下列操作能证明溶液中有SO42-存在的是（） A．加入硝酸酸化_爱下问搜题

搜题请搜索小程序“爱下问搜题”

问题选择题

下列操作能证明溶液中有SO₄^2-存在的是（　　）

A．加入硝酸酸化的Fe（NO₃）₃，产生白色沉淀

B．加入BaCl₂溶液，如果产生白色沉淀，则一定含有SO₄^2-

C．先加入HCl溶液，如果无明显现象再加入BaCl₂，如果产生白色沉淀则有SO₄^2-

D．向溶液中加盐酸酸化的BaCl₂，如果产生白色沉淀，则一定有SO₄^2-

答案

A．加入硝酸酸化的Fe（NO₃）₃，与SO₄^2-不反应，没有明显现象，故A错误；

B．加入BaCl₂溶液，不能排除Ag⁺的影响，可能生成AgCl沉淀，故B错误；

C．先加入HCl溶液，如果无明显现象，说明不含有Ag⁺，并且排除SO₃^2-、CO₃^2-等离子的影响，再加入BaCl₂，如果产生白色沉淀则有SO₄^2-，故C正确；

D．向溶液中加盐酸酸化的BaCl₂，不能排除Ag⁺的影响，可能生成AgCl沉淀，故D错误．

故选C．

解答题

多项式 4x^2m+1y-5x²y²-πx⁵y．（1）填写多项式各项及其系数和次数；（2）若多项式是八次多项式，求m的值．

完形填空

In the past, man did not have to think about the protection of his environment. There were few people on the earth, and natural resources seemed to be____36____ .
Today things are___37_____, the world has become too___38____ . We are using up our natural resources too quickly, and at the same time we are___39____ our environment with dangerous chemicals. If we continue to do this, human life on the earth ___40____survive.
Everyone___41_____ today that if too many fish are taken from the sea, there will soon be none left. Yet, with modern fishing__42_____ , more and more fish are caught. We know that if too many trees are cut down, ___43____will disappear and nothing will grow on the land. Yet, we ___44_____to use bigger and more powerful machines to __45____more and more trees.
We realize that if rivers are polluted with waste products from factories, we will die.___46_____, in most countries wastes are ___47______put into rivers or into the sea, and there are _48_____laws to stop this.
We know, too, that if the___49____ of the world continues to rise at the present rate, in a few years, there will not be enough___50_____. What can we do to solve these problems ?
It we eat more vegetables and less___51____, there will be more food available for everyone. Land that is used to grow crops __52_____five times more people than land where animals are kept.
Our natural resources will _53_____longer if we learn to recycle them. The world population will not rise so quickly if people use modern methods of birth__54_______ .
Finally, if we educate people to think about the problems, we shall have a better and clearer ___55____in the future.

C．Generally speaking