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How Do Aqueous Solutions of Ionic & Molecular Compounds Differ?

Instructor: Dave Hays
Why does sodium chloride conduct an electric current when dissolved in water while sugar does not? In this lesson learn about how the type of bonding of atoms in a compound determines how the compound dissolves in water and how its aqueous solution properties are influenced.

Introduction

Jean-Paul and Simone were sitting in a lab, performing an experiment. Jean-Paul had taken a beaker of distilled water and dissolved a tablespoon of salt inside it. Simone had performed the same operation with a tablespoon of sugar on a separate beaker of distilled water.

Simone then plugged an electric light bulb into a wall socket. The base the light bulb was screwed into had two exposed wires coming out from its base. Simone immersed these wires into the salt water solution and the light bulb lit up. She removed the wires from the salt water solution, cleaned and dried them, placed them in the sugar water solution and the light bulb remained dark. After removing, cleaning, and unplugging the apparatus, she turned to Jean-Paul and said, 'Okay, let's explain what happened.'

Dissociation in Molecular Compounds

After thinking for a moment, Jean-Paul said, 'When I dissolved the salt in the water, the positively charged cations and negatively charged anions in the crystal lattice structure of the salt dissociated, or separated, to form individual cations and anions in the water.'

'That seems reasonable,' replied Simone. 'And how exactly does this process of dissociation occur?'

'Well,' responded Jean-Paul. 'To understand that, I think we need to begin by reviewing what we know of the molecular structure of both water and sodium chloride.'

Bond Dipoles and Polar Bonds

'We know that water molecules are polar, meaning they have positively and negatively charged regions of the molecule.' said Jean-Paul. 'The polar nature of water is due to the formation of polar bonds, which arise due to the differences in electronegativity of the atoms that bond together. Polar bonds form bond dipoles. These bond dipoles create charged regions indicated by using δ+ to indicate the positively charged region, and δ- to indicate the negatively charged region.'

polar water molecule
polar water molecule

'That seems clear enough.' said Simone. 'What about the structure of sodium chloride?'

Dissolving Process of Ionic Compounds

Jean-Paul continued, 'We also know that the sodium chloride crystal lattice is held together by coulombic attractions between the sodium and chloride ions.'

'What are coulombic attractions?' asked Simone.

'Coulombic attractions are those that arise between opposite charges. The sodium cation has a positive charge and the chloride anion has a negative charge so the crystal lattice is held together by the attractions between these charges.'

'Got it,' Simone said. 'So, what happens then when the salt is put into the water?'

'When that happens,' Jean-Paul explained, 'the negatively charged regions of the water molecule become attracted to the sodium cations and the positively charged regions of the water molecule become attracted to the chloride anions by ion-dipole attractions. These ion-dipole attractions are strong enough to overcome the coulombic attractions holding the crystal lattice together and the ions pull away and dissolve. The ions are then said to be hydrated because both are surrounded by water molecules.'

Dissociation of Sodium Chloride
Dissociation of NaCl

'Wow!' said Simone. 'But why does the light go on then when the wires are placed in the solution?'

Electrolyte Formation

'That's somewhat complicated,' said Jean-Paul. 'But in general, what happens is that since the wires are connected to electricity from the wall, they create a voltage difference between them. This voltage difference creates an electric field in the water and the dissolved salt creates a solution that facilitates a current that causes the light to go on. Ionic compounds that conduct an electric current are called electrolytes.'

'Okay, got it.' said Simone. 'So without the ions in the water to form a solution, no current can be formed to complete the circuit and allow the light to go on?'

'Right,' Jean-Paul's replied. 'But why the light did not go on when the wires were placed in the sugar water solution.'

How Molecular Compounds Dissolve

'Following your analysis,' Simone said, 'let's begin by examining the molecular structure of sugar.'

Sugar
Sugar

'The carbon, oxygen, and hydrogen atoms in sugar are covalently bonded, which means the atoms form bonds by sharing electrons, rather than transferring them like with sodium chloride. There's no coulombic attractions between atoms because there's no ions formed.'

'Even so,' Simone continued, 'there's bond dipoles, arising from the different electronegativities of the atoms forming the bonds. Therefore, there can be multiple regions where polar bonds can form, as you explained in the case of the water molecules.'

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