Inorganic Chemistry – The elements of Group 1 and 2

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What is the trend in Ionisation energy down Group 2?
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As you go down group 2 the ionisation energy *decreases*
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Why does Ionisation energy decrease down group 2?
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-extra *inner shielding of electrons* -*outer electrons are further* from the nucleus so becomes easier to remove
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What is the difference between an s-orbital electron being removed from calcium compared to magnesium
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It is a 4s-orbital electron being removed from Calcium which is further away from the nucleus than the 3s-orbital electron removed when Magnesium is ionised
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In the same period is the 1st ionisation energy of the group 2 metal greater than equivalent group 1 metal i.e. Li and Be? *why?*
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Be (group 2) ionisation energy is greater than Li (group 1) in the same period because group 2 atom has a smaller radius than the group 1 atom
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What is the trend in reactivity down group 2?
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As you go down group 2 reactivity increases
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In the same period, is the reactivity of the group 2 metal greater than the equivalent group 1 metal? i.e. Be and Li
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Reactivity of Be (group 2) is less than Li (group 1)
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Why does the reactivity down group 2 increase?
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the sum of the first and second ionisation energies decreases down the group, making it easier to remove the two outer electrons
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What is formed when group 2 elements (m) react with oxygen?
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Group 2 elements and oxygen form *ionic oxides* 2M+O₂ → 2MO
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Complete: Group 2 react when _________ with chlorine
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Group 2 react when *heated* with chlorine
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What is the chemical and word equation when group 2 elements react with chlorine?
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Group 2 element and chlorine produce *ionic chlorides* M + Cl₂ → MCl₂
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When MCl₂ dissolves in _________ they produce solutions that contain __________ cations of formula ___________
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When MCl₂ dissolved in *water* they produce solutions that contain *hydrated* cations of formula [M(H₂O)₆]
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What is the formula of MCl₂ in aqueous solution?
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[M(H₂O)₆]
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What is the name of [M(H₂O)₆]
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hexaquamagnesium
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What happens to [M(H₂O)₆] when it is placed in solvent water (2)
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hexaquamagnesium ion is *partially deprotonated* solution becomes *slightly acidic*
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Why does the solution become slightly acidic?
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H₃O ions are formed
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What is the chemical symbol equation for deprotonation (partially) of hexaquamagnesium ions?
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[Mg(H₂O)₆](aq) + H₂O(l) →[Mg(H₂O)₅(OH)] +H₃O
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What is the word and symbol equation for the reaction of group 2 metals with cold water
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metal + water → metal hydroxide + hydrogen Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)
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What is the word and symbol equation for the reaction of group 2 metals with steam
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metal + steam → metal oxide + hydrogen Mg(s) + H₂O(g) → MgO(s) + H₂
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What is the difference in rate between the reaction of water with group2 and the reaction of steam with group 2
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The reaction of water with group 2 metals is slower
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Group 2 oxides and hydroxides are b____
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Group 2 oxides and hydroxides are *bases*
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*Describe* the reaction of Magnesium *oxide* with water
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-reacts slowly -form slightly alkaline suspension of Magnesium Hydroxide
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What is the symbol equation for the reaction of Magnesium Oxide with water?
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MgO(s) + H₂O(l) → Mg(OH)₂(s)
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What is another name for calcium oxide
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quicklime
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What is the chemical equation for calcium oxide reacting with water?
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CaO(s) + H₂O(l) → Ca(OH)₂(s) *forms alkaline suspension of calcium hydroxide Ca(OH)₂(s) ⇌Ca²⁺(aq) + 2OH⁻(aq)
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What is the chemical equation for the reaction of strontium and barium oxides with water
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BaO(s) + H₂O(l) →Ba²⁺ (aq) + 2OH⁻ (aq) *forms alkaline solutions
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As group 2 oxides and hydroxides are bases what do they form when they react with acids?
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salt and water
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What is the chemical equation when grouup 2 oxide (MO) reacts with acid?
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MO(s) + 2H⁺(aq) → M²⁺(aq) + H₂O(l)
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What is the chemical equation when group 2 hydroxide (M(OH)₂) reacts with acid?
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M(OH)₂(s) + 2H⁺(aq) → M²⁺ + 2H₂O
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What happens to the solubility of group 2 hydroxides as you go down the group?
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The solubility of group 2 hydroxides *increases* down the group
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What happens to the solubility of group 2 sulfates as you go down the group?
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The solubility of group 2 sulfates *decreases* down the group
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What is thermal stability
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is the stability of a molecule at high temperatures; i.e. a molecule with more stability has more resistance to decomposition at high temperatures.
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What does thermal stability depend upon
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Thermal stability depends upon the *polarising power of the cation*
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Define polarising power
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The ability of the cation to distort an anion
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What is the polarising power determined by?
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Polarising power is determined by *charge density*
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When does an ion have a greater charge density?
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The *larger the charge* and the *smaller the radius*
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(Translate sentence using more specific terms) Compounds containing cations that (strongly polarise) the anion are more (easily decomposed) than those with less (polarising cations)
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Compounds containing cation with *high polarising power* are *less thermally stable* than those with *low polarising power*
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As you go down the group what happens to the polarising power?
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Polarising power *decreases* down a group
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As you go across a period what happens to the polarising power?
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Polarising power *increases* across a period
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Why does thermal stability *increase* down group 2? (i.e why does the ease of thermal decomposition decrease)
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-*ionic radius* increases down a group -reduces *polarising power* of the cation
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Why are group 2 compounds less stable to heat and decompose more easily compared to group 1 compounds?
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-group 2 elements have a *charge* of 2+ compared with a charge of 1+ for group 1 elements -group 2 *ions* have a *smaller ionic radius* than group 1 ions -therefore group 2 cations are *more polarising*
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What is the equation for Lithium nitrate decomposing and any other group 1 nitrate decomposing?
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Lithium nitrate → Lithium Oxide + Nitrogen Dioxide (brown gas) + Oxygen 4LiNO₃ → 2Li₂O +4NO₂ + O₂ Group 1 nitrate → molten group 1 nitrate + oxygen 2NaNO₃ → 2NaNO₂ + O₂
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Why does Lithium nitrate produce different products compared to group 1 nitrates
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-Li ion has *small ionic radius* -Therefore Li has *high charge density* -Li can *polarise* the O-N bonds in the nitrate ions to break them -other group 1 cations are *too large* to polarise to this extent
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What is the equation(word and chemical) for group 2 nitrates decomposing?
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Magnesium Nitrate → Magnesium Oxide + Nitrogen Dioxide + Oxygen 2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂
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Which group 1 carbonate decomposes when heated and why?
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Only *lithium carbonate* decomposes when heated because Li⁺ cation is *very small* and polarises the O-C bond in CO₃²⁻ ion sufficiently for it to break and form an O²⁻
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What is the word and chemical equation for Lithium Carbonate decomposing?
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Lithium carbonate → Lithium Oxide and Carbon Dioxide Li₂CO₃ → Li₂O + CO₂
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What is the equation for group 2 carbonates decomposing?
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CaCO₃ → CaO + CO₂
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What is the flame colour for sodium?
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Flame colour for sodium is *yellow*
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What is the flame colour for lithium?
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Flame colour for lithium is *crimson*
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What is the flame colour for Potassium
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Flame colour for potassium is *lilac*
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What is the flame colour for Rubidium
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Flame colour for Rubidium is *red*
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What is the flame colour for Caesium?
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Flame colour for Caesium is *blue*
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What is the flame colour for Magnesium?
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Flame colour for Magnesium is null and void (outside visible spectrum)
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What is the flame colour for Calcium?
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Flame colour for calcium is *brick red*
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What is the flame colour for Strontium?
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Flame colour for Strontium red
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What is the flame colour for Barium
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The flame colour for Barium is *apple green*
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What causes the formation of flame colours by group 1 and 2 compounds?
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Electron transitions
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Describe electron transitions(6)
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-heat causes *electron* to move to a *higher energy level* -electron is *unstable* in this *excited state* -therefore it drops back down -as it drops from high energy level to low energy level *energy is emitted* -the energy emitted is in the form of *visible light* -has the wavelength of observed light
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Describe the experiment to study *thermal decomposition*(4)
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-place *same amount* of each carbonate (or nitrate) in a series of hard glass *test tubes* -fix a *delivery tube* to the test tube and *clamp in a stand* -light the *bunsen burner* and *measure* the time taken for the *gas* evolved to reach the mark on the test tube in the water bath -repeat with the *same amount in moles* of the remaining carbohydrate
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Describe the flame test
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-take *platinum or nichrome* wire -clean by dipping in some *concentrated hydrochloric acid* on a watchglass then place in hottest part of a bunsen flame -repeat until flame is not coloured -dip wire in concentrated hydrochloric acid and then into the *solid* under test -place this in hottest part of the flame and *observe the colour* of the flame
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Hydrated magnesium nitrate, Mg(NO3)2.6H2O, is heated in a boiling tube and the following observations are made. Stage 1 The white solid forms a clear, colourless solution. Stage 2 Condensation forms around the mouth of the boiling tube and a white solid starts to form at the bottom of the tube. Stage 3 As the heating continues, the colourless solution disappears leaving a white solid. Stage 4 The white solid melts. Stage 5 A brown gas forms. Stage 6 A glowing splint reignites when it is placed in the boiling tube. Stage 7 A white solid is left in the boiling tube. *Explain what is happening in stages 1 and 2. (3)
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-The salt dissolves in the water of crystallization -water evaporates -Mg(NO₃)₂ begins to form (crystallizes)
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Suggest why the observations of the flame tests for Magnesium Chloride and Calcium Chloride are different (2)
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-the energy levels are different -for magnesium the energy released is outside the visible spectrum
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Write the equation when barium reacts with cold water (2)
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Ba(s) + 2H₂O(l) → Ba²(aq) + 2OH⁻(aq) + H₂(g)
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Dilute hydrochloric acid is added to barium hydroxide. Dilute sulfuric acid is added to another sample of the solution. How would the appearance of the resulting mixture from dilute sulfuric acid differ from the mixture produced in dilute hydrochloric acid. Explain this difference(2)
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-*white precipitate* is formed with dilute sulfuric acid -BaSO₄ is insoluble
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How could you distinguish between magnesium carbonate and barium carbonate by heating them? (1) Suggest another test other than heating or use of an acid (2)
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Barium Carbonate is more thermally stable than Magnesium Carbonate hence requires more heating. Flame test Magnesium Carbonate – no colour Barium Carbonate – green colour
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Write equations for the decomposition of potassium nitrate (1)
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Potassium nitrate → potassium nitrite and oxygen 2KNO₃→2KNO₂ + O₂
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State two things that would be observed when anhydrous calcium nitrate is heated (2)
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-Brown gas forms -white solid melts (then solidifies)
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Explain why potassium nitrate and calcium nitrate decompose to form different products (3)
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-Ca²+ have a higher charge density -Calcium ions are more polarizing than potassium -of nitrate anion
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Greenhouse gases can absorb infrared radiation. Explain why carbon dioxide absorbs infrared radiation but oxygen cannot (2)
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CO₂ has polar bonds so polarity of carbon dioxide changes
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Use your knowledge of intermolecular forces to suggest why butane has a higher melting temperature than 2-methyl propane (2)
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-Butane has a larger surface area -more contact between neighboring molecules
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Group 2 carbonates both undergo thermal decomposition. The difference in stability to heat can be compared in an experiment. Suggest how this experiment could be carried out (4)
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-reacting gas with limewater OR measuring change of mass OR collecting gas formed -measure time for limewater to go cloudy OR find loss of mass after heating samples for a given time OR measure volume of gas formed in a given time -for a fair comparison distance of flame from boiling tube constant -use carbonates with same particle size -same volume of lime water -keep strength of flame constant
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Explain why the melting temperature of silicon is very much greater than that of white phosphorus (3)
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-Si is a giant covalent structure -has strong *covalent* bonds which require a lot of energy to break -White phosphorus is a *simple covalent molecule* with weak intermolecular forces
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Explain why the melting temperature of argon is the lowest of all the elements of Period 3 (1)
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Argon is *monatomic*

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