Edexcel GCSE additional science chemistry: covalent compounds and separation
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| What is a covalent bond? | A pair of electrons that are shared by two atoms that are either the same or different non-metals
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| How many electrons can there be in a covalent bond? | Only 2
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| Show the covalent bonding for methane | CH4
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| Show the covalent bonding for oxygen | O2
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| What are simple molecular covalent substances and what are some properties of them? | Covalent molecules with a discrete formula. Properties: comparatively low melting and boiling points as there are weak bonds, poor electrical conductors
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| What are giant molecular covalent substances and what are some properties of them? | Covalent substances where there are billions of atoms joined by covalent bonds. Properties: high boiling and melting points due to strong covalent bonds, some conduct electricity (graphite) but most don't
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| What are some properties and uses of diamond? | Diamond is a carbon allotrope, a giant molecular covalent substance (each carbon atom is joined to 4 atoms in a tetrahedral pattern) High boiling/melting points. Very hard- cutting tools. Sparkle and reflects light- jewellery. Doesn't conduct electricity
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| What are some properties and uses of graphite? | Carbon allotrope- giant molecular covalent substance (each C atom is joined to 3 atoms in a layer and delocalised electrons bond layers together) High melting/boiling points. Electrical conductor- electrodes. Rubs away in layers- pencil lead & lubricant
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| What does it mean if liquids are immiscible? | They do not mix when put together, instead they form two layers e.g. oil and water
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| How do you separate immiscible liquids? | Separating funnel- liquids are put in the funnel and are left to separate out, then the tap is opened and the bottom liquid runs out. When the bottom liquid has all run out, the tap is closed.
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| What does it mean if liquids are miscible? | They mix when put together and form one layer e.g. alcohol and water
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| How do you separate miscible liquids? | Fractional distillation- heat the liquids and allow them to evaporate, then put them in a fractionating column and wait for them to condense at different points. Highest boiling point condenses at the bottom. Lowest boiling point condenses at the top
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| How do you separate the components of liquid air? | Air is cooled so water can freeze and be removed. Air is cooled to -200 degrees c and is liquefied, then is wamred to -185 degrees c in a fractionating column. Liquid nitrogen rises to the top and is collected. Liquid oxygen is collected at the bottom.
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| What is paper chromatography used for? | It is used to separate out different solutes dissolved in one solvent e.g. in inks
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| How does paper chromatography work? | The samples are put on a pencil line on chromatography paper and are placed in a beaker with water at the bottom. The water soaks up the paper and carries the solutes with it producing a chromatogram with separated solutes
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| Why do solutes spread out in chromatography? | They all have different solubilities so move at different speeds
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| What is the Rf value, how is it calculated and why is it used? | It is a way of identifying the solutes dissolved in a solvent- Rf value = distance moved by compounds / distance moved by solvent. It is used in forensics and in the food industry to identify specific compounds
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