F.4 chemistry questions

1)
a) physical change
Gaseous hydrogen changes to liquid hydrogen. No new substance is formed.
(All changes of physical state are physical change.)

b) chemical change
Fermentation of glucose gives ethanol and carbon dioxide. Hence, new substancesare formed.

c) chemical change
Explosion is an extremely rapid burning.
(All burning processes are chemical changes.)

d) physical change
Liquid butter changes to solid butter. No new substance is formed.
(All changes of physical state are physical change.)

e) physical change
Just reduce the size of rice grind. No new substance is formed.

f) chemical change
A new substance is formed which is brown in colour.


2.
a) chemical change
Toxicity to human is due to the bad effect on metabolism. Metabolism includes aseries of chemical reactions.

b) physical change
Benzene is easily vaporized in vacuum, and can be solidifies in vacuum whencooled.
However, all changes of physical state are physical changes, as no newsubstance is formed.

c) physical change
Mixing benzene and hexane form a mixture of the two substances. No new substance is formed.

d) physical change
Molar mass of benzene involves benzene only. No other substance is involved.

e) chemical change
Burning of benzene gives carbon dioxide and water. Hence, new substances areformed.
(All burning processes are chemical changes.)

f) chemical change
The reason is identical to (e).

所有燃燒 (burning/combustion/ignition/explosion/flammable/inflammable) 都是 chemical changes。

這是基礎的化學常識，不用每一題都用實驗試吧！況且由於 benzene 的 toxicity，中學實驗是不容許用它做實驗的。Mr. X 的答案（根本就沒有回答！）有點不切實驗吧！

chemical change occurs when a new substance is formed through a chemical reaction.

a) Liquefaction of gaseous hydrogen.
並沒有新物質的產生，氫仍然是氫，所以是 physical change.
b) Fermentation of glucose
有新物質的產生，例如酒(乙醇)，所以是 chemical change.
c) Explosion of fireworks in the sky
放煙花時，有沒有新物質產生呢？
d) Hardening of butter in liquid nitrogen
用液態氮使牛油變硬時，有沒有新物質產生呢？
e) Grinding of rice into rice flour using a mortar and a pestle
把米磨成粉時，有沒有新物質產生呢？
f) Fresh-cut fruits turn brown upon exposure to air
新鮮水果切開後變啡，有沒有新物質產生呢？
這個有點難，答案是有新物質的產生，所以是 chemical change.


如果要試驗該物質是否有這種性質的試驗過程是一個化學反應則該性質為一個化學性質，例如碳是可燃的，這是一個化學性質。為了試驗這個性質，你需要把碳在空氣中加熱，因而會產生新物質(二氧化碳)，所以涉及化學反應，因而是一個化學性質。
a) It has an acute toxicity of 2.99mg/kg
中毒應該是涉及化學反應的，所以是 chemical property.
b) it solidifies easily in vacuum
要測試這個性質，會把(苯)benzene放到真空中，沒有新物質的產生，所以是 physical change.
其實把(苯)benzene放到真空中，它會氣化而不是固化solidifies.
C) it forms a clear homogeneous solution with hexane
苯與己烷(hexane)混合，例如水加酒有沒有新物質的產生呢？是不是化學反應呢？
d) it has a molar mass of 78g/mol
是一個用秤去秤的過程，有沒有新物質的產生呢？是不是化學反應呢？
e)it burns with a pink colored flame
為了驗証，是不是要燒它呢，是不是化學反應呢？
f) it is flammable
為了驗証，是不是要燒它呢，是不是化學反應呢？