An alloy is usually harder than the pure metal 64 l

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·: An alloy is usually harder than the pure metal. 64
l 52. 53. Exercise - Topic 3: Metals B A: False Zn is more reactive than tin (Sn). B: True C: False Sn protects iron from rusting by separating Fe from the contact with air and water . D: False Since Fe is more reactive than Sn, once the Sn layer on · 1he Fe has been broken, rusting is speeded up. Sn cannot prevent iron from - rusting by sacrificial protection . The metal that provides sacrificial protection must be more reactive than Fe. D Fe 2 ' is produced during the first step of the rusting process. Fe'• (NOT Fe 3 •) reacts with rust indicator to give a blue colour. 54 . C Reacti vity seri es: Mg> Zn > Fe > Cu > Ag Rusting occurs less readily when iron is connected to a metal higher than iron in the reac tivi ty series, e.g. Zn and Mg (dish 2 and dish 4) Rusting occurs more read ily when iron is connected to a metal lower than iron in the r ea c ti vi ty seri es, e. g. Ag and Cu (dish 1 and dish 3) 55. A (3 ) is a fact BUT not a reason. Although Al is the most abundant metal in the earth crust (NOT Fe) , it is NOT cheap. This is becau se Al is usually extract ed from Al 2 O 3 in molten state by electrolysis, which involves the use of a lar ge amou nt of el ectrici ty and thu s is expensive. 65 Exercise - Topic 3: Metals 56. D (1): False ·: Fe is stronger than Al. (2): True Al is lightweight due to its low density . (3): True Al has an inert Al 2 0 3 layer firmly adhere to its surface. This oxide layer Is impervious/ impermeable (7,: Ill ;f'1! to both oxygen and water . This prevents the Al from further oxidation more corrosion-resistant than Fe 57 . D A: False B: False C: False D: True 58 . B (1): False (2): True (3): False Stainless steel is an alloy containing Fe, C, Mn and Cr . There is no Al. Anodization is a process used to increase the thickness of Al 2 O 3 layer on Al surface by electrolysis, so as to increase the corrosion resistance. Aluminium is the most abundant METALLIC element in the earth's crust. Oxygen is the most abundant element in the earth 's crust . 4Al(s) + 3O 2 (g) - 2Al 2 O 3 /s) The cost for extracting Al (by electrolysis) is higher than the cost for extracting iron (by carbon reduction) . Al has an inert _ Al 2 O 3 layer firmly adhere to its surface. This oxide layer is impervious/ impermeable (,;:1>1;fcilitl/J) to both oxygen and water . This prevents the Al from further oxidation more corrosion-resistant than Fe Fe is harder than anodized alumi nium . 59.

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