Properties of substances according to their bonding

Common salt melts at $801\ \celsius$: it must be heated immensely to melt it. A small section of its crystal lattice is shown in the figure.

Explain, based on its ionic bonding and its crystal lattice structure, why the melting point of $\text{NaCl}$ is so high.

Water boils at $100\ \celsius$, whereas salt melts at $801\ \celsius$.

1. a) State what structure each substance has: molecules or a crystal lattice?
2. b) Explain, based on this structural difference, why water changes state at a much lower temperature than salt.

$\text{NaCl}$ melts at $801\ \celsius$ and $\text{MgO}$ at $2852\ \celsius$, and both are ionic crystals with the same structure. The melting points are compared in the figure.

1. a) State the charges of the ions in each compound.
2. b) Explain, by comparing these charges, why the electrostatic attraction is much greater in $\text{MgO}$.
3. c) Relate this attraction to its extremely high melting point: $\text{MgO}$ is, in fact, used to line the inside of industrial furnaces.

Household wires are made of copper, a solid that conducts electric current. The sea of electrons model is shown in the figure.

Explain, using the sea of electrons model, why copper conducts electricity in the solid state, unlike a piece of salt.

A salt crystal does not conduct electricity, but if you melt it or dissolve it in water, it does conduct.

1. a) Explain what happens to the ions in each of the three situations.
2. b) What condition must charged particles meet in order to be able to carry an electric current?

You set up a circuit with a battery, a light bulb and two electrodes to test the conductivity of salt. The experimental set-up is shown in the figure.

1. a) Predict what you will observe when placing solid common salt between the electrodes and explain the result.
2. b) Repeat the prediction, first dissolving the salt in water.
3. c) What role does ion mobility play in the difference in results?

Distilled water barely conducts electric current, tap water conducts a little, and seawater conducts a lot.

1. a) Explain which particles carry the current in tap water and where they come from (dissolved $\text{Ca}^{2+}$, $\text{Mg}^{2+}$, $\text{Na}^{+}$, $\text{Cl}^{-}$...).
2. b) Why does distilled water barely conduct, even though the water molecule contains electrons?
3. c) Order the three types of water by their ion concentration and relate this to their conductivity.

Rock drill bits have diamond tips, the hardest natural material known. A fragment of its covalent network is shown in the figure.

Explain, based on its three-dimensional covalent lattice, why diamond is so hard.

The lead of a pencil is made of graphite: when writing, the carbon layers slide over one another and detach onto the paper. The layered structure is shown in the figure.

1. a) Explain, from its layered structure, why graphite is soft and leaves a mark.
2. b) What type of forces hold the layers together? Compare them with the bonds within each layer.

Gold leaf used in decoration consists of extremely thin sheets of gold produced by hammering the metal. Explain, using the metallic bonding model, why metals deform when struck instead of shattering, as a salt crystal would.

Diamond and graphite are pure carbon, but their properties are almost opposite: diamond is extremely hard and does not conduct electricity; graphite is soft and does conduct. The two structures are compared in the figure.

1. a) Explain the difference in hardness based on the structure of each.
2. b) Explain the difference in electrical conductivity.
3. c) Reason why graphite leaves a mark on paper, whereas diamond scratches glass.

A salt crystal shatters cleanly when hit with a hammer, whereas a copper sheet merely dents.

1. a) Draw two layers of ions in your notebook and explain what happens when one layer of the ionic lattice slides over the other: which signs of charge are forced together?
2. b) Contrast this with what happens in a metal when its layers of cations slide.
3. c) Conclude why we say that salts are "brittle" and metals are "malleable".

You can do it on paper and send a photo with your answer.

In the kitchen, salt disappears when you stir it into water, but oil forms droplets and does not mix.

1. a) Explain, in terms of its type of bonding and the interactions it establishes with water, why common salt (NaCl) dissolves in water.

1. b) Using the rule "like dissolves like", explain why oil does not dissolve in water.

At $20\ \celsius$, up to $36\ \text{g}$ of $\text{NaCl}$ can be dissolved per $100\ \text{g}$ of water, but only $0,0002\ \text{g}$ of $\text{AgCl}$.

1. a) Calculate the maximum mass in grams of common salt that can be dissolved in $250\ \text{g}$ of water at $20\ \celsius$.
2. b) Explain, with the help of a diagram in your notebook, how water molecules surround and separate the ions when salt dissolves (hydration of ions).
3. c) Conclude why stating that "ionic substances dissolve in water" is only an approximation.

Beach sand is silica, $\text{SiO}_2$: hard, does not conduct electricity and melts at $1710\ \celsius$.

1. a) Deduce, from these properties, what type of bonding and structure silica has.
2. b) $\text{CO}_2$, with the same type of covalent bonding, is a gas at room temperature: explain how this is possible.
3. c) Reason why sand serves as a raw material for making glass, whereas $\text{CO}_2$ could not be used.