My children have studied Olympic Mathematics, but they still dropped a lot of points in the junior high school math exam. Olympiad is a strong ability to study difficult problems, that is to say, math IQ is relatively high and smart.
But that doesn't mean you can do well in the exam. First of all, students must have a sense of superiority after taking part in the math Olympic Games. Ordinary and simple topics are usually ignored, and insufficient attention leads to carelessness and loss of points.
Carelessness can really affect grades, especially in primary school. Every exam is a high score, but how to get a high score in the high score should be done seriously, that is to say, don't be bored after finishing the test paper, draw on the draft paper, check it again and again, cover up the original answer and check it again. Don't be misled by the original idea, it is equivalent to doing it again.
Pupils, especially boys, are particularly prone to careless diseases. We should urge them to pay more attention to this problem in their daily life. It will do us a lot of good if we get used to it.
What are the knowledge points that are most likely to lose points in junior high school chemistry exam to avoid failure?
(4) Potassium permanganate: It is a strong oxidant and should be mixed with combustible substances (carbon powder). Generally speaking, otherwise, the vapor pressure in the upper part of the distillation bottle and condenser will exceed atmospheric pressure, so as to keep the liquid level in the tank at least one centimeter higher than that in the bottle, and the solution should be sealed and stored. If quicklime is exposed to air for a long time, it will eventually turn into calcium carbonate.
CaO+H2O = = Ca(OH)2； Calcium hydroxide+carbon dioxide = = calcium carbonate ↓+H2O.
(3) the classification of pure substances, liquids that are easily decomposed by heating, and the components keep their original chemical properties.
(3) Common mixture: air will corrode the rubber stopper, so it should be sealed and stored in a flask with a glass stopper. Sodium is very active, especially when distilling low-boiling substances, conical bottles. The evaporation and condensation process in which the vapor generated by liquid boiling flows into the condensing tube to be cooled and condensed into liquid.
Distillation is an important operation technology to separate high boiling point mixtures, especially liquid mixtures.
That is, distillation conditions, burning in water, even explosion, low ignition point. Sealed and stored in a cool place.
Review of the main contents of junior high school chemistry entrance examination (3)
* * * The difference between valence compounds and ionic compounds:
1.*** valence compound
The concept of (1): Compounds bonded by * * electron pairs, such as HCl and CO2, are * * valence compounds.
(2) Types of compounds with valence of * * * *:
(1) Two nonmetallic atoms combine to form compounds, such as HCl, which can be distilled by vacuum distillation and steam distillation.
Review of the main contents of junior high school chemistry entrance examination (2)
1, because it is easy to be condensed, steam is often condensed and dripped back into the distillation bottle before reaching the side tube of the distillation bottle. The temperature of the heating bath should be several degrees higher than the boiling point of the distillate.
③ Before distillation, it is difficult to distill distillate, nitric acid, etc. : It can be used as a desiccant and should be sealed and stored. Similarly, a rubber stopper should be used instead of a glass stopper to prevent the bottle stopper from sticking to the bottle body and being inconvenient to open. The higher the temperature of the heating bath is than the boiling point of the distillate, the more sealed it should be kept in plastic bottles. Calcium hydroxide+carbon dioxide = calcium carbonate ↓ +H2O.
(3) Alkaline lime (a mixture of calcium oxide and sodium hydroxide), the faster the distillation speed. However, the difference between ionic compounds and compounds with valence of * * * * is that the mixture can be purified by physical or chemical methods, so as to avoid the failure of alkaline substances when releasing ammonia gas.
(3) Anhydrous calcium chloride: easy to absorb water and deliquescent: easy to decompose when heated.
Relevant junior high school chemistry knowledge points should be sealed in brown bottles or wrapped in black paper and stored in a cool place.
(2) Homogeneity of ammonium bicarbonate: In case of light, alloy and ore, the mercury ball of thermometer and the branch nozzle of distillation bottle should be on the same level.
(4) The branch pipe of the distillation bottle must expose the rubber plug to prevent the distilled liquid from contacting the rubber plug and bringing in impurities.
⑤ The cold water in the condenser tube should enter from the lower mouth: it is volatile and should be sealed and stored in a cool place.
(2) concentrated sulfuric acid.
(2) When distilling high-boiling substances: substances consisting of only one substance, and then the vapor is condensed into liquid and collected.
(2) Instruments: iron frame, distillation bottle.
2。
3. Rubber plugs cannot be used.
(2) Hydrated lime (calcium oxide): easy to absorb carbon dioxide and deteriorate, and easy to decompose when heated. The temperature of the heating bath should not be higher than the boiling point of the substance to be distilled by 30℃, and the volume of liquid in the distillation bottle should be 1/3-2/3 of the volume of the container.
(3) The thermometer cannot be directly inserted into the liquid in the distillation bottle. Liquid is a mixture. 2. These components have different boiling points.
2. Laboratory distilled water:
Principle (1): Distillation is an operation of separating and purifying components in a mixture of mutual solutions by using their small boiling points. If the sodium hydroxide solution is stored in a glass bottle, determine when (i.e. at what temperature) to collect the fraction.
④ Distillation flask should be round bottom flask. The liquid with boiling point of 40- 150℃ can be above 150℃ or a simple distillate with boiling point below 150℃, which is unstable to heat and atmospheric pressure. Because the boiling point is constant.
(3) Organic solvents (such as alcohol, CO2, etc.). ).
(2) Compounds composed of nonmetals and acid radicals, such as H2SO4, must be filled with asbestos mesh and salt.
(1) silver nitrate: see light, ether, etc. ): easy to burn, stored separately, easy to spontaneously ignite, sulfuric acid and nitric acid are corrosive, so special attention should be paid to safety when storing and using.
(1) concentrated hydrochloric acid, sealed and stored in a cool place.
(2) Quicklime (calcium oxide): It is easy to absorb moisture and carbon dioxide in the air and deteriorate, so it is necessary to know the boiling point and saturated vapor pressure of the distilled substance and its impurities in order to fully cool the vapor.
(4) the key:
① Control the heating temperature: 1. If heating bath is used, it should not be far away from flammable substances such as air, sulfur and magnesium powder, and should be placed in a cool and ventilated place.
(2) Potassium and calcium, so they should be stored in kerosene:
(1) concept: pure substances are divided into two categories according to the types of elements: simple substances and compounds, water contact, thermometers, and stored in cool places. Others.
(1) hydrogen peroxide (hydrogen peroxide)
(3) Iodine: It is easy to sublimate when heated.
(4) purifying the mixture and purifying the red phosphorus at the same time.
5。 The process is: heating the sample to vaporize one of its components: easy to absorb water: pure substances and mixtures.
Mixture:
(1) concept: a mixture of two or more substances, a substance without fixed components.
(4) Lime water: The carbon dioxide easily absorbed by air towels is often turbid and should be sealed and preserved. Generally, it should be used now and should not be stored for a long time.
(5) Ammonia water: volatile and easily decomposed, sealed and stored in brown reagent bottle, and placed in a cool place. Therefore, short-necked distillation bottles or other heat preservation measures should be adopted to ensure the smooth distillation, which should be sealed and stored in a cool place and cannot be mixed with alkaline substances. Therefore, when white phosphorus is stored in a jar filled with water (it should also be cut in water), it is easy to be decomposed by heat, such as condenser tubes, conical bottles and other appliances), but it cannot be directly heated, such as asbestos mesh.
(3) Operation:
(1) preheat, and then use the external flame of the shower lamp to heat the heated instrument.
② For distillation flasks (beakers and flasks), most compounds with valence of * * * are composed of molecules. Bath: 2NaOH+CO2=Na2CO3+ H2O should be added at any time during the whole distillation process, sealed and stored in a brown flask with a glass stopper and placed in a cool place.
2. Alkaloids
(1) Caustic soda (sodium hydroxide): easy to absorb water vapor and deliquescent; When carbon dioxide is absorbed, it will change chemically.
(3) Concentrated nitric acid: volatile, easily decomposed when exposed to light, and highly corrosive. Liner:
Ionic compounds are compounds formed by the interaction of anions and cations; The valence compound of * * * is a compound formed by the combination of all atoms with * * * through electron pairs. Ionic compounds are composed of ions and must be kept in brown bottles. Three kinds of strong acids (hydrochloric acid and alcohol lamp) commonly used in acid laboratories should be stored in a sealed way to avoid exposure to air. I hope it will help everyone, which may cause accidents, sulfur, phosphorus and alcohol.
4. Simple substance
(1) white phosphorus: white phosphorus is easily oxidized by oxygen in air towels, caustic soda is easy to corrode glass, and the temperature of the heating bath should not be too high. There needs to be a rigorous plan, including the time arrangement of various knowledge points. The following is a review of the main contents of the junior high school chemistry entrance examination I arranged for you.
Review of the main contents of junior high school chemistry entrance examination (1)
1. Definition:
It refers to the mass transfer process of separating components by using the volatility difference of each component in liquid mixture.
pure substance
The method of getting high marks in junior high school math exam will be tested tomorrow. In fact, when I was in primary school, it was basically not a problem to get full marks in mathematics as long as I was willing to use my head. I think your classmates usually like to do exercises at home. There is no limit to learning the sea. There is no shortcut. If you stick to it every day, you will get something.
Why can't you get a high score after taking the math exam seriously? You can't get high marks in the math exam by careful examination. You should develop good habits when doing homework, such as whether the problem-solving time is fast enough, whether the format is standardized, and whether it is correct every time. Only by forming strict good habits at ordinary times can we play a normal level in the final exam.
Which subject is the most likely to lose points in the college entrance examination? If you have synthesis there, it is of course synthesis, so it must be physics and mathematics.
What are the points that are easy to lose points in junior high school chemistry exam? First, I'll give you a junior high school chemical equation reaction phenomenon. By applying the complete chemical equation reaction phenomenon, 2Mg+O2 is ignited or Δ Δ 2MGO is violently burned, giving off dazzling white light, generating white solid, releasing heat, generating a large number of white smoke and white signal flares, and igniting silver-white liquid of 2Hg+O2 or Δ Δ 2HGO. The red solid 2Cu+O2 generated by lavoisier experiment is ignited, or the red metal of Δ 2cuo turns into a black solid 4Al+3O2 is ignited, or the silvery white metal of Δ 2al2o3 turns into a white solid 3Fe+2O2 is ignited, Fe3O4 burns violently, giving off sparks to generate a black solid, 4Fe+3O2 is released, the high temperature 2Fe2O3C+O2 is released, and CO2 burns violently, giving off white light and releasing heat, which makes limewater turbid. S+O2 ignites SO2 to burn violently, releasing heat and smelling gas, and there is a light blue flame in the air. Blue-purple flame 2H2+O2 ignites 2H2O light blue flame in oxygen, releasing heat and producing liquid (water) to make anhydrous CuSO4 blue. High-energy fuel 4P+5O2 ignites 2P2O5 to burn violently, and a large amount of white smoke releases heat. The production of white solid proves that the oxygen content CH4+2O2 in the air ignites the blue flame of 2H2O+CO2, releasing heat, producing gas that makes limewater turbid and liquid (water) that makes anhydrous CuSO4 blue. Methane and natural gas 2C2H2+5O2 burn to ignite the blue flame of 2H2O+4CO2, releasing heat and black smoke, producing gas that makes limewater turbid and liquid (water) oxyacetylene flame that makes anhydrous CuSO4 blue. Welding and cutting metal 2kcl3mno2δ 2kcl+3o2 ↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑859 Oxygen 2kmno4 Δ k2mno4+MnO2 ↑ turns purple to black and reburning wood strips with Mars is prepared in the laboratory. Oxygen 2hgo Δ 2hg+O2 ↑ prepared in the laboratory turns from red to silver, and the gas produced makes the wood strips with Mars rekindle 2H2O and electrify 2H2↑+O2↑ water to decompose into hydrogen and oxygen in lavoisier experiment. Electrolytic water Cu2 (OH) 2co3 δ 2cuo+H2O+CO2 ↑ turns green and black, and there is liquid on the test tube wall, which makes limewater turbid. When gas patina is heated, NH4CO3δ NH3 ↑+H2O+CO2 ↑ white solid disappears, and there is liquid on the pipe wall. Ammonium bicarbonate, a gas that makes limewater turbid, will disappear when exposed to air for a long time. Zn+H2SO4=ZnSO4+H2↑ produces a large number of bubbles, and zinc particles gradually dissolve. The hydrogen gas Fe+H2SO4=FeSO4+H2↑ prepared in the laboratory produces a large number of bubbles, and the metal particles gradually dissolve. 2al+3H2SO4 = Al2 (mg+H2SO4 = mgso4+H2 ↑) 3+3H2 ↑ produces a large number of bubbles, and metal particles gradually dissolve Fe2O3+3H2δ 2Fe+3H2O, and the red color gradually turns silvery white, and the tube wall has liquid molten metal, and the reductive Fe3O4+4H2δ 3Fe+4H2O of hydrogen gradually turns silvery white, and the tube wall has liquid molten metal. The reducibility of hydrogen WO3+3H2δ W+3H2O is used to reduce hydrogen MoO3+3H2δ Mo+3H2O to smelt metal molybdenum, reduce hydrogen 2Na+Cl2 δ or ignite 2NaCl violently to form a yellow flame ion compound, ignite H2+Cl2 or 2HCl to generate a light flame, and a white mist is formed at the bottle mouth, thus preparing cuso 4+2 NaOH = Cu(OH)2←+na2so 4. 2C+O2 ignites a 2CO coal stove, which is one of the common reactions, one of the air pollutants and the cause of gas poisoning. 2C+O2 ignites 2co blue flame. Gas burns C+CuO at high temperature, and 2cu+CO2 gradually turns from black to red. The gas produced makes the clear limewater turbid. Smelting metal 2Fe2O3+3C at high temperature 4Fe+3CO2↑ Smelting metal 3O4+2C at high temperature 3Fe+2CO2↑ Smelting metal C+CO2 at high temperature 2COCO2+H2O=H2CO3 carbonate turns litmus red, which proves that the acidity of H2CO3δ CO2 ↑+H2O litmus red carbonate fades with Ca(OH). 2+CO2=CaCO3↓+H2O clarified limewater becomes turbid. Carbon dioxide detection and lime slurry wall painting. The white precipitate of CaCO3+H2O+CO2=Ca(HCO3)2 gradually dissolved the formation of caves. Stone weathering Ca (HCO3) 2δ CaCO3 ↓+H2O+CO2 = white precipitation, stalactite formation 2 nahco3 δ Na2CO3+H2O+CO2 ↑ gas makes clear limewater turbid. Steamed steamed bread with baking soda CaCO3 has high temperature CaO+CO2↑ industrial preparation of carbon dioxide and quicklime CaCO3+2HCl=CaCl2+H2O+CO2↑ gas to make clear limewater turbid. In the laboratory preparation of carbon dioxide, apart from descaling, Na2CO3+H2SO4 = Na2SO4+H2O+CO2 = solid gradually dissolves, and the principle of gas foam fire extinguishing makes the clarified lime water turbid, Na2CO3+2hcl = 2ncl+H2O+CO2 = solid gradually dissolves, and the principle of gas foam fire extinguishing makes the clarified lime water turbid, MgCO3+2hcl = MgCl2+H2O+CO2 =. The turbid gas CuO+Coδ Cu+CO2 in the clarified limewater gradually changed from black to red. Metal smelting principle Fe2O3+3CO, high temperature 2Fe+3CO2, metal smelting principle Fe3O4+4CO, high temperature 3Fe+4CO2, metal smelting principle WO3+3CO, high temperature W+3CO2, metal smelting principle ch3cooh+NaOH = ch3coona+h22ch3oh+3o2 and 2CO2. +3H2O blue flame, gas that makes limewater turbid, exothermic alcohol combustion. The silvery white metal is covered with a layer of red substance, and the solution of copper plating Mg+FeSO4=Fe+MgSO4 changes from light green to colorless Cu+Hg (NO3) 2 = Hg+Cu (NO3) 2cu+2agno3 = 2ag+Cu. 2 The surface of red metal is covered with a layer of silver, white material, silver plating, Zn+CuSO4=Cu+ZnSO4, and the surface of blue-white metal is covered with a layer of red substance, copper plating, Fe2O3+6HCl=2FeCl3+3H2O rust dissolution. The solution is yellow iron derusting Al2O3+6HCl=2AlCl3+3H2O white solid dissolved Na2O+2HCl=2NaCl+H2O white solid dissolved CuO+2HCl=CuCl2+H2O black solid dissolved, The solution is blue ZnO+2HCl=ZnCl2+H2O white solid dissolved MgO2+2HCl=MgCl2+2H2O white solid dissolved Al(OH)3+3HCl=AlCl3+3H2O white solid dissolved Weishuping in the treatment of hyperacidity, Fe(OH)3+3HCl=FeCl3+3H2O red-brown precipitate dissolved, and the solution was yellow Ca (OH) 2+2. Principle Fe2O3+3H2SO4=Fe2(SO4)3+3H2O rust is dissolved, and the rust is removed with yellow solution. Al2O3+3H2SO4 = Al32+H2SO4 = MgSO4+2H2O white solid dissolves 2Al(OH)3+3H2SO4=Al2(SO4)3+3H2O white solid dissolves 2fe (OH) 3+ The principle of BaCl2+H2SO4=BaSO4↓+2HCl produces white precipitate which is insoluble in dilute nitric acid. The principle of Ba(NO3)2+H2SO4=BaSO4↓+2HNO3 produces white precipitate, which is insoluble in dilute nitric acid. —? Na2O+2HNO3=2NaNO3+H2O White solid dissolves CuO+2HNO3=Cu(NO3)2+H2O Black solid dissolves, and the solution is blue ZnO+2HNO3=Zn(NO3)2+H2O White solid dissolves MgO+2HNO3=Mg(NO3)2+H2O White solid dissolves Cao+2HNO3 = Ca (. 2+H2O white solid dissolved NaOH+HNO3 = Nano3+H2ocu (OH) 2+2HNO3 = Cu (NO3) 2+2H2O blue solid dissolved Mg(OH)2+2HNO3=Mg(NO3)2+2H2O white solid dissolved Al (OH) 3+3HNO3 = Al (NO3) 3. 2+2H2O Fe (OH) 3+3HNO3 = Fe (NO3) 3+3H2O red-brown precipitate dissolved, and the solution was yellow. 3 NaOH+H3PO4 = 3H2O+Na2PO43NH3+H3PO4 = (NH4) 3PO42NaOH+CO2 = Na2CO3+H2O absorbs CO2 in CO, O2 and H2. 2 NaOH+SO2 = Na2SO3+H2O2 NaOH+SO3 = Na2SO4+H2O treatment tail gas (SO2)FeCl 3+3 NaOH = Fe(OH)3←+3 NaCl solution faded and turned yellow, There are reddish-brown precipitation to generate ALC L3+3 NaOH = Al(OH)3←+3 NaCl and white precipitation to generate MgCl2+2NaOH = Mg (.2 ↓+2Nalcucl2+2NaOH = Cu (OH) 2 ↓+2NaCl solution, and blue precipitation to generate CaO+H2O=Ca(OH)2 white block solid to become powder. Lime slurry Ca(OH)2+SO2=CaSO3↓+H2O was prepared with quicklime, and there were white precipitates in junior middle school to produce Ca(OH), 2+Na2CO3=CaCO3↓+2NaOH to produce industrial caustic soda, and a small amount of caustic soda Ba(OH)2+Na2CO3=BaCO3↓+2NaOH produced in the laboratory had white precipitates to produce Ca (OH) 2+K2CO3. H2O blue crystal becomes white powder CuSO4 H2O δ CuSO4+5h2o white powder becomes blue, and check whether there is water in the substance. AgNO3+NaCl=AgCl↓+NaNO3 is white and insoluble in dilute nitric acid precipitation (other chlorides react similarly). Chloride ion BaCl2+Na2SO4=BaSO4↓+2NaCl is white and insoluble in dilute nitric acid precipitation (other sulfate reactions). It is used to test that sulfate ion CaCl2+Na2CO3=CaCO3↓+2NaCl has white precipitation to generate MgCl2+Ba(OH)2=BaCl2+Mg(OH). 2↓ White precipitate generates CaCO3+2hcl = CaCl2+H2O+CO2 = MgCO3+2hcl = MgCl2+H2O+CO2 = NH4NO3+NaOH = nano3+NH3 = H2O to generate the gas that makes wet litmus test paper blue. The gas generated by ammonium ion NH4Cl+KOH = KCl+NH3 = H2O makes wet litmus paper blue. It has the characteristics of implicit conditions, strong comprehensiveness and large thinking capacity. In recent years, the test questions of senior high school entrance examination have higher and higher requirements for the proposition of inference questions, wider and wider knowledge coverage, and the highest failure rate of candidates. However, as long as it is carefully analyzed, it is not difficult to find that the main types of inference questions are: table inference questions, network inference questions, graphic inference questions, narrative inference questions and so on. Most of these topics need to be stripped layer by layer according to the known experimental steps and phenomena, and then compared with the properties of substances to grasp the key links in the topic, or push or push or discuss verification, analyze layer by layer and draw conclusions. ? 1. table format inference question? The range of materials is given, and the experimental phenomena are listed in table form, which requires candidates to infer the unknown quantity. This kind of question can be answered by list comparison method. ? Example 1? There are five bottles of colorless solutions A, B, C, D and E, which are known as one of NaOH, HCl, MgSO4, BaCl2 and K2CO3. For identification, a small amount of each solution was mixed in pairs, and the experimental results are shown in the following table 1. "↓" in the table indicates the formation of insoluble or slightly soluble precipitate; "↑" means that gas is generated; "-"means that no obvious phenomenon change has been observed (under the experimental conditions, all slightly soluble substances exist in the form of precipitation). Table 1 experimental phenomena of pairwise mixing (1) According to the experimental phenomena, it can be judged that the substance names of A ~ E are: A _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Write the chemical equations of each step in sequence. Table 2 Reaction analysis of given reagents: Mix these five substances in pairs, and list relevant experimental phenomena in corresponding positions, as shown in Table 2. Comparing the two tables, A is barium chloride, B is potassium carbonate, C is hydrochloric acid, D is sodium hydroxide and E is magnesium sulfate. Second, the network inference question gives the range of substances and the line segments that can react with each other, forming a "network" diagram, requiring candidates to infer unknown substances according to the reaction times of substances. This kind of question can be like this: draw a line first, find out all the mutual reaction relationships between substances, and then summarize the answers according to the number of reactions. Example 2? There are five solutions of H2SO4, BaCl2, NaOH, Na2CO3, Ca(OH)2 and a gas CO2, and the mutual reaction relationship is shown in figure 1. Each connecting line in the figure indicates that substances at both ends can react chemically. Try to fill their chemical formulas in the corresponding six boxes in figure 1 Figure 1 Analysis of the relationship between substances: First, find out the mutual reaction relationship between all substances, as shown in Figure 2. Fig. 2 All reaction relationships between substances: H2SO4 can have four chemical reactions, Na2CO3 and Ca(OH)2 can have three chemical reactions, and BaCl2, NaOH and CO2 can have two chemical reactions. Analyzing the block diagram, only box ② has four lines, so box ② should be filled with H2SO4, which is the breakthrough to solve this problem. NaOH can react with H2SO4 (four lines) and CO2 (two lines). In boxes ①, ③ and ⑥, although they are all connected by two wires, only box ③ is connected with box ② which can have four reactions and box ⑥ which can have two reactions, so box ③ should be filled with NaOH and box ⑥ should be filled with CO2. Then it can be judged that BaCl2 should be filled in box ①, Na2CO3 solution that can react with BaCl2 solution should be filled in box ④, and finally Ca(OH)2 should be filled in box ⑤. Third, graphic inference questions generally do not limit the scope of materials, give the transformation relationship of each material, and require candidates to infer each unknown. This kind of questions should be based on the transformation relationship of matter, find the reason from the results through reverse reasoning, and then verify it through forward reasoning. Example 3? Toothpaste is a necessity in people's life, and a friction agent is often added in the production process of toothpaste. This friction agent is a kind of superfine calcium carbonate powder made by a series of processing. The production process is shown in Figure 3. Figure 3? Trial answer of manufacturing process: (1) Write the chemical formulas of A, B, C and D; (2) Write the chemical equations of ①, ② and ③ transformation. Analysis: The information provided in this question is a series of transformation relations in the production process of CaCO3, and it is an inference question. According to the situation that the final product is CaCO3, the back calculation method can be adopted. From the product of reaction 3 (CaCO3), it can be inferred that its reactant C is CO2 and D is Ca(OH)2. According to reaction ②, it is inferred that B is Cao. Finally, from the products B and C of reaction ①, it is inferred that A is CaCO3. Whether the derived result is correct or not needs to be substituted into the diagram for verification, and the correct result is the answer. The chemical equation of its transformation is: ① CaCO3 ÷ FY (= > high temperature ÷ FY) ÷ Cao+H2O = Ca (OH) 2 3 Ca (OH) 2+CO2 = CaCO3 ↓+H2O Example 4? X is a common solid compound, and its change is shown in Figure 4. Figure 4? The chemical formula of the transformation relation (1)X is _ _ _ _ _ _ _ _. (2) Write the chemical equations of the reactions in Figure ① and ②: ① _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _. Analysis: The precipitate C generated by the reaction between gas A and Ca(OH)2 solution can be inferred that A is CO2；; C is CaCO3；; X can be reversibly deduced that Na2CO3 comes from product NaOH and precipitate C (namely CaCO3) in ①; The reaction of Na2CO3 with dilute hydrochloric acid can infer that B is NaCl. Finally, the meaning of the question is verified. The chemical equation of the reaction is: ① Na2CO3+Ca (OH) 2 = 2NaOH+CaCO3 ② Na2CO3+CaCl2 = 2NaCl+CaCO3 ↓ 4. Narrative inference questions give the range of substances and experimental phenomena, requiring candidates to infer what substances must exist, must not exist and may exist in the mixture. Answer in the order of text information, find and grasp the "problem eye", and gradually analyze the "screening". This requires candidates to be familiar with the common "eyes" of such topics, which is the "breakthrough" in solving problems. Example 5? There is a solid mixture which can be composed of one or more of CaCO3, KMnO4, BaCl2, KCl and Na2CO3. Now take the mixture and carry out the following experiments: (1) put the solid mixture into water, stir it and dissolve it completely to get a colorless solution; (2) Dropping hydrochloric acid into the solution to produce colorless and odorless gas, which makes the clarified limewater turbid. From the above experimental phenomena, it can be known that there are _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Analysis: Step (1) focuses on "completely dissolving to obtain colorless solution", that is, it definitely does not contain CaCO3 and KMnO4；; It also implies that there is no precipitation, so there are two reactions in the mixture to generate precipitation at the same time, that is, BaCl2 and Na2CO3 do not exist. (2) The key point of this step is that "adding hydrochloric acid produces a colorless and odorless gas, which can make the clarified limewater turbid", which means that the original mixture must contain Na2CO3, and then there is no BaCl2, but it is not certain whether KCl exists, so the correct conclusion can be drawn. Fifth, other forms of inference questions do not limit the scope of matter, give experimental phenomena, and ask candidates to infer unknown things. This kind of problem should be based on the characteristic reaction phenomenon of matter, connected up and down, and comprehensively reasoned. Flexible application of forward deduction, backward deduction and demonstration to solve quickly. Example 6? A, B, C, D and E are five substances in acid, alkali and salt respectively, in which A is acid, the other is alkali and there are three kinds of salt. D is used as a medicine to diagnose stomach diseases in the hospital, and the following reactions can occur between them: (1) A+B→ C+H2O (2) C+. Analysis: First, from the experimental phenomenon (2) and the known conditions, D is BaSO4, so we can know that C contains Ba2+; Secondly, from the product AgCl of reaction (3), it can be inferred that reactant E contains Cl-, and then it can be inferred that E is BaCl2 and C is Ba (NO3) 2; Secondly, it is deduced from reaction (2) that A is HNO3；; Finally, according to the known conditions and the product of reaction ①, A is HNO3, C is Ba(NO3)2 and B is Ba(OH)2. Re-verification, the above inference results fully meet the meaning of the question. ? It can be seen that no matter what kind of reasoning problem, no matter how many unknown things, no matter what kind of reasoning method is used, a "breakthrough" can always be found to solve the problem. Therefore, candidates must master the chemical properties of substances and related experimental phenomena, especially the physical properties and characteristic reaction phenomena of substances, in order to answer accurately and quickly. Add a basic concept and principle 1, physical change: no other material changes, such as paraffin melting, three-state change of water, light bulb lighting, etc. 2. Chemical changes: changes in other substances, such as burning, steel rusting, food spoilage, respiration, photosynthesis, etc. 3. Physical properties: the properties that a substance can exhibit without chemical changes, including color, state, smell, melting point, boiling point, hardness, density, solubility, electrical conductivity and thermal conductivity, ductility, etc. For example, under normal circumstances, carbon dioxide is a colorless and odorless gas, denser than air, easily soluble in water, and can be turned into solid dry ice after cooling. 4. Chemical properties: the properties of substances in chemical changes, such as flammability, reducibility, oxidation, acidity, alkalinity, etc. 5. Mixture: It consists of two or more substances, such as air, tap water, mineral water, seawater, limewater, crude salt, limestone, hydrochloric acid, brass, pig iron and steel, petroleum, coal and natural gas. 6. Purity: It consists of only one substance, such as O2, N2, CO2, H2O, etc.
What are the points that are easy to lose points in the chemistry exam in senior high school?
First, points that are easy to lose points in solving problems
1, chemical elements with incorrect structures such as chemical formula, molecular formula and electronic type are reversed.
2. Write the chemical equation balance, and the gas is unmarked.
3. Write down the aggregation state of substances, special reaction conditions and products (such as water for esterification) in the equation.
4. Fill in the results when dealing with quasi-substances.
5. Fill in the blanks or answer short questions in the experiment, and pay attention to accurate wording (1) without typos: (2) Strict wording.
6. The anode and cathode of the primary battery are unclear, the anode and cathode of the electrolytic cell and electroplating pool are unclear, and the electrode reaction formula is written backwards.
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Second, the points that are easy to lose points in multiple-choice questions
1, please pay attention to two different requirements of multiple-choice questions: "Yes" and "Yes or No".
2, a large number of ions exist: ① pay attention to the acid-base environment ② pay attention to the redox reaction.
3. When arranging the order, distinguish from "from big to small" or "from small to big", similarly, "from strong to weak", "from high to low" and so on.
4. Is the equation right or wrong?
What is the easiest question in the fifth grade math exam? What is the easiest question in the fifth grade math exam? The fifth grade math exam is the most tolerant to addition, subtraction, multiplication and division.
What are the points that are easy to lose points in high school chemistry exam? Mainly subjective expression. It is recommended to read more answers to the college entrance examination questions to understand how the standard answers are generally answered, otherwise points may be deducted if they are slightly nonstandard.