问题 选择题

只需用下列试剂中的1种,即可将氯化铝、偏铝酸钠、硝酸银、碳酸钠四种无色溶液一次鉴别。这种试剂是()                                                         

A..氢氧化钠溶液

B.稀硝酸

C.盐酸

D.氯化钡溶液

答案

答案:C

氢氧化钠无法鉴别偏铝酸钠和氯化钠,稀硝酸无法鉴别氯化铝和硝酸银,氯化钡无法鉴别氯化铝和偏铝酸钠,硝酸银和碳酸钠中分别加入氯化钡都会产生白色沉淀无法鉴别。当将盐酸溶液加入四种溶液中时,氯化铝无现象,偏铝酸钠先产生沉淀,再加入盐酸沉淀会溶解,硝酸银产生白色沉淀,再加入盐酸,沉淀不溶解,碳酸钠中加入盐酸会产生气泡,四种物质与盐酸的反应现象不同,所以选C。

阅读理解

Babies born in summer are more likely to become short-sighted in late life, a study has shown.

As many as a quarter of all cases of short-sightedness are caused by too great an exposure to sunlight in the first weeks of life, say eye experts.

They are advising all parents to put sunglasses on their babies during the first weeks.

Scientists had already established that over-exposure to sunlight caused short-sightedness in animals.

Researchers who compared the months in which babies were born with whether they needed glasses later on say the principle also applies to humans.

A study of almost 300,000 young adults─the largest of its kind─showed that those born in June and July had a 25 per cent greater chance of becoming severely short-sighted than those born in December or January. Research leader Professor Michael Belkin, of Tel Aviv University, said it was because prolonged illumination(光照) causes the eyeball to lengthen, causing short-sightedness.

Hence the more light a newborn is exposed to, the more the eyeball lengthens and the worse the short-sightedness will be.

The mechanism which lengthens the eyeball is associated with levels of melatonin(褪黑激素), a pigment (色素) which protects the skin against harmful rays of the sun.

In young babies not enough melatonin is released as protection, meaning they are more vulnerable to sunburn and changes to eyeball shape.

Sight expert Professor Daniel O’Leary, of Anglia Ruskin University in Cambridge, said “At the moment we don’t know the precise cause of why light exposure affects sight, but the evidence seems to prove that it is one of the reasons for people becoming short-sighted.”

55. Babies born in summer are more likely to be short-sighted ____________.

A. because the summer sun is too strong for babies

B. because babies born in summer have lengthened eyeballs

C. if they are exposed to much sunlight in the first weeks after they are born

D. if parents don’t know a proper way to protect their babies’ eyes

56. Melatonin is a kind of material to ___________.

A. prevent the eyes from becoming near-sighted

B. protect the skin from harmful sun rays

C. make our body strong

D. protect babies’ eyes from summer sun

57.   From what Professor Daniel O’Leary says we can conclude that ___________.

A. there is no evidence that short-sightedness is related to exposure to sunlight

B. whether light exposure affects sight still needs to be further proved

C. he believes that light exposure can cause short-sightedness

D. he tries to give the cause of why light exposure affects sight

58.   The underlined word “vulnerable” in the passage probably means __________.

A. easy to be harmed                          B. resistant

C. protective                                     D. changeable

单项选择题