Quanzhou junior high school chemistry knowledge is about 20 1 1 year, so it's better to follow the rules and meet the rapid demand.

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 will make bronzes in Shang Dynasty and steel in Spring and Autumn and Warring States Period.

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

① Four characteristics P6 (raw materials, conditions, zero emission, products) ② Core: eliminating 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 to be tested: cover the upper part of the flame with a dry and cold beaker with water mist in it.

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 (the components of inhaled air and exhaled air are the same).

6. An important way to learn chemistry-scientific inquiry

General steps: ask questions → guess and assume → 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 are 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.

Non-heatable instruments. Measuring cylinders, funnels, gas cylinders

(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. The measuring cylinder with the measuring range of 10 ml can generally only read 0. 1 ml.

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

Note: (1) Adjust the zero point first (2) The position of the weighed object and weight is "left object and right code". (3) The weighed objects cannot be directly placed on the tray. When weighing general drugs, put a piece of paper with the same size and quality in each tray on both sides 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" when using alcohol lamps: ① Do not add alcohol to the burning alcohol lamps; 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 holder, 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 outer flame of an alcohol lamp.

(4) If the alcohol lamp is accidentally knocked 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. Don't press your thumb on the short handle of the tube clamp.

When the test tube clamps the test tube, the test tube clamp should be erected 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 prevent the filtrate from splashing.

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

(a) Access to medicines

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 the dose is not specified, take the minimum amount, cover the solid with the bottom of the test tube, and the liquid is 1~2mL.

Don't put the extra reagent back into 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 spoons or V-shaped paper slots; ② Blocked 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 placed in the center of the hand (to avoid corrosion of the label by residual liquid flow). 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, put the label outward and put it back.

(2) Liquid reagent drop method:

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

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

C, in the process of use, always put the rubber nipple on the floor, so as not to be corroded by reagents.

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

E, rubber dropper must not extend into the container or contact with the container wall when in use, otherwise it will cause reagent pollution.

(2) Connect the instruments and equipment and check the air tightness of the equipment.

Air tightness inspection of the device: first immerse one end of the catheter in water, 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 mouth, release your hand and a water column will rise at the catheter mouth. After the pause, the water column did not fall back, indicating that the device did 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 evenly first, and then heated in the center.

(2) When heating the liquid, the volume of the liquid should 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. To avoid injury, do not point the test tube mouth at yourself or others when heating.

(4) filtering

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 for drainage, 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, stir continuously 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 a large number of solids appear), stop heating, and use the waste heat to evaporate the remaining moisture to prevent the solids from splashing out due to heating.

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

(6) Cleaning of instruments:

(1) Waste residue and liquid are poured into the waste 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 the glass instrument: firstly, clean it with hot sodium carbonate (Na2CO3) solution or washing powder, and then rinse it with clean water.

(4) The glass instrument is attached with water-insoluble alkali, basic oxide and carbonate: it is dissolved with dilute hydrochloric acid first, and then washed with water.

(5) After washing the instrument, it should not be misplaced. After the test tube is washed, it should be inserted upside down in the test tube rack to dry.

Unit 2 "The air around us"

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

2. Composition and composition of air O2 N2 CO2 Rare gas Other gases and impurities Volume fraction 2 1% 78% 0.03% 0.94% 0.03%.

(1) determination of oxygen content in air

A requirements for combustible materials: sufficient quantity; solid products: red phosphorus B; Equipment 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, which does not support combustion, nor can it burn, and 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 items included in the air pollution index include CO, SO2, NO2, O3 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, acid rain, etc.

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

(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 gives off a faint light blue flame in the air and a bright blue-purple flame in oxygen, producing a pungent gas.

Magnesium gives off dazzling white light, giving off heat and producing white solid.

Aluminum gives off dazzling white light, giving off heat and producing white solid.

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 generated on the bottle wall, resulting in gas that makes clear limewater 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 =

2k MnO 4△k2mno 4+MnO 2+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 an example)

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

B, note:

(1) the test tube mouth slightly tilted downward; Prevent the test tube from breaking due to the backflow of condensed water; (2) spreading the medicine on the bottom of the test tube: heating 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 Put a ball of cotton at the mouth of the test tube: prevent potassium permanganate powder from entering the catheter; ⑤ When the drainage method is used for collection, 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. When the exhaust gas is collected, the conduit extends to the bottom of the gas collecting cylinder.

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

Difference: oxygen inspection: insert a wooden stick 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 commonly used gas:

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

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

(3) Rare gases (helium, neon, argon, krypton, xenon, etc.): protective gas, electric light source (energized to emit different colors of light) 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 breath, food decay, wine making.

* * * Similarity: ① Both are oxidation reactions ② Both are exothermic reactions.

Unit 3 Understanding Water in Nature

I. 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 a wooden strip with sparks at the air outlet —— The wooden strip is re-ignited.

H2-the air outlet is equipped with a burning wood-the gas burns to produce a 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) said a substance water this substance.

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

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

(4) indicates the molecular composition of the 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:

(1) water resources

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); The application of pesticides and fertilizers is unreasonable, and domestic sewage is discharged at will

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 filtration and adsorption functions is activated carbon.

(2) Hard water and soft water A. Definition Hard water is water containing more soluble calcium and magnesium compounds; Soft water is water containing no or little soluble calcium and magnesium compounds.

B identification method: soapy water, hard water with scum 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 easy to scale, 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 changes 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 property: 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, measure the purity (method? ) Phenomenon: A light blue flame is emitted, which releases heat and produces water droplets.

(2) Reducibility (use: smelting metal)

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

Phenomenon: black powder turns red, and water droplets form at the mouth of the test tube (summary: combustible and reducing substances H2, C and CO).

3. Laboratory method of hydrogen.

Principle: Zn+H2SO4 = ZnSO4+H2 ↑ Zn+2hcl = ZnCl2+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.

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

Connection: Molecules are made 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 split into atoms and atoms recombine into new molecules.

Four. Composition, composition and classification of substances

Composition: A substance (pure substance) 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、O2、N2、Cl2 .

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, 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.

Salt NaCl CuSO4 Na2CO3

Unit 4 The Mystery of Material Composition Review Study Plan

1, composition of atoms

Understanding of (1) Atomic Structure Diagram

(2) The number of nuclear charges in an atom = the number of protons = the number of electrons outside the nucleus determines the element type: the number of protons (nuclear charges).

(3) The mass of atoms 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 weight ≈ proton number+neutron number determines atomic weight: nucleus.

Note: tHe chemical properties of the outermost layer are not necessarily the same as the same number of electrons (Mg, he outermost electron number 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:

* A substance composed of the same element is not necessarily a simple substance, such as a mixture of O2 and O3 or a mixture of diamond and graphite, and cannot be a compound.

(2) Representation method-element symbol-Latin initials capitalized

First, the writing method:

B, meaning

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

* Adding a number before an element symbol can only have a microscopic meaning, not a macroscopic meaning. For example, 3O means only three oxygen atoms.

C. periodic table of elements

* Discovery: Mendeleev

* Arranging basis: Arranged according to the nuclear charge number of atoms.

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

Classification: metal and nonmetal

E, most elements: crust: O, Si, Al, Fe cells: O, C, H.

3. Ions: charged atoms or atomic groups.

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

(2) Understand the schematic diagram of ion structure

Note: Different from the schematic diagram of atoms, the number of protons = the number of electrons is the schematic diagram of atomic structure.

* 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 >; Electron number proton number

Particles are not electric, they are positive and negative.

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

Second, the appearance of material composition:

1, valence

A, writing and meaning: the valence of Mg: 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 are usually represented by elemental symbols;

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 (composition of water above) 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 elements in the substance.

Unit 5 knowledge of chemical equations

A, the law of conservation of mass:

1, content: the total mass of substances participating in the chemical reaction is equal to the total mass of substances generated after the 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. Microscopic explanation: before and after the chemical reaction, the kind, 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.