Unit 1 Entering the Chemical World

1. Chemistry is a basic science to study the composition, structure, properties and changing laws of matter.

2. Working people in China made bronzes in Shang Dynasty and steel in Spring and Autumn Period and Warring States Period.

3. Green chemistry-environment-friendly chemistry (the combination reaction conforms to the green chemical reaction)

①P6 Four Features (Raw Materials, Conditions, Zero Emissions, Products) ② Core: Eliminate pollution from the source by using chemical principles.

4. Candle burning experiment (product names are not allowed to be used when describing phenomena)

(1) flame: flame core, inner flame (brightest) and outer flame (highest temperature).

(2) Compare the temperature of each flame layer: put a matchstick flat in the flame. Phenomenon: both ends are carbonized first; Conclusion: The external flame temperature is the highest.

(3) H2O: Cover the upper part of the flame with a dry, cold beaker with water mist.

CO2: Take off the beaker, pour in the clarified lime water, and shake well to become turbid.

(4) After fire extinguishing: there is white smoke (paraffin vapor). When the white smoke was lit, the candle was rekindled. Explain paraffin steam combustion.

5. Comparison between inhaled air and exhaled air Conclusion: Compared with inhaled air, the amount of O2 in exhaled air decreases, while the amounts of CO2 and H2O increase (inhaled air and exhaled air have the same composition).

6. An important way to learn chemistry-scientific inquiry

General steps: ask questions → guess and hypothesis → design experiments → experiment verification → record and conclusion → reflection and evaluation.

The characteristics of chemistry learning: paying attention to the essence, change, change process and phenomenon of matter;

7, chemical experiment (chemistry is a science based on experiments)

A, commonly used instruments and methods of use

(1) Instruments for heating-test tubes, beakers, flasks, evaporating dishes and conical bottles.

Instruments that can be directly heated include test tubes, evaporating dishes and burning spoons.

Instruments that can only be heated indirectly include beakers, flasks and conical bottles (padded with asbestos mesh-heated evenly).

Instruments that can be used for solid heating include test tubes and evaporating dishes.

Instruments that can be used for liquid heating include test tubes, beakers, evaporating dishes, flasks and conical bottles.

Do not heat the instrument. Measuring cylinder, funnel, gas cylinder

(2) Measuring container-measuring cylinder

When measuring the volume of liquid, the measuring cylinder must be placed smoothly. The line of sight is flush with the scale line and the lowest point of concave liquid level in the measuring cylinder.

The measuring cylinder cannot be used for heating or as a reaction vessel. A measuring cylinder with a measuring range of 10 ml can generally only read 0. 1 ml.

(3) Weigher-pallet scale (used for rough weighing, generally accurate to 0. 1 g). )

Note: (1) First adjust the zero point (2) The position where the object and weight are weighed is "left object and right code". (3) The weighed object cannot be directly placed on the tray. When weighing general drugs, put a piece of paper with the same size and quality on both sides of each tray and weigh it on the paper. Wet or corrosive drugs (such as sodium hydroxide) are weighed in covered glassware (such as small beakers and watches). (4) Clamp the weight with tweezers. When adding weights, add the weights with large mass first, and then add the weights with small mass (large mass first, then small mass) (5) After weighing, reset the roaming code to zero. Put the weight back in the weight box.

(4) heating container-alcohol lamp

(1) Pay attention to the "three noes" in the use of alcohol lamp: ① Do not add alcohol to the burning alcohol lamp; ② Light the alcohol lamp from the side with a match, instead of directly lighting another alcohol lamp with a burning alcohol lamp; (3) Extinguish the alcohol lamp should be extinguished together with the lamp cap and cannot be blown out.

(2) The amount of alcohol in the alcohol lamp should not exceed 2/3 of the volume of the alcohol lamp and should not be less than 1/4.

(3) The flame of alcohol lamp is divided into three layers: outer flame, inner flame and flame core. Heat an object with the external flame of an alcohol lamp.

(4) If the alcohol lamp accidentally falls over when burning and alcohol burns on the experimental platform, it should be covered with sand or put out the flame with a wet rag in time, and cannot be washed with water.

(5) Bracket-iron clamp and test tube clamp

The position where the iron clamps the test tube should be near the test tube mouth 1/3. Do not press the short handle of the test tube holder with your thumb.

When the test tube clamp holds the test tube, the test tube clamp should be supported from the bottom of the test tube; The clamping position is near the tube mouth 1/3; Hold it in your hand.

(6) Instruments for separating substances and adding liquids-Funnels and long-necked funnels

When filtering, the nozzle at the lower end of the funnel should be close to the inner wall of the beaker to avoid splashing filtrate.

The lower nozzle of the long-necked funnel should be inserted below the liquid level to prevent the generated gas from escaping from the long-necked funnel.

Second, the basic operation of chemical experiment

Access to drugs

1. Storage of drugs: generally, solid drugs are put in wide-mouth bottles, liquid drugs are put in narrow-mouth bottles (a small amount of liquid drugs can be put in drop bottles), metal sodium is stored in kerosene, and white phosphorus is stored in water.

2, the general principles of drug access

① Dosage: Take the medicine according to the needs of the experiment. If there is no prescribed dose, the minimum dose should be taken, the solid should cover the bottom of the test tube, and the liquid should be 1~2mL.

Don't put the extra reagent back in the original bottle, don't throw it around, and don't take it out of the laboratory, but put it in the designated container.

② "Three noes": you can't take medicine with your hands, taste medicine with your tongue, and smell medicine directly with reagents (if you need to smell the smell of gas, you should gently fan the bottle mouth with your hands, so that only a very small amount of gas can enter your nostrils).

3. Obtaining solid drugs: ① Powdered and granular drugs: medicine spoon or V-shaped paper slot; ② Block and strip drugs: clamped with tweezers.

4. Obtain liquid medicine

① Dumping method of liquid reagent: Take off the bottle cap and pour it on the table (to avoid drug contamination). The label should be located in the center of the hand (to avoid the residual liquid flow from corroding the label). Pick up the reagent bottle, put the bottle mouth close to the edge of the test tube mouth, slowly inject the reagent, pour it out, cover the bottle cap, and put the label back to its original place outwards.

(2) Liquid reagent drop method:

How to use the dropper: a, first expel the air in the dropper, and then suck the reagent.

B, dropping reagent, dropper should be kept vertically hanging above the mouth of the container.

C, in the process of use, always keep the rubber nipple on the floor to avoid being corroded by reagents.

D wash the dropper with water immediately after use (except the dropper on the dropper bottle).

E, when using the rubber dropper, be sure not to put your hand into the container or contact with the container wall, otherwise it will cause reagent pollution.

② Connect instruments and devices, and check the air tightness of devices.

Air tightness inspection of the device: First, immerse one end of the catheter in water, and hold it close to the outer wall of the container by hand, and stop for a moment. If there are bubbles coming out of the catheter, release your hand and a water column will appear at the catheter. After the suspension, the water column will not fall back, which means that the equipment will not leak.

(3) heating of substances

(1) When heating solids, the mouth of the test tube should be slightly inclined downward, and the test tube should be heated uniformly first, and then concentrated.

(2) When heating the liquid, the volume of the liquid shall not exceed 1/3 of the volume of the test tube. When heating, make the test tube form an angle of about 450 with the desktop. When heating, heat the test tube evenly first, then heat the middle and lower part of the liquid in the test tube, and move the test tube up and down from time to time. In order to avoid injury, do not aim the test tube mouth at yourself or others when heating.

④ filtration

1. Operation notes: "One post, two lows and three inclines"

"One stick": the filter paper clings to the inner wall of the funnel "two lows": (1) The edge of the filter paper is lower than the funnel mouth (2) The liquid level in the funnel is lower than the edge of the filter paper.

"Three leads": (1) The nozzle at the lower end of the funnel is close to the inner wall of the beaker. (2) When the glass rod is used for drainage, the lower end of the glass rod gently leans against one side of the three-layer filter paper. (3) When the glass rod is used to drain water, the mouth of the beaker is close to the middle of the glass rod.

2. The possible reasons why the filtrate is still turbid after filtration are: ① the beaker receiving the filtrate is not clean; ② When pouring liquid, the liquid level is higher than the edge of filter paper; ③ The filter paper is damaged.

⑤ Evaporation

Note: (1) During the heating process, keep stirring with a glass rod (function: accelerate evaporation and prevent droplets from splashing due to excessive local temperature).

(2) When the liquid is close to evaporation (or when a large number of solids appear), stop heating and use the waste heat to evaporate the remaining water to prevent the solids from splashing out due to heating.

(3) The hot evaporating dish shall be clamped by crucible tongs. If the hot evaporating dish needs to be put on the experimental platform immediately, it should be padded with asbestos net.

(6) Cleaning the instrument:

(1) Waste residue and waste liquid are poured into the waste liquid tank, and useful substances are poured into the designated container.

(2) Standard for cleaning glass instruments: Water attached to glass instruments neither gathers into water drops nor flows down in strands.

(3) Grease adhesion on glass instruments: firstly, clean with hot sodium carbonate (Na2CO3) solution or washing powder, and then rinse with water.

(4) Water-insoluble alkali, basic oxide and carbonate are attached to the glass instrument: firstly dissolved with dilute hydrochloric acid, and then washed with water.

(5) The instrument should not be misplaced after cleaning. After cleaning the test tube, it should be inserted into the test tube rack upside down to dry.

Unit 2 "The air around us"

1, the first chemist to explore the composition of air: lavoisier (the first person to make quantitative analysis with a balance).

2. The composition of air and the volume fraction of other gases and impurities that make up O2, O2 N2 CO2 rare gas are 2 1% 78% 0.03% 0.94% 0.03%.

(1) Determination of Oxygen Content in Air

A requirements for combustible materials: sufficient quantity and solid products: red phosphorus B is selected; Device requirements: good air tightness.

C. Phenomenon: a large amount of white smoke is produced, and the liquid level in the tank rises by about 1/5 volume.

D. conclusion: air is a mixture; O2 accounts for about 1/5, which can support combustion; N2 accounts for about 4/5, it does not support combustion, nor can it burn, and it is insoluble in water.

E. inquiry: ① liquid level rise is less than 1/5. Cause: Air leakage, insufficient red phosphorus and incomplete cooling.

② Can iron and aluminum be used instead of red phosphorus? There is no reason: iron and aluminum can't burn in air. Can carbon and sulfur replace red phosphorus? No reason: the product is gas, and it can't produce pressure difference.

(2) Air pollution and prevention: The main pollutants that pollute the air are harmful gases (CO, SO2, nitrogen oxides) and smoke. At present, the air pollution index includes carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone and inhalable particles.

(3) Harm and protection of air pollution:

Hazards: seriously damaging human health, affecting crop growth, destroying ecological balance, global warming, ozone layer destruction and acid rain, etc.

Protection: strengthen air quality monitoring, improve environmental conditions, use clean energy, and the waste gas generated by the factory can only be discharged after treatment, and actively plant trees, trees and grass.

(4) Current environmental pollution problems:

Ozone layer destruction (Freon, nitrogen oxides, etc. ) and greenhouse effect (carbon dioxide, methane, etc. )

Acid rain (NO2, SO2, etc. ) and white pollution (plastic waste, etc. )

6. Oxygen

(1) Chemical properties of oxygen: unique properties: supporting combustion and supplying breathing.

(2) the reaction of oxygen with the following substances

Material phenomenon

Carbon stays red-hot in the air and emits white light in oxygen, and the generated gas makes the clear limewater turbid.

Phosphorus produces a lot of white smoke.

Sulfur emits a faint light blue flame in the air, and a bright blue-purple flame in oxygen, producing a gas with a pungent smell.

Magnesium gives off dazzling white light, gives off heat and produces white solids.

Aluminum gives off dazzling white light, gives off heat and produces white solids.

Iron burns violently, and sparks are everywhere, producing black solid (Fe3O4).

Paraffin wax burns in oxygen to give off white light, and water droplets are produced on the bottle wall, and gas is produced to make clear lime water turbid.

* When burning iron and aluminum, put a small amount of water or fine sand at the bottom of the gas container to prevent the overflowing high-temperature melt from cracking the bottom of the bottle.

* Iron and aluminum are nonflammable in air.

③ Preparation of oxygen:

Industrial oxygen production. Method of separating liquid air (principle: different physical changes of boiling points of nitrogen and oxygen)

Principle of oxygen production in laboratory: 2h2o 2mno22h2o+O2 =

2KMnO4 △ K2MnO4 + MnO2 + O2↑

2KClO3 MnO2 2KCl+3O2↑

(4) Selection of gas taking and gathering devices

Generating devices: solid-solid heating type and solid-liquid unheated type collecting devices: according to the density and solubility of substances,

(5) Operation steps and matters needing attention in oxygen production (taking potassium permanganate oxygen production and drainage collection as examples)

A. Steps: check-install-set point-receive-move-extinguish

B, note:

① The test tube mouth is slightly inclined downwards: to prevent the test tube from breaking due to the backflow of condensed water; ② The medicine is laid flat on the bottom of the test tube: heated evenly; ③ The distance between the iron clip and the nozzle is about1/3; ④ The rubber plug should be slightly exposed from the catheter: it is convenient for gas discharge; ⑤ A ball of cotton should be placed at the mouth of the test tube: potassium permanganate powder should be prevented from entering the catheter; ⑤ When collecting by drainage method, when bubbles emerge uniformly and continuously, they will be collected (the air in the test tube will be discharged first). ⑦ At the end of the experiment, move the catheter first, and then put out the alcohol lamp to prevent the water from being sucked back and the test tube from breaking. (8) When the gas is collected by exhaust, the conduit extends to the bottom of the gas collection cylinder.

(6) Oxygenation: put a piece of wood with Mars at the mouth of the gas container.

Difference: oxygen check: insert a piece of wood with sparks into the gas container.

7. Catalyst: A substance that can change the chemical reaction rate of other substances in a chemical reaction, but its own quality and chemical properties have not changed before and after the reaction. The role of catalysts in chemical reactions is called catalysis.

8, the use of common gases:

① Oxygen: used for breathing (such as diving and medical first aid) to support combustion (such as fuel combustion, steelmaking and gas welding).

② Nitrogen: inert protective gas (chemically inactive), important raw materials (nitric acid, fertilizer) and liquid nitrogen freezing.

(3) Rare gases (helium, neon, argon, krypton, xenon, etc. ): shielding gas, electric light source (which emits different colors when electrified) and laser technology.

9, common gas inspection methods

① Oxygen: wooden strips with Mars. Phenomenon: wood reburning.

② Carbon dioxide: clarifying limewater phenomenon: clarifying limewater becomes turbid.

③ Hydrogen: ignite the gas and cover it with a dry and cold beaker above the flame; Phenomenon: Water drops appear on the inner wall of beaker.

Or, hot copper oxide is introduced first, and then anhydrous copper sulfate is introduced: phenomenon: white solid turns blue.

9. Oxidation reaction: chemical reaction between substance and oxygen (oxygen element).

Violent oxidation: combustion

Slow oxidation: rust, human respiration, food decay, wine brewing.

Common ground: ① Both are oxidation reactions ② Both are exothermic reactions.

Unit 3 Understanding Water in Nature

One. water

1, composition of water:

(1) Electrolyzing water experiment A. Device-water electrolyzer B. Power supply type -DC.

C. The purpose of adding sulfuric acid or sodium hydroxide is to enhance the conductivity of water.

D. chemical reaction: 2h2o = = 2h2 =+O2 =

Generation position negative electrode positive electrode

Floor area ratio 2: 1

Mass ratio 1: 8

F. Inspection: O2-Put the wooden strip with Mars in the air outlet-The wooden strip is rekindled.

H2-air outlet is equipped with burning wood-gas combustion produces light blue flame.

(2) Conclusion: ① Water is composed of hydrogen and oxygen.

② A water molecule consists of two hydrogen atoms and 1 oxygen atom.

(3) In chemical changes, molecules can be separated, but atoms cannot be separated.

According to the chemical formula H2O of water, what information can you read?

The meaning of chemical formula H2O

(1) a substance that represents water.

② indicates the composition of the substance. Water consists of hydrogen and oxygen.

(3) A molecule representing this substance, water molecule.

④ indicates the molecular composition of this substance. A water molecule consists of two hydrogen atoms and one oxygen atom.

2. Chemical properties of water

(1) charged decomposition 2H2O = = = 2H2 =+O2 =

(2) Water can react with some oxides to generate alkali (soluble alkali), such as H2O+CaO = = Ca(OH)2.

(3) Water can react with some oxides to generate acids, such as H2O+CO2==H2CO3.

3. Water pollution:

Water resources (1)

A.7 1% of the earth's surface is covered with water, but less than 1% of fresh water is available for human use. The ocean is the largest reservoir on the earth. Seawater contains more than 80 elements. The most abundant substance in seawater is H2O, the most metal element is sodium, and the most element is oxygen. ..

C. The distribution of water resources in China is uneven, and the per capita possession is small.

② Water pollution

A water pollutants: industrial "three wastes" (waste residue, waste liquid and waste gas); Unreasonable application of pesticides and fertilizers, and random discharge of domestic sewage.

B, prevent water pollution: industrial wastewater should be treated and discharged up to standard, and zero discharge should be advocated; Domestic sewage should be treated centrally to meet emission standards and advocate zero discharge; Rational use of pesticides and fertilizers, advocate the use of farmyard manure; Strengthen water quality monitoring.

(3) Care for water resources: save water and prevent water pollution.

4. Purification of water

(1) The purification effect of water from low to high is: standing, adsorption, filtration and distillation (all physical methods), among which distillation operation has the best effect; The water purifying agent with both filtration and adsorption functions is activated carbon.

(2) Hard water and soft water A. Definition Hard water refers to water containing more soluble calcium and magnesium compounds; Soft water is water without or without soluble calcium and magnesium compounds.

B identification method: soapy water, hard water with foam or less foam and soft water with more foam.

C. methods of softening hard water: distillation and boiling.

D. Disadvantages of long-term use of hard water: waste of soap and inability to clean clothes; Boilers are prone to scale formation, which not only wastes fuel, but also easily deforms pipelines and even causes boiler explosion.

5. Others

(1) water is the most common solvent and the oxide with the smallest relative molecular weight.

(2) Water inspection: anhydrous copper sulfate, if it turns from white to blue, indicates the existence of water; Copper sulfate +5H2O = copper sulfate? 5H2O

Water absorption: commonly used concentrated sulfuric acid, quicklime, solid sodium hydroxide and iron powder.

Second, hydrogen H2

1, physical properties: the gas with the lowest density (downward exhaust method); Insoluble in water (drainage method)

2, chemical properties:

(1) combustibility (application: high-energy fuel; Hydrogen and oxygen flame welding, cutting metal)

2H2+O2 = = = 2H2O Before ignition, test the purity (method? ) Phenomenon: It emits a light blue flame, emits heat and produces water droplets.

(2) Reducibility (use: smelting metal)

H2+CuO = = Copper +H2O Hydrogen "Go out early and return late"

Phenomenon: Black powder turns red, and water drops are formed at the mouth of the test tube (summary: combustible and reducing substances H2, C and CO).

3, laboratory hydrogen method

Principle: zinc+sulfuric acid = zinc sulfate+H2 = zinc+hydrochloric acid = zinc chloride+H2 =.

The reason why concentrated hydrochloric acid cannot be used is that concentrated hydrochloric acid is highly volatile; The reason why concentrated sulfuric acid or nitric acid cannot be used is that concentrated sulfuric acid and nitric acid have strong oxidizing properties.

4. Hydrogen energy: three advantages: no pollution, high heat release and wide sources.

Three, molecules and atoms

Molecular atom

Definition: Molecules are the smallest particles that maintain the chemical properties of substances, and atoms are the smallest particles that change chemically.

Features: small size, small mass; Move on; Have a gap

Connection: Molecules are made up of atoms. Molecules and atoms are particles that make up matter.

Difference: In chemical changes, molecules can be separated, but atoms cannot be separated.

The essence of chemical reaction: in chemical reaction, molecules are split into atoms, and atoms are recombined into new molecules.

Four. Composition, composition and classification of substances

Composition: Matter (pure matter) consists of elements.

Atoms: composed of atoms, such as metals, rare gases, carbon, silicon, etc.

Molecules: For example, hydrogen chloride is composed of hydrogen chloride molecules. H2, oxygen, N2 and chlorine.

Ions: NaCl plasma compounds, such as sodium chloride, consist of sodium ions (Na+) and chloride ions (Cl-).

Classification:

1, mixture (multiple substances)

2. Pure substance (1) Simple substance (an element): metal, nonmetal and rare gas.

(2) Compound (one substance): organic compounds CH4, C2H5OH, C6H 12O6, starch and protein (multiple elements).

Inorganic compounds: oxide H2O, copper oxide, carbon dioxide acid, hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide, calcium hydroxide and potassium hydroxide.

Sodium chloride copper sulfate Na2CO3 salt

Unit 4 mystery review study plan composed of materials

1 composition of atoms

Understanding of (1) Atomic Structure Diagram

(2) The number of nuclear charges in atoms = the number of protons = the number of electrons outside the nucleus determines the type of elements: the number of protons (nuclear charges).

(3) Atomic mass is mainly concentrated on the nucleus. (5) The chemical properties of elements are determined by the number of electrons in the outermost layer.

(4) Relative atomic mass ≈ proton number+neutron number determines atomic mass: nucleus.

Note: The chemical properties of the outermost layer are not necessarily the same as the same number of electrons (mg, the number of electrons in the outermost layer is 2).

The chemical properties of the outermost layer may be similar, but the number of electrons is different (he and Ne are both stable structures).

2. Elements

(1) Definition: The general term for a class of atoms with the same nuclear charge number (proton number).

The essential difference between one element and another is that the number of protons is different.

note:

* substances composed of the same elements are not necessarily simple substances, such as a mixture of O2 and O3 or a mixture of diamond and graphite. They cannot be compounds.

(2) Representation Method-Element Symbol-Capitalization of Latin Names

First, the writing method:

mean

Note: * Some elemental symbols can also represent simple substances, such as iron, helium, carbon and silicon.

* Adding numbers before element symbols can only have microscopic significance, but not macroscopic significance. For example, 3O means only three oxygen atoms.

C. periodic table of elements

* Discovery: Mendeleev

* Arrangement basis: arranged in order of atomic nuclear charge number.

* Note: Atomic number = proton number = nuclear charge number.

Classification: metal and nonmetal

E, most elements: crust: oxygen, silicon, aluminum, iron cells: oxygen, carbon, hydrogen.

3. Ions: charged atoms or atomic groups.

(1) Representation method and significance: For example, Fe3+: an iron ion has a positive charge of 3 units.

② Understanding the schematic diagram of ion structure

Note: The difference from the atomic diagram is that the number of protons = the number of electrons is the atomic structure diagram.

* number of atoms ≠ number of electrons is the schematic diagram of ion structure.

(3) Differences and connections with atoms

Particle type: atomic ion: cationic anion

The number of protons used to distinguish the particle structure = the number of electrons and protons; Electron number proton number

Particles are not electric, but positive and negative

Symbols are represented by element symbols, cation symbols and anion symbols.

Second, the characterization of material composition:

1, price

A, writing and meaning: Mg: The valence of magnesium is+2; The valence of magnesium in magnesium chloride is +2.

B, the meaning of several numbers: Fe2+ Each ferrous ion has two unit positive charges: 3fe2+:3 ferrous ions.

2H2O is two water molecules, each containing two hydrogen atoms.

C, the algebraic sum of the positive and negative valence of each element in the compound is zero.

D, valence is the nature of the atoms of elements when they form compounds, so the valence of elements in simple molecules is 0.

2. Chemical formula

(1) Writing:

A simple substance: metals, rare gases and most solid nonmetals usually use elemental symbols to represent their chemical formulas;

The molecules of nonmetallic gases such as oxygen, hydrogen, nitrogen and chlorine are composed of two atoms, and their chemical formulas are O2, H2, N2 and Cl2.

Compound B: Positive valence elements are in the front and negative valence elements are in the back (except NH3 and CH4), such as MgCl2.

(2) Meaning: For example, the meaning of chemical formula H2O: write 4 points (the composition of water above) and the meaning of chemical formula Fe: write 3 points.

(3) Calculation:

A, calculate the relative molecular mass = the relative atomic mass of each element × the sum of the number of atoms

B, calculate the mass ratio of the constituent elements of the substance: the ratio of relative atomic mass × atomic number.

C, calculating the mass fraction of an element in a substance.

Unit 5 knowledge of chemical equations

A, the law of conservation of mass:

1, content: the total mass of substances participating in chemical reaction is equal to the total mass of substances generated after reaction.

Description: ① The law of conservation of mass only applies to chemical changes, but not to physical changes;

② The mass of substances that do not participate in the reaction and the mass of substances that are not products cannot be counted as "sum";

(3) Consider whether substances in the air participate in the reaction or whether substances (such as gas) are omitted.

2. Micro-explanation: Before and after the chemical reaction, the species, quantity and quality of atoms remain unchanged (the "three invariants" of atoms).

3.( 1) must remain unchanged before and after the chemical reaction. Macro: the total mass of reactants and products remains unchanged; The type and quality of elements remain unchanged.

Microscope: The species, quantity and quality of atoms remain unchanged.

(2) Macro must be changed: the types of substances must become micro; The types of molecules must be changed.

(3) Possible changes: The total number of molecules may change.