f4 chem acid and alkali

Hygroscopicsubstances 是指「可吸濕的物質」，是能夠從空氣中吸收水蒸氣的物質。有些 hygroscopic substances 是 deliquescent substances，有些不是。

有些 hygroscopic substances (例如 sodium hydroxide)，會溶於所吸收的水中，形成 aqueous solution。這些hygroscopic substances 亦可稱為 deliquescent substances (可潮解的物質)。但 hygroscopic substances 中不是deliquescent substances 的 (例如 anhydrous calcium chloride)，便不會溶於所吸收的水中。

無論是否 hygroscopic substances，一般 hydroscopic substances 可作 drying agents (乾燥劑)。


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"Notice that a drying agent should not react withthe material being dried. For example, we don't use anhydrous CaCl2and conc. H2SO4 to dry NH3 (NOT NH3+) because ammonia reacts with both of them. Wecan use CaO to dry NH3."

以上一段的意思是：與要 dried 的material 有 reaction 的 substance，不可用作這 material 的 drying agent。例如，不能用 anhydrousCaCl2 和 conc. H2SO4 去 dry NH3，因為 ammonia 與它們都有 reaction。我們可用CaO 去 dry NH3。

Anhydrous CaCl2 should NOT be used as the drying agent of ammonia,because anhydrous CaCl2 reacts with ammonia gas，產生一個 complex，因而吸收了ammonia gas。
CaCl2 + xNH3 →[Ca(NH3)x]Cl2

Concentrated H2SO4 should NOT be used as the drying agentof ammonia, because concentrated H2SO4 reacts withammonia gas，產生一個 salt，因而吸收了 ammonia gas。
H2SO4­ + NH3 → (NH4)HSO4


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Can acid conduct electricity?

Pure acid CANNOT conduct electricity. The aqueous solution of acid can conduct electricity.


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Does conc. HClhave properties of a typical acid?

Yes, conc. HCl has properties of a typical acid. Such properties includes :
• Having a sour taste.
• Change the colour of indicators (e.g. change bluelitmus to red).
• React with metal (more reactive than copper) to give hydrogen gas.
• React with metal carbonates or metal hydrogencarbonates to give carbon dioxidegas.
• Neutralize with metal oxides and metal hydroxides.

Besides, conc. HCl is a weak reducing agent, but NOT an oxidizing agent.

2014-08-05 14:19:34 補充：
補充在《意見欄》。

2014-08-05 14:20:38 補充：
• acids 是 molecular compounds，可在 aqueous solution 中 dissociate to give hydrogen ions。常見例子有：HCl、HNO3、H2SO4、CH3COOH。

• acidic properties，其實就是 aqueous hydrogen ions 的 properties。因此，acids 的水溶液可導電，但 acids 的 (molten) liquid 不可導電，因為是 molecular。

2014-08-05 14:25:56 補充：
• base 是可與 acid 產生 neutralization 的物質。常見的 base 有 metal oxides, metal hydroxides 和 NH3 三種。neutralization 的例子：
CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
NH3(aq) + HNO3(aq) → NH4NO3(aq)

2014-08-05 14:27:20 補充：
• 除了 ammonia 是 molecular 外，其餘常見的 bases 都是 ionic substances，它們的 molten liquids 和 aqueous solutions 都可導電。但因為 ammonia 是 molecular，它的 aqueous solution 可導電，但 liquid 不可導電。

• 若一個 base 可溶於水，而在水中產生 hydroxides ions 是唯一的 anions，這個 base 亦稱為 alkali。常見的 alkalis 有 NaOH, KOH, Ca(OH)2 and NH3。

2014-08-05 14:28:48 補充：
• 既不屬於 acids 又不屬於 bases 的 substances，則稱為 neutral substances。

• If a drying agent 同時又係一個 acid，則稱為 acidic drying agent。例子有：conc. H2SO4、P4O¬10 等。

2014-08-05 14:29:02 補充：
• If a drying agent 同時又係一個 base，則稱為 basic drying agent。例子有：CaO、NaOH、soda lime 等。

• If a drying agent 同時又係 neutral，則稱為 neutral drying agent。例子有：anhydrous CaCl2、silica gel 等。

2014-08-06 00:19:07 補充：
anhydrous calcium chloride 可以用來 dry 除了 NH3 以外所有中學常見的氣體。

1)q:Notice that a drying agent should not react with the material being dried.For example, we don't use anhydrous CaCl2 and conc. H2SO4 to dry NH3+ because ammonia reacts with both of them.We can use CaO to dry NH3+.

a:First of all, there are three types of drying agents:acidic(conc.H2SO4),neutral(CaCl2) or basic(CuO)

To dry an acidic sustances, we cant use an alkaline drying agent as neutralization will happen.
To dry an alkaline sustances, we cant use an acidic
drying agent as neutralization will happen

In your question,i think you want to mention NH3, as there is no NH3+ in the world.
When NH3 dissloves in water( you need to dry it,it means that it contains water)
it is alkaline as hydroxide ion is formed :
NH3+H2O----> NH4+ + OH-

so, we cant use H2SO4 to dry it as neutralization will happen

For the anhydrous calcium chloride, it can absorb NH3 but forming a complex

CaCl2 + nNH3---->[Ca(NH3)n]Cl2(s)

So, anhydrous calcium chloride cant dry the NH3

Finally, CuO is an basic drying agent, it has no reaction with NH3 ,so it can dry NH3



1)Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically changed somewhat, by an increase in volume, stickiness, or other physical characteristic of the material

你好勤力呀，中四暑假就溫書！係好事黎既

hygroscopic: 吸水 (absorb water from the atmosphere)

deliquescent : absorb water & dissolve in the water abosrbed. 即係吸水同溶係自己吸既水裏

另一條唔知:(

2014-08-04 21:18:26 補充：
acid CAN conduct electricity only when they are in molten state or in aqueous

通電要素:1mobile 2 charged particles (e.g. ions, electrons

so when acid is dissolved in water or become in molten state, 就有mobile ions 幫手通電

2014-08-04 21:21:12 補充：
HCl has the properties of a typical acid, it can react with a base to form salt and water.

HOWEVER, 你睇到的資料講話H2SO4 同HNO3 可同unreactive metals react 只限於when 前者是concentrated & 後者為dilute or concentrated. dilute sulphuric acid and very dilute nitric acid have NO reaction with metals below Pb (lead) in

2014-08-04 21:25:12 補充：
properties of a typical acid include 'sour', 'can be electrolytes' 'can turn blue litmus paper red', 'can react with metals above Pb' , 'can react with metal carbonates or hydrogencarbonates' and 'can react with metal oxide or metal hydroxide'

2014-08-05 12:32:47 補充：
Alkali係咪咩情况都通電?因為acid要喺molten state或in aqeous solution form 先conduct到嘛!

原理一樣，不過要留意既係alkali 大多都係ionic compound only ammonia isn't ionic compound in my textbk), so they dissociate instead of ionize in water.