铁的化合物是高中教材里典型的无机化合物，氢氧化亚铁的制备是中学阶段十分重要的学生实验，也是高考的热点和难点。氢氧化亚铁是白色沉淀，教材中常用少量的硫酸亚铁与氢氧化钠溶液反应来制备，但在实际操作中往往观察到这样的现象：（1）沉淀不是纯白色，而是带有很浅绿色，（2）接着，局部地迅速变成灰绿色，（3）随着时间的延长，灰绿色逐渐加深，变成深绿色，（4）大约经过40天之后，变成黄褐色。即使改进实验条件，优化实验方案，也很难长时间观察到白色沉淀。沉淀的颜色变化吸引了很多研究者的关注，由于研究水平的限制，人们对绿锈和层状双金属氢氧化物（LDHs）缺乏足够的认识，导致这个问题一直没有得到很好的解决。本文是结合LDH的发展，用实验揭示了相变过程颜色变化的根本原因。 绿锈是层状双金属氢氧化物（layered double hydroxides，LDHs，中文也有翻译成“水滑石”的）的一种，其化学式一般被认为是FeII4FeIII2(OH)12SO4.mH2O。而LDHs是一类离子晶体，其通式为[M2+1-xM3+x(OH)2]x+(An-x / n)·mH2O]，其中x通常为0.20-0.33，对应于M2+/ M3+比值为4-2。近年来，LDHs因其特殊的结构、优异的性能在环保、催化、医药、功能材料等方面取得了长足的发展。在合成LDH时，除了形成M2+/ M3+≥ 2的无缺陷的LDH层板，M2+/ M3+<2具有晶体缺陷的情况也很常见。以最为常见的Mg-Al-LDH 为例，当Mg/Al≥ 2， Mg2 +和Al3 +的分布是均匀的，不存在Al3+-Al3+紧密接触；而对于Mg/Al< 2，Al3 +-Al3 +紧密接触会产生八面体空位，Mg2 +取代骨架Al3 +并填充空的八面体孔。当Mg/Al< 2时，其XRD上也会表现出LDH一样的层状结构，得到纯相LDH的XRD图谱，因此很难通过XRD来估计Mg/Al的比值。所以，绿锈并不是一类具有确定Fe2+/Fe3+比值的LDHs，将绿锈认的化学式认为FeII4FeIII2(OH)12SO4.mH2O不够严谨。 本工作是向硫酸亚铁中加入氢氧化钠溶液制备氢氧化亚铁，利用透射电子显微镜（TEM）、X射线衍射（XRD）和傅里叶变换红外光谱（FTIR）表征沉淀的结构和成分；用X射线光电子能谱（XPS）表征铁的化合价；利用手持技术探究反应机理。结果表明，颜色变化的主要原因为氧气的氧化，导致Fe(OH)2（白色）? GR2(SO42-)（硫酸根型绿锈，蓝绿色）?γ- FeOOH和α-FeOOH（黄色）。而灰绿色与微粒的大小不均匀有关。结合LDH的发展，本文提出新的反应机理：首先将硫酸根型绿锈的化学式写成[Fe2+1-xFe3+x(OHˉ)2]x+[(SO42ˉ)x/2·mH2O]xˉ，只要Fe3+出现，就会变成LDH而显出绿色。随着反应不断进行，x不断增大，直到Fe2+/Fe3+比值小于2，尽管不能称作LDH，但是仍会有阴离子存在于层间，表现出类似于LDH的结构。当Fe2+/Fe3+比值为0的时候，即所有的Fe2+转化为Fe3+，此时产物为FeOOH。原来存在层间的大量的阴离子（SO42ˉ），则会阻碍其形成，因此沉淀变黄需要很长的时间才能实现。综上所述，为了防止沉淀变色，可以采取以下措施：（1）选择氯化亚铁与氢氧化钠反应，（2）选择无水环境下，亚铁盐与碱反应，（3）保证碱过量，（4）整个过程尽量避免氧气。本文提出一种新的实验方案：在几乎无氧的情况下，用新制的硫酸亚铁和氢氧化钠溶液反应，最后成功地制备出白色的氢氧化亚铁沉淀，且可以保持数分钟不变色。 化学是一门以实验为基础的学科，化学教育不能只谈教育，还应结合化学学科的特点，在实验的基础上谈化学教育更有意义。本工作的意义在于建立正确的知识体系；为教师教学和教研提供有利的参考依据；帮助学生理解从氢氧化亚铁到氢氧化铁的相变过程和机理，拓宽视野。 STSE教育是新课改的主要方向之一，教材的编写要体现STSE教育的理念，应与时俱进，不断加以补充、完善、发展。

Iron compounds are typical inorganic compounds in senior school textbooks. The preparation of ferrous hydroxide is a very important experiment in senior school, which plays a key role in the college entrance examination. Fe(OH)2 precipitate is white, prepared by adding a small amount of ferrous sulfate to sodium hydroxide. However, in practice, such a phenomenon is often observed: (1) The precipitate is not pure white, with light green, (2) rapidly turns grayish green, (3) as time goes by, the grayish green gradually deepens and turns dark green, (4) After about 40 days, it turns yellowish brown. Even though the experimental conditions are improved and the experimental scheme is optimized, it is still difficult to observe the white precipitate for a long time. The color change of the precipitate attracted the attention of many researchers (mostly teaching and research personnel) and triggered a series of conjectures. This problem has not been well solved due to the lack of sufficient knowledge of green rust and layered double hydroxides (LDHs). This paper combines the development of LDH and reveals the root cause of the color change in the phase change process. Green rust belongs to the family of layered double hydroxides with the chemical formula is generally considered as FeII4FeIII2 (OH)12SO4.mH2O when the anion is sulfate. LDHs are a class of ionic solids with a general formula[M2+1-xM3+x(OH)2]x+(An-x/n)·mH2O],where x is normally 0.20–0.33, corresponding to M2+/M3+ ratios of 4–2. In recent years, LDHs have made great progress in environmental protection, catalysis, medicine, and functional materials due to their special structure and excellent performance. In the synthesis of LDHs, in addition to the formation of the “perfect” brucite layer for M2+/M3+ ≥2, M2+/M3+ <2 with crystal defects were common. Taking the most common Mg-Al-LDH as an example, when Mg/Al ≥ 2, the distribution of Mg2 + and Al3 + is uniform, there is no close contact of Al3+-Al3+; When Mg/Al< 2, based on the gibbsite-like layer, where Al3+-Al3+close contacts would give octahedral vacancies. Mg2+ substitution for skeletal Al3+ and filling of the vacant octahedral holes both take place. It is difficult to estimate Mg/Al ratios for a LDH phase, because Mg/Al < 2 may give an XRD pattern of pure-phase LDH. Therefore, Fe2+/Fe3+ ratios of green rust are not fixed. It is not rigorous to considered green rust as FeII4FeIII2 (OH)12SO4.mH2O. In this work, ferrous hydroxide was prepared by adding sodium hydroxide solution to ferrous sulfate, and the structure and composition of the precipitate were analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The valence of iron was characterized by X-ray photoelectron spectroscopy (XPS), and the mechanism is explored by hand-held technique. The results show that the main cause of color change is oxidation of oxygen, resulting in Fe(OH)2(white)? GR2(SO42-)（blue-green）?γ- FeOOH（yellow）. Gray-green is related to the uneven size of the particles. Combined with the development of LDH, this paper proposes a new reaction mechanism: firstly, the chemical formula of green rust is written as [Fe2+1-xFe3+x(OHˉ)2]x+[(SO42ˉ)x/2·mH2O]xˉ. As long as Fe3+ appears, it becomes LDH and appears green. As the reaction progresses, x continues to increase until the Fe2+/Fe3+ ratio is less than 2, although it cannot be called LDH, but exhibiting a structure similar to LDH. When the Fe2+/Fe3+ ratio is zero, all the Fe2+ is converted to Fe3+, and the end product is FeOOH. The existence of a large number of interlayer anions (sulfate) hinders its formation, so it takes a long time for the precipitate to turn yellow. In summary, in order to prevent precipitate from turning green, the following measures can be taken: (1) Select ferrous chloride to react with sodium hydroxide (2) select ferrous salt to react with alkali in the absence of water, (3) ensure excess alkali, (4) avoid oxygen as much as possible. In this paper, we propose a new method, in the case of almost no oxygen, the new ferrous sulfate reacted with sodium hydroxide solution, and finally the white ferrous hydroxide precipitate is successfully prepared, which keeps for a few minutes without discoloration. Chemistry is an experiment-based discipline.Chemistry education can not only talk about education, but also combines the characteristics of chemistry, and it is more meaningful to talk about chemistry education on the basis of experiments.The significance of this work is to establish a correct knowledge system; to provide a favorable reference for teachers and help students understand the process and mechanism of phase transition from ferrous hydroxide to ferric hydroxide, broaden their horizons. STSE education is one of the main directions of the new curriculum reform. The preparation of teaching materials should reflect the concept of STSE education, and should be kept up to date, constantly supplemented, improved and developed.