问题 选择题

用质量分数为5%的氯化钠溶液配制50克质量分数为1%的氯化钠溶液,必不可少的一组仪器是(  )

A.托盘天平、药匙、玻璃棒

B.量筒、烧杯、酒精灯

C.量筒、烧杯、玻璃棒

D.量筒、玻璃棒、漏斗

答案

A、溶液加水稀释操作中,因为不涉及固体物质的取用,所以在选择仪器时托盘天平、药匙等与取用固体物质相关的仪器就不需要了;

B、加水进行稀释,操作中不涉及加热,因此本组的酒精灯也属于不需要的仪器;

C、把5%的食盐溶液稀释成1%的溶液,使用量筒量取一定量5%的氯化钠溶液和一定量的水,然后倒入烧杯中用玻璃棒充分搅拌均匀,因此此组为必不可少的仪器;

D、漏斗在过滤操作中是必需的仪器,而稀释操作与漏斗无关.

故选C.

填空题
阅读理解

“Mom, can I bake some bread?” We were 15; my best friend, Hanna, and I, determined to try our hands at creating some beautiful bread.

“It’s not worth the trouble,” my mother said. “It takes lots of time and makes a big mess. Our bakery bread is delicious without all that effort.”

Begging was useless. Mom’s “no” meant “No!”

But several weeks later, opportunity knocked. My parents were going out for the evening. I immediately invited Hanna to be my partner in bread-baking crime.

We studied the recipe. That was easy. “Mix oil into flour then beat in four eggs, one at a time, with remaining sugar and salt.”

We were not good at breaking eggs. I tried to learn from my mother.

“Gradually add eight cups of flour. When dough (面团) holds together, squeeze it.”

We took turns working like that. “Is the dough ‘holding together’?” we asked each other.

I remembered my neighbor’s instructions: “If it’s too sticky, add some flour; if too dry, add water.”

We added water. Then more flour. Then more water. By then, the mass of our dough had grown very much.

“Place dough on floured surface and squeeze till smooth,” the recipe instructed.

We took turns burying our hands in the damp dough, pinching, squeezing, and feeling it leak between fingers. “Clean and oil bowl, and then return dough to bowl. Cover and let dough rise in a warm place for one hour.”

This was good news—we’d have a break. On the dirty kitchen chairs, we dreamed about our beautiful bread. “See?” we would tell my mom. “Isn’t it worth the work?”

Hanna and I couldn’t help glancing at the rising process every few minutes. But nothing happened.

“Maybe something will happen in the hot oven,” I said.

Unfortunately, when we removed the loaves from the oven, they were like hard stones.

Mom was right; it takes time and effort. It sometimes makes a mess. But still it feels good, somehow, to be part of that long, ongoing chain of bread bakers. Since that night, both Hanna and I have learned to do it right.