问题 填空题

亚硝酸钠(化学式NaNO2)是一种外观极像食盐,且具有食盐一样咸味的有毒物质,因此这种工业用盐常被冒充食盐掺假或被误作食盐作用,导致多起中毒事故发生.请你阅读下表有关资料,并结合已有化学知识,用三种方法鉴别亚硝酸钠和氯化钠.

项目亚硝酸钠氯化钠
溶解性15℃时溶解度为81.5g15℃时溶解度为35.8g
熔点271℃801℃
沸点(320℃时发生分解主出有臭味气体)1413℃
水溶液酸碱性碱性中性
方案选取的试剂实验现象和结论
(1)

(2)

(3)

答案

根据题目中的表中有关资料:亚硝酸钠的溶解性、熔点、沸点和水溶液酸碱性,并结合已有化学知识,利用:在同一块金属片上加热两种物质,先熔化者为亚硝酸钠;各取少量分别溶于水,用PH试纸,pH>7的是亚硝酸钠;各取少量分别溶于水,分别滴加硝酸银溶液,有白色沉淀产生的是氯化钠.

故答为:

 方案 选取的试剂和方法 实验现象和结论
 (1) 在同一块金属片上加热两种物质 先熔化者为亚硝酸钠
 (2)各取少量分别溶于水,用PH试纸  pH>7的是亚硝酸钠
 (3)各取少量分别溶于水,分别滴加硝酸银溶液 有白色沉淀产生的是氯化钠

单项选择题
问答题

Ragtime is a musical form that synthesizes folk melodies and musical techniques into a brief quadrille-like structure, designed to be played—exactly as written—on the piano. (46) A p analogy exists between European composers like Ralph Vaughan Williams and Edward Grieg, who combined folk tunes and their own original materials in larger compositions and the pioneer composers in the United States. Composers like Scott Joplin and James Scott were in a sense collectors or musicologists, collecting dance and folk music in Black communities and consciously shaping it into brief suites or anthologies called piano rags.

It has sometimes been charged that ragtime is mechanical. For instance, Wilfred Mellers comments, "rags were transferred to the pianola roll and, even if not played by a machine, should be played like a machine, with meticulous precision." (47)However, there is no reason to assume that ragtime is inherently mechanical simply because commercial manufacturers applied a mechanical recording method to it, the only way to record pianos at that date. Ragtime’s is not a mechanical precision, and it is not precision limited to the style of performance. It arises from ragtime’s following a well-defined form and obeying simple rules within that form.

The classic formula for the piano rag disposes three to five themes in sixteen-bar strains, often organized with repeats. (48)The rag opens with a bright, memorable strain or theme, followed by a similar theme, leading to a melody of marked lyrical character, with the structure concluded by a lyrical strain that parallels the rhythmic developments of the earlier themes. The aim of the structure is to rise from one theme to another in a stair-step manner, ending on a note of triumph or exhilaration. Typically, each strain is divided into two 8-bar segments that are essentially alike, so the rhythmic-melodic unit of ragtime is only eight bars of 2/4 measure. (49) Therefore, not concerned with development of musical themes, the ragtime composer instead sets a theme down intact, in finished form, and links it to various related themes that are brief with clear melodic figures. Tension in ragtime compositions arises from a polarity between two basic ingredients: a continuous bass—called by jazz musicians a boom-chick bass--in the pianist’s left hand, and its melodic, syncopated counterpart in the right hand.

Ragtime remains distinct from jazz both as an instrumental style and as a genre. Ragtime style stresses a pattern of repeated rhythms, not the constant inventions and variations of jazz. (50)As a genre, ragtime requires strict attention to structure, not inventiveness or virtuosity, existing as a tradition, a set of conventions, a body of written scores, separate from the individual players associated with it. In this sense ragtime is more akin to folk music of the nineteenth century than to jazz.

(47)However, there is no reason to assume that ragtime is inherently mechanical simply because commercial manufacturers applied a mechanical recording method to it, the only way to record pianos at that date.