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Comparing
Concentrated and Dilute Acids
F.4 Chemistry
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Physical Property
Characteristic: No new substance will be formed in the
testing process
Examples: pH value, melting point, boiling point, density,
colour, magnetic property, conductivity, solubility …..
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Chemical Property
Characteristic: new Substance(s) will be formed in the
testing process.
Examples: Chemical reactions with other reagents., i.e.
reaction of acids with metals to form hydrogen.
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Physical Properties
1. pH Paper pH paper
Dilute HCl(aq) Conc. HCl(aq)
Reason: related to the concentration of H +(aq)
After dipping
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2. Electrical Conductivity
Dilute HCl(aq) Conc. HCl(aq)
dim bright
Reason: related to the concentration of ions present
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Chemical Properties
1. Reaction with carbonates
• Same for all mineral acids, dilute or concentrated
• Observations
•Gas bubbles evolve
•Solids dissolve
Reason: Concentrated acids have ahigher concentration of Hydrogenion so they react faster and bubblesare given out at a faster rate
carbonateCO 3
2-
+ 2H+
H2O + CO 2
CO 2
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Chemical Properties
2. Reaction with oxides and hydroxides e.g. copper(II) oxide
• same for all acids(dilute or concentrated)
• Observationblack powder dissolvesa green/blue solution remains
Reason: black copper(II) oxidedissolves to form Cu 2+(aq),which is blue/green in colour. Before After
Copper(II) oxide
CuO (s) + 2HCl (aq) CuCl 2 (aq) + H 2O (l)
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Chemical Properties
3. Reaction with metals
Dilute Concentrated
HydrochloricAcid Gives HydrogenGas Gives HydrogenGasSulphuric Acid Gives Hydrogen
GasGives Sulphur Dioxide Gas
Nitric Acid Gives NitrogenMonoxide Gas
Gives NitrogenDioxide Gas
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Observations
1. Hydrogen GasPop
• Colourless gas evolves
• Gives a ‘pop’ sound with a burning splint
• The metal dissolves
N.B.: Not for metals below lead in the Reactivity Series
HCl(aq)
Zinc metal
Zn (aq) + 2 HCl (aq) ZnCl 2 (aq) + H 2 (aq)
Video
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Observations
2. Sulphur Dioxide Gas
• Colourless gas evolves
• The metal dissolves
N.B.: The reaction is faster when hot acid is used.
H2SO
4(l)
Zinc metal
K 2Cr 2O7/H+(aq)
Zn (s) + 2 H 2SO 4 (l) ZnSO 4 (aq) + SO 2 (g) + 2H 2O (l)
• It turns orange potassium dichromate paper green
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Observations
2a. Nitrogen Dioxide Gas
• Brown gas (NO 2) evolves just above liquid level
• The metal dissolves
N.B.: The reaction is faster when hot acid is used.
HNO 3(conc.)Zinc metal
Zn (s) + 4 HNO 3 (conc.) Zn(NO 3)2 (aq) + 2NO 2 (g) +2H 2O(l)
Brown gas
a) Conc. Nitric acid
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Observations
2b. Nitrogen Dioxide Gas
• Brown gas (NO 2) appears at the upper part of thetest tube.
• The metal dissolves
N.B.: colourless nitrogen monoxide forms which reacts withair oxygen to form brown nitrogen dioxide gas.
HNO 3 (dilute)Zinc metal
3Zn (s) + 8 HNO 3 (conc.) 3 Zn(NO 3)2 (aq) + 2 NO (g) +4 H 2O (l)
2NO (g) + O 2 (g) 2 NO 2 (g)
Brown gas
b) Dilute Nitric acid
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Other reaction
Dehydrating property of conc. Sulphuric acid
a. Reaction with sugar
• Black spongy solid of carbon forms
C12
H22
O11
(s) 12 C (s) + 11 H2O (l)
Reason: H and O atomsare removed from sugar
molecules in the ratio 2:1.Water is formed andcarbon solid remains
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