membershipInspection and identification, removal of impurities, and coexistence of substances
Knowledge is a must-have. 1. Inspection and identification of substances
1. Examination of substances
PrincipleAccording to the different characteristics of the substance, take advantage of the difference in the characteristics to choose the appropriate and practical method to detect the substance, and the phenomenon is obvious.
Test Method
1) Physical methods: According to the special physical properties of the substance, such as color, odor, water solubility, etc., through observation, analysis, judgment, and conclusions.
2) Chemical method: use the special chemical properties of the substance and the characteristic phenomena produced during the reaction to test the given substance.
Physical methods
1) Substances with a special color are common (ions represent solutions containing such ions).
Solid substance: cu: red (or purplish-red) color;cuso4ยท5h2o: blue;S: Pale yellow.
Precipitate: Cu(OH)2: blue;Fe(OH)3: reddish-brown.
Solution: Fe2: light green;Fe3: Yellow;cu2: blue;mno4: fuchsia.
2) Substances with special odors are common.
Ammonia: pungent ammonia gas;SO2: pungent odor gas.
Chemical methods
1) Inspection of common gases.
2) Detection of common ions.
2. Identification of substances
Principles of Substance Identification:The operation is simple, the reagent types are few, and the phenomenon is obvious.
Methods of substance identification: It should be simple and then complex, and use physical methods first and then chemical methods.
1) Physical methods: Observation, analysis, judgment and conclusions are drawn based on the special physical properties of the substance (such as color, state, odor, solubility, and heat absorption and release phenomena during dissolution).
2) Chemical method: Identification is carried out according to the special reaction phenomena (such as discoloration, gas release, precipitation generation, heat release, etc.) generated during the reaction between substances.
Forms of substance identification
1) Identification of a reagent: select a reagent, add it to the substance to be tested, and observe the experimental phenomenon.
2) Identification of a variety of reagents: select a variety of reagents and add them to the substances to be tested in turn, and there will be obvious differences.
3) Reagent-free identification: Substances are identified by using different physical properties of substances or mutual reactions between substances (including reactions with products) to produce different phenomena.
Identification of common substances
Second, the removal of impurities
The principle of impurity removal: No increase, no decrease, easy to divide.
1) The main is not reduced: the selected reagent cannot react with the purified substance (main component).
2) Impurities do not increase: while removing impurities, no new impurities can be generated.
3) Easy separation: the state of the product generated after impurity removal should be different from the state of the purified substance, which is convenient to remove.
Impurity removal method
1) Removal of impurities from gases.
Method 1: The impurity gas is removed through the absorption and reaction of the substance, such as the water vapor in oxygen can be removed by the absorption of concentrated sulfuric acid, and the carbon dioxide in carbon monoxide can be absorbed and removed by sodium hydroxide solution.
Method 2: Convert impurity gases into main gases, such as carbon monoxide in carbon dioxide, which can be converted into carbon dioxide by calcitrant copper oxide.
Examples of common gas impurities
2) Removal of impurities in solids.
Method 1: Remove impurities into gases, such as carbon powder in copper oxide by burning in the air.
Method 2: Convert impurities into solutions to remove, such as iron in copper can be removed by reaction with hydrochloric acid.
Common examples of solid impurities
3) Impurity removal in liquid: that is, the removal of impurity ions.
Removal of impurities through ion coexistence problems.
Step 1: Compare the two substances and find the same ions and different ions (ion a, impurity ions) in the two substances;
Step 2: According to the chemical properties of impurity ions, find out ion B that does not coexist with impurity ions, and combine ion B with ion A, which is the impurity removal reagent.
Take the removal of sodium carbonate (Na2CO3) from sodium hydroxide (NaOH) solution as an example
Impurity removal by displacement reaction:
That is, the removal of impurities is carried out by using the order of metal activity.
This type of impurity removal is generally the same anion, and the cation is two metal ions with different activities.
Step 1: Compare the two substances and find the same ions and different ions (ion a, impurity ions) in the two substances;
Step 2: According to the chemical properties of impurity ions, find out ion B that does not coexist with impurity ions, and combine ion B with ion A, which is the impurity removal reagent.
Examples of common liquid impurities].
3. The coexistence of matter
Conditions of coexistence
1) The substances in the group are soluble.
2) In the same solution, there is no reaction between substances (i.e. the ions they contain), i.e. no precipitate, gas or water is generated.
Ions that cannot coexist
The implicit conditions for the coexistence of matter
1) pH 7 or acidic solution, then H must be present in the solution, and there are no substances (ions) that can react with H in the solution.
Common descriptions are: acidic solution, pH 2 (pH 7) solution, solution that can make purple litmus solution red, solution that can release hydrogen gas by adding active metals, and solution that can release gas by adding carbonate or bicarbonate, etc.
2) pH>7 or alkaline solution, there must be OH in the solution, and there is no substance (ion) that can react with OH in the solution.
Common descriptions are: alkaline solution, pH 11 (pH 7) solution, solution that makes colorless phenolphthalein solution red, solution that makes purple litmus solution blue, etc.
3) If the solution is colorless and transparent, there must be no colored ions in the solution, such as Cu2, Fe2, Fe3, Mno4, etc. The solution must also not form turbidity or precipitation, i.e. the ions in the given mixture that can react to form a precipitate cannot coexist.