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Tuesday, 30 June 2015

ACIDS, BASES AND SALTS notes Part 2

ACIDS, BASES AND SALTS


On the basis of there chemical properties, all the compounds can be classified into three groups.
  1. Acids,
  2. Bases, and
  3. Salts
Indicator for Testing Acids and Bases
An indicator is a ‘dye’ that changes color when it is put into an acid and base. An indicator tells as whether the substance we are testing is an acid or a base by change in its color.
The three common indicators to test for acids and bases are: Litmus, Methyl orange and Phenolphthalein.
  1. Litmus: -- Litmus can be used in the form of litmus solution or in the form of litmus paper (Blue litmus and Red litmus). Litmus is natural indicator extracted from ‘lichen’ plant. The natural color of lichen is purple.
->An acid turns blue litmus to red.
->A base (or alkali) turns red litmus to blue.
  1. Methyl orange: -- Methyl orange is synthetic indicator. The natural color of methyl orange is orange.
->Methyl orange indicator gives red color in acid solution.
->Methyl orange indicator gives yellow color in basis solution.
  1. Phenolphthalein: -- Phenolphthalein is synthetic indicator. The natural color of phenolphthalein is colorless.
->Phenolphthalein indicator is colorless in acid solution
->Phenolphthalein indicator gives pink color in basic solution.
Turmeric is also a natural indicator. It contains a yellow dye. It turns red in basic solution. The red cabbage extract (obtain from red cabbage leaves) is also a natural indicator. It is red in acidic solution but turns green on adding to basis solutions.

Olfactory Indicators:
Those substances whose smell (or odour) changes in acidic or basic solutions are called olfactory indicators. Onion and vanilla extract are olfactory indicators.
  1. Onion has a characteristic smell. In basic solution the onion smell can not be detected. An acidic solution dose not destroys the smell of onions.
  2. Vanilla extract has a characteristic pleasant smell. In basis solution the pleasant smell cannot be detected. An acid solution dose not destroys the smell.
ACIDS
Acids are those substances which have a sour taste. Acids change the color of blue litmus to red.
The acids present in plant materials and animals are called organic acids. Examples are Acetic acid (ethanoic acid), Citric acid, Lactic acid, Formic acid (methanoic acid).
The acids prepared from the minerals of the earth are called mineral acids. Examples are hydrochloric acid, Sulphuric acid and Nitric acid.
Strong Acids and Weak Acids
All the acids are divided into two groups: strong acids and weak acids.
(i)All the mineral acids are strong acids. Hydrochloric acid, sulphuric acid and nitric acid are strong acids.
(ii)The organic acids are weak acids. Acetic acid, formic acid, tartaric acid and carbonic acid are weak acids.
Concentrated and dilute acids
A concentrated acid is one which contains the minimum possible amount of water in it. A dilute acid is one which contains much more of water in it.
Diluting Acids
The dilution of concentrated acid should always be done by adding concentrated acid to water gradually with string and not by adding water to concentrated acid.
Properties of Acids
The important characteristic of acids is:
  1. Acids have a sour taste
  2. Acids turns blue litmus to red
  3. Acids solutions conduct electricity
  4. Acids reacts with metals to form hydrogen gas
Metal + Acid Salt + Hydrogen gas
Most of the acids react with metals to form salts and evolve hydrogen gas. Hydrogen is common to all acid.
Curd and sore food stuffs should not be kept in metal vessels because curd and other food stuffs contain acids which can reacts with metal of the vessel to form poisonous metal compounds which can cause food poisoning.
  1. Acids reacts with metal carbonate (and metal hydrogen carbonates) to form carbon dioxide gas.
Metal carbonate + Acid Salt + Carbon dioxide + Water
  1. Acids react with bases (or alkali) to form salt and water.
Acid +Base Salt + Water
The reaction between an acid and base to form salt and water is called a netralisation reaction.
  1. Acids react with metal oxides to form salt and water.
Metal oxide + Acid Salt + Water
The acids also react with metal hydroxides to form salt and water.
  1. Acids have corrosive nature.
The mineral acids cause severe burns on the skin and attack and eat up materials like cloth, wood, metal structures and stonework, so they are said to be corrosive. Acids are never stored in metal container because they gradually corrode and eat up metal container so they stored in glass and ceramics container. The strong bases (or alkali) such sodium hydroxide are also very corrosive, and attack and destroy our skin.

WHAT DO ALL ACIDS HAVE IN COMMON

An acid is a substance which dissociates (or ionizes) on dissolving in water to produce hydrogen ions [H+(aq)ions].
HCl(aq) H+(aq) + Cl-(aq)
A common thong in all the acids is that they produce hydrogen ions [H+(aq)ions] when dissolved in water.
The aqueous solutions glucose and alcohol do not show acidic character because hydrogen does not separate out as hydrogen ions [H+(aq)ions] on dissolving in water.

The aqueous solution of acid conducts electricity due to the presence of charge particles called ions in it. Due to the absence of ions, glucose solution do not conducts electricity. Distilled water dose not conducts electricity because it dose not contain any ionic compound (like acids, bases of salts) dissolved in it. Due to the presence of carbonic acid (which provides ions to rain water), the rain water conducts electricity.

Acids Do Not Show Acidic Behavior in the Absence of Water
In the absence of water, a substance will not form hydrogen ions and will not show its acidic behavior. Dry HCl gas dose not change the color of dry litmus paper because it has no hydrogen ions [H+(aq)ions] in it. The HCl gas turns ‘wet’ blue litmus paper red because it dissolves in the water present in wet litmus paper to form hydrogen ions, which can turns blue litmus paper to red.

Strong Acids
An acid which is completely ionised in water and produce s a large amount of hydrogen ions is called a strong acid. Hydrochloric acid is completely ionized in water, so it is a strong acid. Sulphuric acid and nitric acid is also strong acids. Strong acids react very rapidly with other substance (such as metals, metal carbonates and metal hydro carbonates).

Weak Acids
An acid which is partially ionized in water and thus produces a small amount of hydrogen ions is called a weak acid. Acetic acid (CH3COOH) is partially ionized in water to produce only small amount of hydrogen ions, so it is weak acid. Carbonic acid (H2CO3) and sulphurous acid (H2SO3) are also weak acids.
When a concentrated solution of acid is diluted by mixing water, then the concentration of hydrogen ions, H+ (aq) [or hydronium ions, H3O+ ] per unit volume decreases.

BASES
Bases are those chemical substances which have bitter taste. It changes the color of red litmus blue. Bases are chemical opposite of acids. Base is a chemical substance which can neutralize an acid.

Water Soluble Bases: Alkalis

Most bases do not dissolve in water but some dissolve in water with out any chemical reaction. A base which dissolves in water is called an alkali.

What do All the Bases have in Common?

A common property of all the bases (or alkali) is that they all produce hydroxide ions (OH- ions) when dissolved in water. NaOH, KOH, Mg(OH)2, NH4OH are all bases.

Strong Bases
A base which completely ionized in water and thus produces a large amount of hydroxide ions (OH- ions) is called a strong base (or a strong alkali). Sodium Hydroxide (NaOH), potassium hydroxide (KOH) is strong bases.

Weak Bases
A base which is partially ionized in water and thus produces a small amount of hydroxide ions (OH- ions) is called a weak bases (or weak alkali). Ammonium hydroxide (NH4OH), calcium hydroxide [Ca(OH)2] are weak acids.

Properties of Bases

  1. Bases have bitter taste
  2. Bases feel soapy to touch
  3. Bases turn red litmus to blue
  4. Bases conduct electricity in solution(They are electrolytes)
  5. Bases react with some metals to form hydrogen gas
All the metal does not react with bases to form salts and hydrogen gas.
  1. Bases react with acids to form salts and water.
When an acid and base combine then real the neutralization reaction occurs due to combination of hydrogen ions present in acid and hydroxide ions presents in base to form water. neutrslisation
H+(aq) + OH-(aq) H2O(l)
  1. Bases react with non metal oxide to form salt and water
Non-metal + Base Salt + Water

STRENGH OF ACID AND BASE SOLUTIONS: pH SCALE

Acidic solution has excess of hydrogen ions but it also contains hydroxide ions.
Basic solution has excess of hydroxide ions but it also contains hydrogen ions.
Both acidic solutions as well as basic solution contain hydrogen ions. In 1909 Sorenson devised a scale (Known as pH scale) on which acid solutions as well as basic solutions could be represented by making use of the hydrogen ion concentration in them.
The pH of a solution is inversely proportional to the concentration of hydrogen ions in it.
The strength of an acid or base is measured on a scale of numbers called the ph scale. The pH scale has values from 0 to 14, pure number, no units.
According to pH scale: ---
  1. Neutral substances have a pH of exactly 7.
  2. Acids (or acidic solution) have a pH pf less than 7. Lower the pH, the stronger the acid.
  3. Bases (or basic solution) have a pH of more than 7. Higher the pH, the stronger the base (or alkali).

Universal Indicator

To obtain the idea of how acidic or basic substance is, universal indicator is used.
Universal indicator is a mixture of many different indicators (or dyes) which gives different color in different pH values of the entire pH scale. Like litmus which can be produced in the form of solution or paper.
Water will produce green color with universal indicator.

Important of pH in everyday life

  1. pH in our digestive system
Dilute hydrochloric acid helps in digesting our food. Excess acid in the stomach causes indigestion. Being basic in nature, antacids react with excess in stomach and neutralise it. Magnesium hydroxide (milk of magnesia) and sodium hydrogen carbonate (baking soda) are used as antacids.
  1. ph change as the cause of tooth decay
The bacteria present in our mouth breaks down the sugar to form acids. Tooth decay starts when the pH of acid formed in the mouth falls 5.5. The best way to prevent tooth decay is to clean the mouth thoroughly after eating food.
  1. Prints and Animals are Sensitive to pH Changes
    1. Soil pH and Plant Growth. Most of the plants grow best when pH of the soil is close to 7. If the soil is too acidic or too basic, the plant grows too badly or do not grow at all. If the soil is too acidic, then it is treated with materials like quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).
    2. pH Change and Survival of Animals. Our body works well within a narrow range of 7.0 to 7.8. If this pH range gets disturbed in the body, then many ailments can occur.
  2. self defense by Animals Plants by through Chemical Warfare
An ant’s string injects methanoic acid into the skin of a person causing burning pain. When a person happens of a nettle plant accidentally, the stinging hair leaves injects methanoic acid into the skin of the person causing burning pain.

SALTS

A salt is a compound formed from an acid by the replacement of hydrogen in the acid by a metal. Salts are formed when acids reacts with bases. Just like acids and bases, solutions of salts in water conduct electricity. Salts are ionic compounds.

Family of salts

The salts having the same positive ions (or same negative ions) are said to belong to the family of salts. For example sodium chloride (NaCl) and sodium sulphate (Na2SO4) belong to the same family of salts called ‘sodium salts’.

The pH of Salt Solutions
Though the aqueous solutions of many salts are neutral (having a ph of 7), some salts produce acidic or basic solutions when dissolve in water
The acidic nature and basic nature of some salt solutions can be explained on the basis of hydrolysis of salts.
  1. The salts of strong acid and strong bases give neutral solutions
  2. The salts of strong acid ands and weak bases give acidic solution
  3. The salts of weak acids and strong bases give basis basic solutions

COMMON SALT (SODIUM CHLORIDE)

The chemical name of common salt is sodium chloride (NaCl).
Sodium chloride can be prepared by the combination of sodium hydroxide and hydrochloric acid:
NaOH(aq) + HCl(l) NaCl(aq) + H2O(l)

How Common Salt is obtained

  1. Common Salt from Sea-water – Common salt obtains from sea-water by the process of evaporation.
  2. Common Salt from Underground Deposits – Rock is mined from the underground deposits just like coal.
Uses of Common Salt
  1. Common salt is used as a raw material for making chemicals like: sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, hydrochloric acid , hydrogen chlorine, and sodium metal.
  2. Common salt is used in cooking food.
  3. Common salt is used as a preservative.
  4. Common salt is used in the manufacturing of soap.
  5. Common salt used to melt ice which collects in the roads during winter in cold countries.

Chemicals from Common Salt

SODIUM HYDROXIDE
Sodium hydroxide is commonly known as caustic soda.
When electricity is passed through a concentrated solution of sodium chloride, it decomposes to form sodium hydroxide, chlorine and hydrogen.
2NaCl (aq) + 2H2O(l) 2NaOH(aq) + Cl2(g) H2(g)
Chlorine gas is produced at the anode and hydrogen gas is produced at the cathode. Sodium hydroxide solution formed near the cathode. The process of sodium chloride solution is called chlor-alkali process. The three very useful products obtained by the electrolysis of sodium chloride solution called brine (or chlor-alkali process) are sodium hydroxide, chlorine and hydrogen.

Uses of Sodium Hydroxide
  1. Sodium hydroxide is used for making soaps and detergents
  2. Sodium hydroxide is used for making artificial textile fibers
  3. Sodium hydroxide is used in the manufacturing of paper
  4. Sodium hydroxide is used in purifying bauxite ore from aluminum metal is extracted
  5. Sodium hydroxide is used de-greasing metals, oil refining, and making dyes and bleaches
Uses of Chlorine
  1. Chlorine is to used to sterilize drinking water
  2. Chlorine is used in the production of bleaching powder
  3. Chlorine is used in the production of hydrochloric acid
  4. Chlorine is used to make plastics
  5. Chlorine is used for making solvents for dry-cleaning
Uses of Hydrogen
  1. Hydrogen is used in the hydrogenation of oils to obtain solid fats
  2. Hydrogen is used in the production of hydrochloric acid
  3. Hydrogen is used to make ammonia for fertilizers
  4. Hydrogen is used to make methanol
  5. hydrogen is used as a fuel for rockets
Uses of Hydrochloric acid
  1. Hydrochloric acid is used for cleaning iron sheets before tin-plastic or galvanization
  2. Hydrochloric acid is used in the preparation of chlorides such as ammonium chloride
  3. Hydrochloric acid is used in medicines and cosmetics
  4. Hydrochloric acid is used is textile, dyeing and tanning industry
  5. Hydrochloric acid is used in making plastics
WASHING SODA
Washing soda is sodium carbonate containing 10 molecules of water of crystallization.
Washing is produced from sodium chloride in following three steps
  1. A cold and concentrated solution of sodium chloride is reacted with ammonia and carbon dioxide to obtain sodium hydrogen carbonate.
NaCl + NH3 + H2O CO2 NaHCO3 + NH4Cl
  1. On heating sodium hydrogen carbonate decomposes to form sodium carbonate
2NaHCO3 Na2CO3 + CO2 + H2O
  1. Anhydrous sodium carbonate is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystal satin.
Na2CO3 + 10H2O Na2CO3.10H2O

Properties of washing soda
  1. Washing soda is a transparent crystalline solid
  2. Washing soda is one of the metal carbonates which are soluble in water
  3. The solution of washing soda in water is alkaline which turns red litmus to blue
  4. Detergent properties (or cleansing properties)

Use of sodium carbonate
  1. Sodium carbonate is used as a “cleansing agent” for domestic proposes.
  2. Sodium carbonate is used for removing permanent hardness of water.
  3. Sodium carbonate is used in the manufacturing of glass, soap and paper.
  4. Sodium carbonate is used in the manufacturing of sodium compounds such as borax.
BAKING SODA
The chemical name of backing soda is sodium hydrogen carbonate and the formula is NaHCO3.
Production of sodium hydrogen carbonate
Sodium carbonate is produced by reacting a cold and concentrated solution of sodium chloride with ammonia and carbon dioxide
NaCl + NH3 + H2O + CO2 NaHCO3 + NH4Cl
Properties of sodium hydrogen carbonate
  1. Sodium hydrogen carbonate consists of white crystals which are sparingly soluble in water.
  2. Sodium hydrogen carbonate is a mild, non-corrosive base and water is mildly alkaline.
  3. When solid hydrogen carbonate is heated, then it decomposes to give sodium carbonate and carbon dioxide.
Heat
2NaHCO3 ----------- > Na2CO3 + CO2 + H2O


Uses of sodium hydrogen carbonate
  1. Sodium hydrogen carbonate is used as an antacid in medicine to remove acidity of the stomach.
  2. Sodium hydrogen carbonate is used in making baking power
  3. Sodium hydrogen carbonate is used in fire extinguishers.

BLEACHING POWER

Bleaching power is calcium oxychloride and the formula is CaOCl2.
Preparation of bleaching power
Bleaching power is prepared by passing chlorine gas over dry slaked lime.
Ca(OH)2 + Cl2 CaOCL2 + H2O

Properties of bleaching power
  1. Bleaching power is a white power which gives a strong smell of chlorine.
  2. Bleaching power is soluble in cold water.
  3. Bleaching power reacts with dilute acids to produce chlorine.
CaOCl2 + H2SO4 CaSO4 + Cl2 + H2O
Uses of Bleaching Power
  1. Bleaching power is used for bleaching cotton and linen in textile industry
  2. Bleaching is used for disinfectant drinking water.
  3. Bleaching power is used for the manufacturing of chloroform (CHCl3).
  4. Bleaching power is used for making wood unshrinkable.
  5. Bleaching power is used as an oxidizing agent in many chemical industry.

PLASTER OF PARIS

Plaster of Paris is calcium sulphate hemi hydrate (calcium carbonate half-carbonate) and the formula is CaSo4.1/2H2O.
Preparation of Plaster of Paris
Plaster of Paris is prepared by heating gypsum (CaSO4.2H2O calcium sulphate dihydrate) to a temperature of 100o C.
CaSO4.2H2O CaSO4.1/2H2O + 1.1/2H2O

Properties of Plaster of Paris
  1. Plaster of Paris is a white power.
  2. Plaster of Paris has a very remarkable property of setting into a hard mass on wetting in water.
  3. Plaster of Paris should be stored in a moister proof container.

Uses of plaster of Paris
  1. Plaster of Paris is used in hospitals for setting fractured bones in the right position.
  2. Plaster of Paris is used in making toys, decorative materials, cheap ornaments, black-board chalks and cast for statues.
  3. Plaster of Paris is used as a fire-proofing material.
  4. Plaster of Paris is used in chemical laboratories for sealing air-gaps in apparatus.
  5. Plaster of Paris is used for making surfaces smooth before painting them.

WATER OF CRYSTALISATION

The water molecules which form part of the structure of a crystal (of a salt) are water of crystallization. The salt contain water of a crystallization are called hydrated salt.
Every hydrated salt has a “fixed number” of molecule of water of crystallization in its one formula unit. For example copper sulphate crystal contain 5 molecules of water of crystallization per formula unit and hence written as CuSO4.5H2O.
Water of crystallization is a part of ‘crystal structure’ of a salt. Since water of crystallisation is not free water, it does not wet the salt.
The water of crystallization gives the crystals of the salts their ‘shape’ and, in some cases, imparts them ‘color’.

ACTION OF HEAT ON HYDRATED SALTS

When hydrated salts are heated strongly, they lose their water of crystallization. The salts which have lost their water of crystallization are called anhydrous salts.



QUESTION FOR PRACTICE

  1. Which gas is usually liberated when an acid reacts with a metal?
  2. While diluting an acid, why is it recommended that the acid should be added to water and not water to acid?
  3. What happens when an acid reacts with a metal carbonate?
  4. What happens when an acid reacts with a metal hydrogen carbonate?
  5. What happens when an acid reacts with a base? Give equation of the reaction involved. What is the special name of this reaction?
  6. What happens when an acid reacts with a metal oxide? Write the equation of the reaction involved.
  7. What happens when carbon dioxide gas passed through lime water: (a) for a short time, and (b) for a considerable time? Write equations of the reactions involved.
  8. Complete and balance the following chemical reactions:
(a) Zn(s) + HCl(aq)
(b) Na2CO3(s) + HCl(aq)
(c) NaHCO3(s) + HCl(aq)
(d) NaOH(aq) + HCl(aq)
(e) CuO(s) + HCl(aq)
  1. What are organic acids and mineral acids? Give examples of each.
  2. Which element is common to all acids?
  3. Why do HCl, H2SO4, HNO3 etc... show acidic character in aqueous solution while solutions of compounds like glucose and alcohol do not show acidic character?
  4. Why does dry HCl gas not change the color of the dry litmus paper?
  5. Why does distilled water not conduct electricity whereas rain water does?
  6. Give the name and formulae of two strong acids and two weak acids.
  7. Fill in the following statement with a suitable word:
Substances do not show their acidic properties without……..
  1. What does pH of a solution signify? Three solutions A, B, and C have pH value of 6, 4 and 10 respectively. Which of the solution is highly acidic?
  2. Two solutions X and Y have pH=4 and pH=8, respectively. Which solution will give alkaline reaction and which one acidic.
  3. The pH of a cold drink is 5. What will be its action an blue and red litmus solution?
  4. A solution turns red litmus blue, its pH is like to be:
(a) 1 (b) 4 (c) 5 (d) 10
  1. (a) Which is more acidic, pH = 2 or pH = 11?
(b) Which is more basic pH = 8 or pH = 11?
  1. Fresh milk has a pH of 6. How do you think the pH would change a it turns into curd? Explain your answer.
  2. What effect does the concentration of H+(aq) ions have on the nature of the solution?
  3. Name the indicator which can give us an idea of how strong or weak an acid or base is?
  4. How does a universal indicator work?
  5. Two solutions X and Y are tasted with universal indicator. Solution X turns orange where as solution Y turns red. Which of the solution is a stronger acid?
  6. Under what soil conditions do you think a farmer would treat the soil of his fields with quicklime or slaked lime or chalk?
  7. How does an antacid work? Name two antacid.
  8. Explain the pH change as the tooth decay. How can tooth decay caused by pH change be prevented?
  9. What happens during a bee sting? What is its remedy?
  10. The pH of a soil A is 7.5 while that of soil B is 4.5. Which of the two soils, A or B, should be treated with powered chalk to adjust its to adjust its pH and why?
  11. A salt X when dissolved in distilled water gives a clear solution which turns red litmus blue. Explain the phenomenon.
  12. Explain why, an aqueous solution of sodium chloride is neutral but an aqueous solution of sodium carbonate is basic.
  13. What is a salt? Give the names and formulae of any two salts. Also name the acids and bases from which these salts may be obtained.
  14. What is meant by a family of salts? Explain with examples.
  15. Write the chemical name and formula of common salt.
  16. Name the major salt present in sea-water?
  17. Name three chemicals made from common salt.
  18. Give any two uses of common salt.
  19. Name the salt which is used as a preservative in pickle, and in curing meat and fish.
  20. What happens when a concentrated solution of sodium chloride is electrolyzed? Write the equation of the reaction involved.
  21. The electrolysis of an aqueous solution of sodium chloride gives us three products. Name them.
  22. Explain why, chlorine is used for sterilizing drinking water supply.
  23. Name the product formed when Cl2 and H2 produced during the electrolysis of brine are made to combine.
  24. Write the chemical formula of sodium carbonate.
  25. Name a metal compound which has detergent properties.
  26. Name a sodium compound used for softening hard water.
  27. What is the common name Na2CO3.10H2O?
  28. Name the metal whose carbonate is known as washing soda.
  29. What is the chemical name of baking soda?
  30. What is the chemical formula of baking soda?
  31. What is the common name of sodium hydrogen carbonate?
  32. Name an acid which is present in baking powers.
  33. What happens when a solution of sodium hydrogen carbonate is heated? Write equation of the reaction involved.
  34. Give the chemical formula of washing soda and baking soda.
  35. What is the chemical name of bleaching power?
  36. What is the chemical formula of bleaching power?
  37. What are the materials used for the preparation of bleaching power?
  38. Name the substances obtained by the action of chlorine on solid slaked lime.
  39. Name one compound of calcium which is used for bleaching cloth.
  40. Which is the real bleaching agent present in bleaching power?
  41. What is plaster of Paris? Write the chemical composition of plaster of Paris.
  42. Name the raw material used for the preparation of plaster of Paris.
  43. What is the commercial name of calcium sulphate hemi-hydrate?
  44. What will happen if heating is not controlled while preparing plaster of Paris?
  45. State two important uses of plaster of Paris.
  46. What is meant by ‘water of crystallization’
  47. What is meant by ‘hydrated’ and ‘anhydrous’ salts? Explain with an example.
  48. What happens when a copper sulphate crystal are heated strongly/ explain with the help of an equation.
  49. Write the names, formula and colors of any two hydrated salts.
  50. What happens when copper sulphate crystals are added to anhydrous copper sulphate? Explain with the help of an equation.



Chemistry for class 10....
acids-and-alkalis
carbon-and-its-compound-notes
periodic-classification-of-elements
unsaturated-hydrocarbon
metal-and-non-metals-notes
chemical-reaction-and-equations-notes-2
periodic-classification-of-elements


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