Properties of Materials (OCR GCSE Combined Science A (Gateway): Chemistry): Flashcards
Exam code: J250
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What are allotropes?
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What are allotropes?
Allotropes are different structural forms of the same element. They contain the same type of atom but have different bonding arrangements, giving them different physical properties.
How many covalent bonds does each carbon atom form in diamond, and what shape does this produce?
Each carbon atom in diamond forms four covalent bonds with other carbon atoms, producing a tetrahedral arrangement. This creates an extremely hard giant covalent lattice.
True or False?
Graphite can conduct electricity but diamond cannot.
True.
Graphite conducts electricity because it has one free electron per carbon atom. Diamond does not conduct electricity because all four of each carbon's electrons are involved in covalent bonds — there are no free electrons.
In graphite, the layers are held together by .......... intermolecular forces, which allows the layers to .......... over each other, making graphite soft and slippery.
In graphite, the layers are held together by weak intermolecular forces, which allows the layers to slide over each other, making graphite soft and slippery.
What is graphene and what makes it structurally unusual?
Graphene is a single layer of graphite — a one-atom-thick sheet of carbon atoms covalently bonded in a hexagonal arrangement. It is essentially a two-dimensional molecule.
Give two uses of diamond based on its physical properties.
Diamond is used in jewellery and for coating cutting tools such as drill bits and saw blades. Both uses exploit its extreme hardness.
True or False?
Buckminsterfullerene is a giant covalent structure made of 60 carbon atoms.
False.
Buckminsterfullerene (C60) is a spherical molecule consisting of 60 carbon atoms arranged in hexagons and pentagons. It is used as a lubricant, in drug delivery, and in nanotechnology.
Fullerenes can trap other molecules, making them useful for targeted .......... delivery systems, and have a huge surface area that makes them useful as ...........
Fullerenes can trap other molecules, making them useful for targeted drug delivery systems, and have a huge surface area that makes them useful as catalysts.
Why can carbon form such a large variety of compounds?
Carbon is in Group 4 and can form four covalent bonds. This allows it to form long chains and rings, giving rise to a huge number of natural and synthetic organic compounds.
Define melting point.
The melting point is the specific temperature at which a solid changes into a liquid. It is unique to each pure substance and is the same temperature as the substance's freezing point.
Why do simple molecular substances have lower melting points than giant ionic or giant covalent substances?
Simple molecular substances have weak intermolecular forces between their molecules that need little energy to overcome. Giant ionic and giant covalent structures have much stronger bonds, requiring far more energy to break.
True or False?
Evaporation only occurs at the boiling point of a liquid.
False.
Boiling point is the temperature at which a liquid changes to a gas throughout the liquid. Evaporation occurs at the surface of a liquid at temperatures below the boiling point, when the most energetic particles escape.
.......... is when a solid changes directly into a gas without passing through the liquid state.
Sublimation is when a solid changes directly into a gas without passing through the liquid state.
A substance has a melting point of 50 °C and a boiling point of 150 °C. What is its physical state at 80 °C?
At 80 °C the substance is a liquid. This temperature is above the melting point (50 °C) but below the boiling point (150 °C).
True or False?
The melting point and freezing point of a pure substance are the same temperature.
True.
Melting is the change of state from solid to liquid. Freezing is the reverse — liquid to solid. Both occur at the same temperature (the melting point/freezing point) for a pure substance.
What happens to the particles of a gas when it condenses?
When a gas condenses, its particles lose energy on cooling. They no longer have enough energy to bounce apart when they collide, so they group together to form a liquid.
A substance is in the .......... state at temperatures below its melting point and in the .......... state at temperatures above its boiling point.
A substance is in the solid state at temperatures below its melting point and in the gas state at temperatures above its boiling point.
What are bulk properties?
Bulk properties are physical properties such as electrical conductivity and melting point that arise from many atoms, ions or molecules acting together. Individual atoms do not possess these properties.
Why are metals malleable while ionic and giant covalent structures are brittle?
In metals, atoms are arranged in layers that can slide over each other when a force is applied, allowing the shape to change without breaking. In ionic and giant covalent structures, applying force breaks the bonds, causing the material to crack.
True or False?
Ionic compounds can conduct electricity when dissolved in water or melted.
True.
When dissolved or melted, the ions in an ionic compound are free to move and carry charge through the liquid. In the solid state they are fixed in a lattice and cannot move, so solid ionic compounds do not conduct.
The intermolecular forces between polymer chains are .......... than those between simple covalent molecules but .......... than ionic or covalent bonds.
The intermolecular forces between polymer chains are larger than those between simple covalent molecules but weaker than ionic or covalent bonds.
Why do giant ionic and giant covalent structures have high melting and boiling points?
Both structures have high melting and boiling points because they contain many strong bonds throughout the structure that require a large amount of energy to break. In giant ionic structures, strong electrostatic attractions act between oppositely charged ions. In giant covalent structures, many strong covalent bonds link all atoms together.
True or False?
Only covalent compounds can form giant structures.
False.
Ionic compounds, metals, and non-metals with giant structures (like diamond and silicon dioxide) all have high melting points. This is because they contain many strong bonds throughout their structures that require large amounts of energy to break.
Why are simple molecular substances poor conductors of electricity?
Simple molecular substances have no free electrons and no ions — all electrons are held in covalent bonds within the molecules. There are no charged particles free to move and carry a current.
Most polymers are .......... at room temperature because the intermolecular forces between their chains require .......... energy to break than those between simple molecules.
Most polymers are solid at room temperature because the intermolecular forces between their chains require more energy to break than those between simple molecules.
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