing a solid into smaller pieces usually speeds dissolving because the total surface area increases. Heating can also change the rate of dissolving. These changes do not automatically mean that the final solubility has changed by the same amount.
Concentration Compared with Solubility
Concentration and solubility are related, but they answer different questions.
| Idea | Question it answers | Example |
|---|---|---|
| Concentration | How much solute is actually present in a given amount of solution? | A solution contains 25 g of solute per litre. |
| Solubility | What is the maximum amount of this solute that can dissolve under specified conditions? | At a stated temperature, only a certain maximum amount can remain dissolved. |
| Saturation | Has the solution reached the solubility limit? | Extra solid remains undissolved after mixing. |
| Dilution | What happens when solvent is added? | The same solute is spread through a larger volume, so concentration decreases. |
A concentrated solution is not automatically saturated. If the solute has a high solubility, a solution may be concentrated and still able to dissolve more solute. A saturated solution is not automatically "very concentrated" compared with every other solution, because some substances have low solubility.
A useful thought experiment
Imagine two different solids, X and Y, placed in separate beakers of water at the same temperature. Solid X has high solubility, while solid Y has low solubility.
A beaker containing a moderate amount of X might be unsaturated even though it looks concentrated. A beaker containing only a small amount of dissolved Y might already be saturated. This shows why concentration and solubility must be kept separate in your reasoning.
Crystallization and Recovery of Solute
When conditions change so that a solution can no longer hold all of its dissolved solute, particles can come together to form crystals. This process is called crystallization.
One method is to make a warm saturated solution and then let it cool. If the solute is much less soluble at the lower temperature, crystals may grow. Another method is to let some solvent evaporate so that the remaining solution becomes more concentrated and eventually reaches saturation.
Crystallization is used in laboratories and industry to separate and purify substances. In a school activity, only teacher-approved materials should be used, and you should never taste laboratory chemicals.
Planning a Fair Solubility Investigation
A fair investigation changes one independent variable while controlling other important variables.
For example, you could investigate how water temperature affects the amount of a safe, teacher-approved solid that dissolves.
Independent variable: the temperature of the water.
Dependent variable: the maximum mass of solute that dissolves under the chosen procedure.
Control variables: amount of water, type of solute, stirring method, particle size, container type, and the rule used to decide when no more solute is dissolving.
A strong method also uses repeated measurements where practical and records data in a table with units.
Example method outline
- Prepare safely: Wear eye protection and follow teacher instructions, especially when using warm water.
- Measure the solvent: Use the same mass or volume of water for every trial.
- Set the temperature: Measure the water temperature with a suitable thermometer.
- Add the solute gradually: Add small measured portions and stir in the same way each time.
- Identify the endpoint: Stop when added solid remains undissolved after the agreed mixing time.
- Record evidence: Write down the total mass that dissolved and repeat for other temperatures.
Do not taste solutions from a laboratory, even when the starting substances are familiar food materials.
Working with Data
Good chemistry uses measurements, units, patterns, and explanations together. A table lets you organize results, while a graph makes trends easier to see.
Suppose your solubility measurements rise strongly with temperature. You can say that the data show a positive relationship over the tested range. You should not claim that temperature is the only factor affecting solubility unless your investigation was designed to test other factors too.
When results do not fit the overall pattern, check for possible measurement uncertainty, incomplete mixing, temperature changes during the trial, or a different judgement of the saturation point. Do not simply delete an unusual result without a scientific reason.
Example concentration calculations
Example A: 15 g of solute is present in 0.50 L of solution.
Concentration = 15 g ÷ 0.50 L = 30 g/L
Example B: A solution has a concentration of 8 g/L and a volume of 2 L.
Mass of solute = 8 g/L × 2 L = 16 g
Example C: A 200 mL sample contains 4 g of dissolved solute. First convert 200 mL to 0.200 L.
Concentration = 4 g ÷ 0.200 L = 20 g/L
Everyday and Scientific Applications
Understanding concentration and solubility helps you explain many real situations.
Food and drinks: Sugar and salts are dissolved at controlled concentrations to produce consistent products. Cooling or evaporation can cause crystals to form.
Water and the environment: Natural water contains dissolved substances. Changes in salt concentration can affect water density and living organisms.
Laboratories: Scientists prepare solutions with known concentrations so that experiments can be compared and repeated.
Manufacturing: Solubility information helps decide how much material can be dissolved, how mixtures should be heated or cooled, and when crystallization may occur.
Cleaning: The choice of solvent matters because different substances dissolve differently in different solvents.
Common Misconceptions
Misconception: Dissolved means disappeared. The particles are still present; they are dispersed through the solvent.
Misconception: Clear means pure water. Many solutions are clear even though they contain dissolved substances.
Misconception: Concentrated means saturated. Concentration describes the actual amount present, while saturation means the solubility limit has been reached.
Misconception: Stirring increases solubility. Stirring usually increases the rate of dissolving, but it does not necessarily change the equilibrium solubility.
Misconception: Hotter water always dissolves more of every substance. Temperature effects depend on the substance. Many solids become more soluble as temperature rises, but not all do, and gases often show the opposite trend.
Misconception: A larger beaker must contain a more concentrated solution. Container size does not determine concentration.
Interactive Tasks
Quiz: Test Your Knowledge
Which statement best defines a solute? (The substance that is dissolved) (!The substance that does the dissolving) (!The container holding the mixture) (!The solid left after filtration)
What happens to concentration when only more solvent is added to a solution? (The concentration decreases) (!The concentration always doubles) (!The concentration becomes zero immediately) (!The solute changes into a solvent)
Which statement best describes solubility? (The maximum amount of a solute that can dissolve under stated conditions) (!The speed at which a liquid is stirred) (!The total volume of an empty container) (!The color of a pure substance)
What is true of a saturated solution? (It has reached the solubility limit under the stated conditions) (!It contains no solvent) (!It must always be very dark in color) (!It always contains more water than solute)
A solution contains 20 grams of solute in 2 litres of solution. What is its concentration? (10 grams per litre) (!2 grams per litre) (!20 grams per litre) (!40 grams per litre)
Which change usually makes a solid dissolve faster without necessarily changing its final solubility? (Stirring the mixture) (!Using a larger label) (!Changing the shape of the beaker only) (!Turning off the room lights)
What does an unsaturated solution allow under the same conditions? (More solute can still dissolve) (!No more solute can dissolve) (!The solvent must evaporate immediately) (!The solute must become a gas)
Which statement correctly compares concentration and solubility? (Concentration is the actual amount present while solubility is a maximum under stated conditions) (!Concentration and solubility always have exactly the same value) (!Solubility is the color of a solution while concentration is its temperature) (!Concentration applies only to solids while solubility applies only to gases)
Why is temperature usually stated with a solubility value? (Solubility can change with temperature) (!Temperature tells you the color of the solute) (!Temperature fixes the size of the container) (!Solubility is measured only when water freezes)
What can happen when a warm saturated solution cools and the solute is less soluble at the lower temperature? (Crystals can form) (!The solute must vanish) (!The solvent must turn into a solid immediately) (!The concentration must become zero)
Memory Game
| Solute | Substance that is dissolved in a solution |
| Solvent | Substance that does the dissolving |
| Concentration | Amount of solute in a given amount of solution |
| Solubility | Maximum amount that can dissolve under stated conditions |
| Saturation | State reached when the solubility limit is met |
| Dilution | Lowering concentration by adding solvent |
| Crystallization | Formation of ordered solid crystals from a solution |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Actual amount of solute per volume | Concentration |
| Maximum amount that can dissolve | Solubility |
| Adding solvent to lower concentration | Dilution |
| Maximum dissolved amount has been reached | Saturation |
| Solid particles form from a solution | Crystallization |
...
Crossword Puzzle
| Solute | What do you call the substance that is dissolved? |
| Solvent | What do you call the substance that does the dissolving? |
| Solution | What is a homogeneous mixture containing dissolved material? |
| Saturation | What state is reached when no more solute can normally dissolve under the same conditions? |
| Dilution | What process lowers concentration by adding more solvent? |
| Solubility | What property describes the maximum amount that can dissolve under stated conditions? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Solution Hunt: Find four safe examples of solutions at home or school, photograph or sketch them, and identify the likely solute and solvent in each.
- Concentration Labels: Examine labels on three household products that report a percentage or concentration, record what each number describes, and explain why concentration information is useful.
- Particle Storyboard: Draw a four-panel particle-model storyboard showing a solid solute before dissolving, during dissolving, in a uniform solution, and after some solvent has evaporated.
- One-Minute Explainer: Record a one-minute video that explains the difference between dilute and concentrated solutions using a safe visual model such as colored water.
Standard
- Temperature Investigation: With teacher approval, investigate how water temperature affects how much of a safe solute dissolves, record measurements with units, and graph the results.
- Crystal Observation: Grow crystals from a teacher-approved solution or observe a prepared crystallization demonstration, keep a dated visual record, and explain how concentration and saturation change.
- Interview a User of Solutions: Interview a cook, laboratory worker, cleaner, gardener, or other person who works with solutions and summarize how concentration or solubility matters in that work.
- Solubility Curve Report: Choose a provided solubility curve, read values at several temperatures, describe the trend, and write three evidence-based predictions.
Advanced
- Fair Test Design: Design a controlled investigation that separates the effect of stirring on dissolving rate from the effect of temperature on solubility, including variables, controls, measurements, and safety steps.
- Water Scenario Model: Create a poster or digital model explaining how evaporation can increase dissolved-salt concentration in a body of water and predict two possible consequences.
- Evaluate a Claim: Test the claim that hot water always dissolves more of every substance by researching trustworthy data for at least three solutes and presenting a reasoned conclusion.
- Solution Science Project: Produce a three-to-five-minute video or presentation that connects concentration, solubility, saturation, and crystallization in one real-world process and supports the explanation with data or diagrams.
Learning Assessment
- Concentration Reasoning: Two students dissolve different masses of sugar in different final volumes; calculate each mass concentration and justify which solution is more concentrated.
- Solubility Curve Analysis: Use a supplied solubility graph to estimate the maximum amount that can dissolve at two temperatures and explain what may happen during cooling.
- Rate Versus Solubility: Explain why crushing and stirring can make a solid disappear faster without proving that its solubility has increased.
- Investigation Critique: Read a short experimental method with uncontrolled variables, identify at least three weaknesses, and redesign it as a fair solubility test.
- Saturation Scenario: Predict what will happen when additional solute is added to an unsaturated solution and then to a saturated solution, and explain both predictions using the particle model.
- Transfer Challenge: Explain how evaporation can change the concentration of dissolved salts in a pond or salt pan and connect this change to saturation and possible crystallization.
Evidence of Learning
Knowledge: You can accurately use the terms solution, solute, solvent, concentration, solubility, dilution, saturation, supersaturation, and crystallization, and you can explain how the ideas are related.
Skills: You can calculate simple mass concentrations, convert common volume units, read a solubility curve, design a fair investigation, record data with units, graph results, identify variables, and distinguish evidence from assumptions.
Products: Strong evidence may include a labelled particle diagram, a correct calculation set, an investigation report, a solubility graph, a crystal-growth record, an interview summary, a poster, or an explanatory video.
Scientific reasoning: You can explain why concentration is not the same as solubility, why stirring affects dissolving rate, how temperature can affect solubility, and why crystals may form when conditions change.
Transfer: You can apply the ideas to unfamiliar examples in food preparation, environmental water, cleaning, laboratory work, and manufacturing without assuming that every solute behaves in the same way.
OERs on the Topic
Explore these English Wikipedia articles to extend your learning:
Linked Learning Areas
Concentration and solubility connect the particle model of matter with measurement, graphs, mixtures, separation methods, and practical chemistry.