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E2.1 · Use solution, solubility, concentration, ionization, and pH terminology

Learn to use solution, solubility, concentration, ionization, and ph terminology through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Solutions and Solubility

SCH3U study topic E2.1

A spoonful of table salt can disappear when stirred into water, while sand stays visible and settles. The salt has not vanished: its particles have spread through the water. Chemists use terms such as solution, solubility, and concentration to describe what is happening. They use ionization to describe how some particles form ions in water, and pH to describe whether a solution is acidic, neutral, or basic. This lesson builds those terms from familiar observations.

What you will learn

Define solution, solute, solvent, and solubility.
Describe concentration as the amount of solute in a stated amount of solution.
Explain ionization using a particle-level model and distinguish it from separating existing ions.
Use pH terminology to describe acidic, neutral, and basic solutions.

1. From a mixture to a solution

A mixture contains two or more substances together. In some mixtures, the different substances remain visibly separate. In other mixtures, one substance spreads evenly through another. A uniform mixture is called a solution. In salt water, the water and dissolved salt form one uniform liquid.

The substance that does the dissolving is the solvent. The substance that dissolves is the solute. In salt water, water is the solvent and salt is the solute. A solution can have more than one solute, and the solvent is not always water.

At the particle level, stirring helps salt particles separate and spread among water particles. The particles are too small to see individually. The solution looks uniform because the dissolved particles are spread throughout it. Dissolving does not mean that the solute has been destroyed.

Solubility describes how much of a solute can dissolve in a stated amount of solvent at a stated temperature. Solubility depends on the substances involved and the conditions, so it is useful to state the temperature when comparing solubilities. A solute described as soluble can dissolve appreciably; a substance described as insoluble dissolves only a very small amount under the stated conditions.

A saturated solution contains as much dissolved solute as it can hold under the stated conditions. If extra solute remains undissolved after mixing, the solution may be saturated. Do not confuse solubility with how quickly a substance dissolves: solubility concerns how much can dissolve, not how fast the process happens.

A solution is a uniform mixture.
The solvent does the dissolving; the solute dissolves.
Solubility is about the amount that can dissolve under stated conditions.

2. Describing how much solute is present

Two salt solutions can both look clear but contain different amounts of dissolved salt. Concentration describes how much solute is present in a stated amount of solution. A solution with more solute in the same volume is more concentrated. A solution with less solute in the same volume is more dilute.

Concentration statements need a basis for comparison. One common measure is mass concentration: the mass of solute divided by the volume of the whole solution. It can be reported in grams per litre. Another common measure is amount concentration, reported in moles per litre. A mole is a counting unit for particles; its use in calculations is studied with other chemistry tools. In either case, say which units are being used.

The volume in a concentration calculation is the volume of solution, not just the volume of solvent added at the start. When a solute dissolves, the final solution volume may differ from the original solvent volume. Keep the units consistent and include them in the answer.

A concentrated solution is not automatically saturated. A solution can be concentrated and still have room for more solute to dissolve. Saturation is about the limit under stated conditions; concentration describes the amount present relative to a stated amount of solution.

c_m = \frac{m_{\text{solute}}}{V_{\text{solution}}}

Concentration compares solute amount with solution volume or another stated basis.
State concentration units clearly.
Concentration and saturation describe different things.

3. Ions in aqueous solutions

Some solutions conduct electricity better than pure water because they contain charged particles that can move. These charged particles are ions. An ion is an atom or group of atoms with a net electric charge. This observation is one clue that certain solutes produce ions in water.

Ionization is the formation of ions from neutral particles. For example, hydrogen chloride molecules form hydronium ions and chloride ions when they ionize in water. Hydronium is a positively charged ion formed when a water molecule gains a hydrogen ion. The equation shows the particles involved; the charges and atom counts are conserved.

Ionic compounds already contain ions. When an ionic compound dissolves, its ions separate and spread through the water. This is different from ionization, which forms ions from neutral particles. The distinction matters: dissolving is not always ionization.

The label aqueous, written as (aq), means that a substance is dissolved in water. In an equation, an aqueous ion is shown with its charge and state. For example, a dissolved chloride ion is written as

\mathrm{Cl^- (aq)}

. The charge is part of the particle's identity and must not be dropped.

\mathrm{HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)}

An ion has a net charge.
Ionization forms ions from neutral particles.
Dissolving an ionic compound separates ions that were already present.

4. Using pH language

pH is a scale used to describe how acidic or basic an aqueous solution is. In the usual introductory scale, values below 7 are acidic, 7 is neutral, and values above 7 are basic. These descriptions apply to the solution, not to how concentrated it looks or how much solute was added.

At the particle level, an acidic solution has more hydronium ions than a neutral solution. A basic solution has fewer hydronium ions than a neutral solution. This gives a particle connection for the pH descriptions without requiring you to see the ions directly. A pH value is a way to report the solution's acidity or basicity; it is not a concentration unit such as grams per litre.

When comparing pH values, a lower value means a more acidic solution, and a higher value means a more basic solution. Use the words acidic, neutral, or basic carefully. A solution can contain dissolved substances and still be neutral. Also, pH alone does not tell you the identity of the solute.

The useful order is to describe what is present, identify the solution and its solute and solvent, state concentration with units when known, and use pH terminology for acidity or basicity. These terms answer different questions, so one should not be substituted for another.

Below 7 is acidic; 7 is neutral; above 7 is basic on the usual introductory scale.
Lower pH means more acidic; higher pH means more basic.
pH, concentration, and solubility describe different properties.

Worked example

Finding mass concentration

A student dissolves 6.0 g of sugar and makes 0.300 L of solution. Find the mass concentration in grams per litre.

Choose the relationship
Mass concentration compares the mass of dissolved solute with the volume of the complete solution. Use the given solute mass and solution volume.
$c_m = \frac{m_{\text{solute}}}{V_{\text{solution}}}$
Substitute with units
Substitute $6.0\ \mathrm{g}$ for the solute mass and $0.300\ \mathrm{L}$ for the solution volume. Dividing grams by litres gives grams per litre.
$c_m = \frac{6.0\ \mathrm{g}}{0.300\ \mathrm{L}}$
Calculate and report
The quotient is $20\ \mathrm{g/L}$ . The given mass has two significant digits, so report the result with two significant digits. Keep the concentration unit in the answer.
$c_m = 20\ \mathrm{g/L}$

Answer: The mass concentration is

20\ \mathrm{g/L}

Check: The result is a mass divided by solution volume, so its unit is grams per litre. A value of

20\ \mathrm{g/L}

means each litre of this solution contains 20 g of sugar.

Common mistakes and how to avoid them

Calling any clear liquid a solution.

Correction: A solution is a uniform mixture, but clear appearance alone does not establish what is dissolved. Use the substance information or evidence available.

Treating solubility and concentration as synonyms.

Correction: Solubility describes how much can dissolve under stated conditions. Concentration describes how much is present relative to a stated amount of solution.

Calling the separation of ions from an ionic compound ionization.

Correction: The ions already exist in the ionic compound. Their separation on dissolving is different from ionization, which forms ions from neutral particles.

Saying that a higher pH is more acidic.

Correction: A lower pH is more acidic. A higher pH is more basic on the usual introductory scale.

Using solvent volume instead of solution volume for mass concentration.

Correction: Use the volume of the finished solution in the denominator and state the units.

Lesson summary

A solution is a uniform mixture of solute and solvent.
Solubility describes the amount of solute that can dissolve under stated conditions.
Concentration describes the amount of solute relative to a stated amount of solution.
Ionization forms ions from neutral particles; dissolving an ionic compound separates ions that already exist.
pH describes acidity or basicity: lower values are more acidic, while higher values are more basic.

Check your understanding

Question 1

A mixture contains dissolved salt spread evenly through water. Which statement correctly identifies the parts?

Salt is the solvent and water is the solute.
Water is the solvent and salt is the solute.
Both salt and water are solutes because the mixture is uniform.
Salt is the solution and water is the solvent.

Show answer and explanation

Water is the solvent and salt is the solute.

Water does the dissolving, so it is the solvent. Salt is the substance dissolved, so it is the solute.

Question 2

Two solutions have the same volume. Solution A contains more dissolved solute than solution B. Which has the greater concentration?

Solution A
Solution B
They must have equal concentration because their volumes match.
There is not enough information to compare their concentrations.

Show answer and explanation

Solution A

With equal solution volumes, the solution containing more solute has the greater concentration.

Question 3

Which statement describes ionization?

Neutral particles form ions in water.
Existing ions change into neutral atoms whenever they dissolve.
A solute becomes visible after it mixes with solvent.
A solution reaches its maximum possible concentration.

Show answer and explanation

Neutral particles form ions in water.

Ionization means that ions form from neutral particles. The other choices describe different or incorrect ideas.

Question 4

Which solution is more acidic: one with pH 3 or one with pH 5?

The solution with pH 3
The solution with pH 5
Both are neutral.
They are equally acidic because both are below 7.

Show answer and explanation

The solution with pH 3

Both values are acidic, but the lower pH value indicates the more acidic solution.

Key terms

Solution: A uniform mixture in which one or more solutes are spread through a solvent.
Solute: A substance that dissolves in a solvent.
Solvent: The substance that dissolves a solute.
Solubility: How much solute can dissolve in a stated amount of solvent under stated conditions.
Concentration: The amount of solute relative to a stated amount of solution or solvent.
Ion: An atom or group of atoms with a net electric charge.
Ionization: The formation of ions from neutral particles.
pH: A scale used to describe how acidic or basic an aqueous solution is.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation E2.1. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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