Electrolytes, Ionisation, and Conductivity Revision Notes for Grade 10 NSC Matric Physical Sciences

Electrolytes, Ionisation, and Conductivity

What are electrolytes?

When ionic substances dissolve in water, they break apart into individual ions. These ions are free to move around in the solution, which allows the solution to conduct electricity. This is a crucial concept for understanding chemical reactions in water.

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Definition: Electrolyte

An electrolyte is a substance that contains free ions and behaves as an electrically conductive medium.

Water itself is a polar molecule, which means it can dissolve ionic substances effectively. When ions are present in water, the solution becomes able to conduct electricity. We call this solution an electrolyte.

Types of electrolytes

Not all electrolytes conduct electricity equally well. We can classify them into three main types based on how well they conduct electricity:

Strong electrolytes

These are substances where many ions are present in the solution. Strong electrolytes are excellent conductors of electricity. Most ionic compounds like sodium chloride (NaCl) and potassium bromide (KBr) are strong electrolytes.

Weak electrolytes

These are substances where few ions are present in the solution. Weak electrolytes are poor conductors of electricity. Some acids and bases fall into this category.

Non-electrolytes

These substances do not conduct electricity at all because they don't form ions when dissolved in water. Most molecular compounds like sugar, alcohol, and oil are non-electrolytes.

Factors affecting conductivity

Conductivity in aqueous solutions measures water's ability to conduct an electric current. The conductivity of an electrolyte depends on several important factors:

Concentration of ions

The higher the concentration of ions in solution, the higher its conductivity will be. More ions means more charge carriers available to conduct electricity.

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Think of it this way: more ions = more conductivity. It's like having more workers to carry electrical charge through the solution.

Type of substance

Whether a material is a strong electrolyte (like potassium nitrate, KNO₃), a weak electrolyte (like acetic acid, CH₃COOH), or a non-electrolyte (like sugar, alcohol, oil) significantly affects the conductivity. Strong electrolytes form ions easily, weak electrolytes do not form ions easily, and non-electrolytes do not form ions in solution at all.

Temperature

The warmer the solution, the higher the solubility of the material being dissolved and therefore the higher the conductivity as well.

Investigating electrical conductivity

Understanding conductivity is best achieved through practical investigation. Here's how we can test different substances:

Aim

To investigate the electrical conductivities of different substances and solutions.

Apparatus

Solid salt (NaCl) crystals
Different liquids such as distilled water, tap water, seawater, sugar, oil and alcohol
Solutions of salts (e.g. NaCl, KBr, CaCl₂, NH₄Cl)
A solution of an acid (e.g. HCl) and a solution of a base (e.g. NaOH)
Torch cells
Ammeter
Conducting wire, crocodile clips and 2 carbon rods

Method

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Setting up the Conductivity Test

Follow these steps carefully to ensure accurate results:

Set up the experiment by connecting the circuit as shown in the diagram below. In the diagram, X represents the substance or solution that you will be testing.
When you are using the solid crystals, the crocodile clips can be attached directly to each end of the crystal. When you are using solutions, two carbon rods are placed into the liquid and the clips are attached to each of the rods.
In each case, complete the circuit and allow the current to flow for about 30 seconds.
Observe whether the ammeter shows a reading.

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Safety Reminder

Always ensure proper connections and handle electrical equipment carefully. Allow current to flow for exactly 30 seconds to get consistent readings.

Understanding the results

When you complete this experiment, you'll notice some important patterns. Solutions that contain free-moving ions are able to conduct electricity because of the movement of charged particles. Solutions that do not contain free-moving ions do not conduct electricity.

For electricity to flow, there needs to be a movement of charged particles (ions). With solid NaCl crystals, there was no flow of electricity recorded on the ammeter. Although the solid is made up of ions, they are held together very tightly within the crystal lattice and therefore no current will flow.

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Why Don't Some Substances Conduct?

Distilled water, oil and alcohol also don't conduct a current because they are covalent compounds and therefore do not contain ions.

The ammeter should have recorded a current when the salt solutions and the acid and base solutions were connected in the circuit. In solution, salts dissociate into their ions, so that these are free to move in the solution.

Key dissociation equations

Here are important examples of how different substances break apart in water:

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Worked Example: Dissociation Equations

Dissociation of potassium bromide: $\text{KBr (s)} \rightarrow \text{K}^+ \text{(aq)} + \text{Br}^- \text{(aq)}$

Dissociation of table salt: $\text{NaCl (s)} \rightarrow \text{Na}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}$

Ionisation of hydrochloric acid: $\text{HCl(ℓ)} + \text{H}_2\text{O(ℓ)} \rightarrow \text{H}_3\text{O}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}$

Dissociation of sodium hydroxide: $\text{NaOH (s)} \rightarrow \text{Na}^+ \text{(aq)} + \text{OH}^- \text{(aq)}$

Note that some molecular substances will form ions when they dissolve. Hydrogen chloride, for example, can ionise to form hydrogen and chloride ions when dissolved in water.

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Key Points to Remember:

Electrolytes contain free ions and can conduct electricity when dissolved in water
Strong electrolytes have many ions and are good conductors; weak electrolytes have few ions and are poor conductors
Non-electrolytes don't form ions and cannot conduct electricity at all
Higher ion concentration leads to higher conductivity
Ionic compounds dissociate into separate ions when dissolved, while covalent compounds generally don't form ions
The conductivity experiment uses an ammeter to detect current flow, which only occurs when free ions are present

Electrolytes, Ionisation, and Conductivity (Grade 10 NSC Matric Physical Sciences): Revision Notes