问题 推断题

(1)物质X、Y是中学化学中的常见物质,X既能与稀硫酸反应,又能与氢氧化钠溶液反应。

①若X为氧化物,X的化学式是________;

②若X与稀硫酸反应生成无色无味的气体A,与浓氢氧化钠溶液加热反应生成气体B。等物质的量的A和B在水溶液中反应又生成X。X与足量稀氢氧化钠溶液反应的离子方程式是____________;

③若X为金属单质,写出X与氢氧化钠溶液反应的化学方程式___________;若Y为非金属单质,Y不能与 浓硫酸或稀硫酸反应,却能与某种酸反应放出氢气,并与氢氧化钠溶液反应放出氢气。写出Y与氢氧化钠溶液反应的化学方程式并用单线桥法标明电子的转移________________。

(2)在宇航器中,可以不断地将座舱内的空气通过盛有金属过氧化物(以过氧化钠为例)的容器,以除去 A。反应的化学方程式是________________。科学家设计出新的航天飞船内的空气更新系统。其中的一步反应是A 和氢气在200℃~ 250℃,镍作催化剂的条件下,生成甲烷和另一种物质。该反应的化学方程式

是________________。

答案

(1)①Al2O3;②NH4++HCO3-+2OH-=NH3·H2O+CO32-+H2O;③2Al+2NaOH+2H2O=2NaAlO2+3H2↑ ;

(2)2CO2+2Na2O2=2Na2CO3+O2;CO2+4H2CH4+2H2O

单项选择题
单项选择题

Workforce is defined as the total number of people who are available to work and earn incomes. The definition includes everyone who is employed or seeking paid employment, so it includes employers and the self-employed.

Although the size of the workforce depends a great deal on the size of the total population, there are several other influences which also affect it. The age distribution of the total population has a very marked effect on the available workforce. If the population has a high proportion of very young people or of those too old to work, then the available workforce would be lower than if there were an evenly spread age distribution. If the population grows rapidly from natural increase, i.e., the number of births greatly exceeds the number of deaths, then as the total population increases, the proportion in the workforce declines

Sometimes a population is described as aging, which means that the birth rate is either falling or growing very slowly, and as people retire from the workforce, there are not enough young people entering it to replace those who are leaving it. The population is top-heavy with older people. So the percentage of the population in the workforce declines when there is either a rapid increase in births or a falling birth rate.

The age distribution of the population has several important effects on the economy. If the population is aging and there is an increase in the number of people retiring without a corresponding increase in the number entering the workforce, this raises the problem of the ability of the economy to provide a reasonable level of social services to the retired group. If the aged are to be cared for in special homes or hotels, finances must be available for that purpose. If the size of the workforce is small relative to the total population, then the government tax receipts are relatively low and either the government has less money available to it or the workforce members have to be taxed more heavily.

It can be concluded from Paragraph 2 that ().

A. a population growth leads to a greater proportion of workforce

B. a large population does not necessarily mean a high proportion of workforce

C. the size of the aged determines the proportion of workforce

D. the proportion of the very young determines the size of workforce