Exercises: 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.

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.

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.

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?

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.

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.

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?

$\text{NaCl}$ 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.

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.

High-voltage power lines are made of aluminium and domestic wiring of copper; their densities are $2,70\ \text{g/cm}^3$ and $8,96\ \text{g/cm}^3$, respectively.

1. a) Explain why both metals conduct electric current.
2. b) Reason, using their densities, what advantage there is in stringing kilometres of aluminium cable hanging from pylons.
3. c) Copper conducts better than aluminium for the same cross-sectional area: reason which material is preferable inside a wall, where weight does not matter.

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.

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.

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.

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.

Three substances display these properties. X: melts at $1538\ \celsius$, conducts in solid and liquid states and is malleable. Y: melts at $801\ \celsius$, only conducts when molten or dissolved and is brittle. Z: sublimes at $-78\ \celsius$, does not conduct under any circumstances and forms soft crystals.

1. a) Deduce, giving reasons, the predominant type of bonding in each substance.
2. b) Suggest a real substance that could correspond to each one.
3. c) For each listed property, state which structural feature of the substance explains it.

In a laboratory there are three unlabelled bottles containing white powders: common salt ($\text{NaCl}$), sugar (sucrose, a molecular substance) and sand ($\text{SiO}_2$).

1. a) Design a procedure with three tests — solubility in water, conductivity of the solution and behaviour upon strong heating — to identify the contents of each bottle.
2. b) Predict the result for each substance in each test.
3. c) Explain each result based on the bonding and structure of the substance.

Consider these substances and their melting points: $\text{CH}_4$ ($-182\ \celsius$), $\text{H}_2\text{O}$ ($0\ \celsius$), $\text{NaCl}$ ($801\ \celsius$), $\text{CaO}$ ($2572\ \celsius$) and $\text{MgO}$ ($2852\ \celsius$). The melting points are shown in the figure.

1. a) Order them from lowest to highest melting point.
2. b) Explain why methane and water, molecular substances, have such low melting points compared to ionic lattices.
3. c) Compare $\text{NaCl}$ with $\text{MgO}$ and $\text{CaO}$: what role do charges $1+/1-$ versus $2+/2-$ play?
4. d) Without knowing its value, predict whether the melting point of $\text{KCl}$ (ions $\text{K}^{+}$ and $\text{Cl}^{-}$) will be closer to that of $\text{NaCl}$ or $\text{MgO}$, and justify your answer.

You dissolve $1\ \text{mol}$ of $\text{NaCl}$ in one litre of water and $1\ \text{mol}$ of $\text{CaCl}_2$ in another litre. Given: $N_{\text{A}} = 6,022\cdot10^{23}\ \text{mol}^{-1}$.

1. a) Calculate the total number of moles of ions in each solution.
2. b) Calculate how many individual ions each solution contains; express the result in standard form with two decimal places in the significand.
3. c) Reason which of the two solutions will conduct electric current better.

Drill bits that drill oil wells have synthetic diamond inserts, manufactured by subjecting graphite to huge pressures and temperatures.

1. a) Explain why diamond is the ideal material for cutting rock, based on its structure.
2. b) Reason why graphite, being the same element, would not be suitable for that tip.
3. c) Diamond does not conduct electricity and melts above $3500\ \celsius$: explain both properties by looking at where its electrons are located.

The negative electrode of lithium-ion mobile phone batteries is made of graphite: when charging the phone, $\text{Li}^{+}$ ions are accommodated between its layers, and the electrode must also conduct current. The accommodation of ions between the layers is shown in the figure.

1. a) Explain, based on its layered structure, why graphite can accommodate ions between its layers.
2. b) Explain why graphite satisfies the requirement of conducting electricity, unlike other covalent materials.
3. c) Reason why diamond could not be used as a battery electrode.

Gold is so ductile that with $1\ \text{g}$ a wire about $2\ \text{km}$ long can be produced.

1. a) Explain, using the sea of electrons model, this extreme malleability and ductility.
2. b) Jewellery is made of $18$-carat gold, an alloy with $25\ \%$ of copper and silver, because pure gold is too soft: reason why atoms of a different size inserted into the lattice make it harder for the layers to slide, thus hardening the alloy without taking away the shine or conductivity characteristic of metals.
3. c) Contrast this behaviour with that of an ionic crystal when deformed.

Essay: "Bonding explains the material world".

1. a) Choose salt, water, copper and diamond and, for each substance, explain its melting point, electrical conductivity and hardness based on the type of bonding and structure it forms.
2. b) Conclude by stating in your own words a general rule connecting the structure of a substance (molecules or crystal lattice) with its properties.

Unit challenge. How does microscopic structure determine the household use of materials? Design a comparative study among three substances from your home: table salt ($NaCl$), ethanol ($CH_3-CH_2-OH$), and aluminum foil ($Al$).

1. Theoretical classification: Justify the electronic stability of their atoms and ions according to the octet rule, identify the type of bonding present in each substance, and describe the properties of the functional group in ethanol.

2. Home data collection: Using a kitchen thermometer ($\pm 0,5\ ^\circ\text{C}$), measure the temperature evolution as a napkin soaked in water evaporates compared to one soaked in ethanol. Record the temperature every $30\text{ s}$ for $3\text{ min}$ ($t \pm 1\text{ s}$) and check the official enthalpy of vaporization of both in the NIST Chemistry WebBook database. Caution: use a small amount of ethanol (flammable), in a well-ventilated area, away from flames, lit stoves or heat sources.

3. Data processing: Construct a table with the values and uncertainties and plot the graph $T = f(t)$. Calculate the average cooling slope ($\Delta T / \Delta t$) and relate volatility to intermolecular bonds.

4. Final product: Prepare a reasoned report (maximum one page) comparing the volatility, conductivity, and melting points of the three materials according to their bonding (ionic, molecular covalent, and metallic).

Evaluation criteria:

• Rigor when explaining electronic stability, bonding types, and the functional group.

• Precision in experimental data collection, expression of uncertainties, and graphic quality.

• Consistency in justifying macroscopic properties from chemical bonding.