Chemistry

Why do ionic substances not conduct electricity?

Because ionic compounds hold ions together in a lattice when solid. They do conduct electricity when they are molten or in an aqueous solution as the charged particles are free to move.

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Why do ionic substances have high melting/boiling points?

Ions are held together in a lattice by very strong electrostatic forces, meaning a lot of energy is needed to break the bonds.

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Why do simple molecular covalent substances have low melting/ boiling points and do not conduct?

Because there are weak forces between the molecules, and there are no charged particles free to move.

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Why do giant molecular covalent substances have high melting/boiling points?

Because the covalent bonds between molecules are strong, and it takes a lot of energy to break these bonds, thats why they're very hard.

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Does graphite conduct electricity? Explain.

Graphite is made up of layers of carbon atoms, and there are weak forces between these layers, allowing charged electrons to move freely between the layers> Therefore even though graphite is a giant molecular covalent substance it is a conductor.

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What is the difference between miscible/immiscible?

Miscible liquid can mix completely with each other, whereas immiscible liquid separate.

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What apparatus can be used to separate immiscible liquids?

Separating funnel.

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How can miscible liquids be separated?

Fractional Distillation: The mixture is heated and the vapours pass into a fractionating column. As the gases cool down, the ones with the highest boiling points condense first. Different liquids are collected at different places in the column.

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Explain why the following temps used in a fractionating column are used to separate nitrogen and oxygen from air. (a) -200°c for the air going into the column (b) -185°c at the bottom of the column.

(a) Nitrogen and oxygen need to be liquids as they go into the column, and this temp is below the b.p's of both. (b) this is below the b.p in oxygen so it stays a liquid/can be piped off; but above the b.p of nitrogen so it evaporates/goes up column.

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What are the uses of chromatography.

Discovering what dye is used in food, in museums to detect forgeries, forensic science to analyse DNA samples or or paints and inks from crime scenes.

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Chemistry

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