CBSE Class 6 Science Sorting Materials into Groups Advanced Exam Notes

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Sorting Materials into Groups
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus.Familiar examples of elements include gold, iron, copper, carbon, silicon, mercury, sodium, calcium, hydrogen, nitrogen, chlorine, and neon. A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds. Chemical compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or complexes held together by coordinate covalent bonds.

A mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically. Mixture refers to the physical combination of two or more substances the identities of which are retained and are mixed in the form of alloys, solutions, suspensions, and colloids. Mixtures are the product of a mechanical blending or mixing of chemical substances like elements and compounds, without chemical bonding or other chemical change, so that each ingredient substance retains its own chemical properties and makeup.Nevestheless, despite there are no chemical changes to its
constituents, the physical properties of a mixture, such as its melting point, may differ from those of the components. Some mixtures can be separated into their components by physical (mechanical or thermal) means. Azeotropes can be considered as a kind of mixture which usually pose considerable difficulties
regarding the separation processes required to obtain their constituents (physical or chemical processes or, even a blend of them). Mixtures can be either homogeneous or heterogeneous. A homogeneous mixture is a type of mixture in which the composition is uniform. A heterogeneous mixture is a type of mixture in which the components can easily be identified, as there are two or more phases present. Air is a homogeneous mixture of the gaseous substances nitrogen, oxygen, and smaller amounts of other substances. Salt, sugar, and many other substances dissolve in water to form homogeneous mixtures. A homogeneous mixture in which there is both a solute and solvent present is also a solution.

The following table shows the main properties of the three families of mixtures.

The following table shows examples of the three types of mixtures.

MIXTURES AND COMPOUNDS
Mixtures are heterogeneous forms of matter. Mixtures are composed of variable proportions of moleculesand atoms.
Compounds are homogeneous forms of matter. Their constituent elements (atoms and/or ions) are always present in fixed proportions (1:1 depicted here).

Examples of mixtures:
· soil
· ocean water and other solutions
· air
· the cytosol of a cell 

Examples of compounds:

• water (H₂O) 
• table salt (NaCl)
• sucrose (table sugar, C₁₂ H₂₂ O₁₁ )

Properties of Mixtures

• The composition of a mixture is variable.
• Each of its components retains its characteristic properties.
• Its components are easily separated.

Properties of Compounds

• The relative proportions of the elements in a compound are fixed.
• The components of a compound do not retain their individual properties. Both sodium and chlorine are poisonous; their compound, table salt (NaCl) is absolutely essential to life.
• It takes large inputs of energy to separate the components of a compound.

Separating the Components of a Mixture 

Most laboratory work in biology requires the use of techniques to separate the components of mixtures. This is done by exploiting some property that distinguishes the components, such as their relative

• size
• density
• solubility
• electrical charge

Dialysis 

Dialysis is the separation of small solute molecules or ions (e.g., glucose, Na+, Cl-) from macromolecules (e.g., starch) by virtue of their differing rates of diffusion through a differentially permeable membrane. Chromatography 

Chromatography is the term used for several techniques for separating the components of a mixture. Follow the links below for examples.

1. paper chromatography, where the molecules are separated by size and solubility
2. exclusion chromatography, where the molecules in a mixture are separated by size.
3. affinity chromatography, where molecules are separated on the basis of their attraction to material in the chromatographic column.

Electrophoresis

Electrophoresis uses a direct electric current to separate the components of a mixture by the differing

electrical charge.

 radical, in chemistry, group of atoms that are joined together in some particular spatial structure and that take part in most chemical reactions as a single unit. Important inorganic radicals include ammonium, NH+  

(+1); carbonate, CO²₃- ; chlorate, ClO₃- , and perchlorate, ClO₄- ; cyanide,- ; hydroxide, OH– ; nitrate, NO₃ ; phosphate, PO³₄ ; silicate, SiO₃ (meta) or SiO²₄- (ortho); and sulfate, SO²₄- . 

A chemical equation is symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side.The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers. The first chemical equation was diagrammed by Jean Beguin in 1615

A chemical equation consists of the chemical formulas of the reactants (the starting substances) and the chemical formula of the products (substances formed in the chemical reaction). The two are separated by an arrow symbol (®, usually read as “yields”) and each individual substance’s chemical formula is separated from others by a plus sign.

As an example, the formula for the burning of methane can be denoted: 

CH₄ + 2 O₂ → CO₂ + 2 H₂O

This equation indicates that oxygen and CH react to form H O and CO . It also indicates that two oxygen molecules are required for every methane molecule and the reaction will form two water molecules and one carbon dioxide molecule for every methane and two oxygen molecules that react. The stoichiometric coefficients (the numbers in front of the chemical formula) result from the law of conservation of mass and the law of conservation of charge 

Common symbols 

Symbols are used to differentiate between different types of reactions. To denote the type of reaction[1]:

• “ = “ symbol is used to denote a stoichiometric relation.

• “ → ” symbol is used to denote a net forward reaction.
• “ ←” symbol is used to denote a net backward reaction.

• “ ‡ˆ ˆˆ †ˆ ” symbol is used to denote an equilibrium. 

Physical state of chemicals is also very commonly stated in parentheses after the chemical symbol, especially for ionic reactions. When stating physical state, (s) denotes a solid, (l) denotes a liquid, (g) denotes a gas and (aq) denotes an aqueous solution.

If the reaction requires energy, it is indicated above the arrow. A capital Greek letter delta (Δ) is put on the reaction arrow to show that energy in the form of heat is added to the reaction. hν is used if the energy is added in the form of light.

Four basic types of chemical reactions: A. Synthesis (composition):

• two or more elements or compounds may combine to form a more complex compound.

• Basic form: A + X → AX

Examples of synthesis reactions:

1. Metal + oxygen → metal oxide

EX. 2Mg(s) + O2(g) → 2MgO(s)

2. Nonmetal + oxygen → nonmetallic oxide

EX. C(s) + O2(g) →  CO2(g)

3. Metal oxide + water →  metallic hydroxide

EX. MgO(s) + H2O(l) →  Mg(OH)2(s)

4. Nonmetallic oxide + water →  acid

EX. CO2(g) + H2O(l) →  ; H2CO3( aq)

5. Metal + nonmetal →  salt

EX. 2 Na(s) + Cl2(g) →  2NaCl(s)

6. A few nonmetals combine with each other.

EX. P4(s) + 6Cl2(g) →  4PCl3(g)

These two reactions must be remembered:

 1. N2(g) + 3H2(g) →2NH3(g)

2. NH3(g) + H2O(l) → NH4OH(aq)

B. Decomposition:

• A single compound breaks down into its component parts or simpler compounds.

• Basic form: AX → A + X

Examples of decomposition reactions:

1. Metallic carbonates, when heated, form metallic oxides and CO2(g).
EX. CaCO3(s) → CaO(s) + CO2(g)

2. Most metallic hydroxides, when heated, decompose into metallic oxides and water.
EX. Ca(OH)2(s) → CaO(s) + H2O(g)

3. Metallic chlorates, when heated, decompose into metallic chlorides and oxygen.
EX. 2KClO3(s)→ 2KCl(s) + 3O2(g)

4. Some acids, when heated, decompose into nonmetallic oxides and water.
EX. H2SO4 → H2O(l) + SO3(g)

5. Some oxides, when heated, decompose.
EX. 2HgO(s) → 2Hg(l) + O2(g)

6. Some decomposition reactions are produced by electricity.
EX. 2H2O(l) → 2H2(g) + O2(g)
EX. 2NaCl(l) → 2Na(s) + Cl2(g)

C. Replacement

Displacement Reaction

• a more active element takes the place of another element in a compound and sets the less active one free.

• Basic form: A + BX → AX + B or AX + Y → AY + X

Examples of displacement reactions:

1. Replacement of a metal in a compound by a more active metal.

EX. Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

2. Replacement of hydrogen in water by an active metal.

EX. 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
EX. Mg(s) + H2O(g) → MgO(s) + H2(g)

3. Replacement of hydrogen in acids by active metal
EX. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

4. Replacement of nonmetals by more active nonmetal
EX. Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(l)

D. Ionic Reactions:

• A ionic reaction will occur when a pair of ions come together to produce at least one of the following:

1. a precipitate
2. a gas
3. water or some other non-ionized substance.

• Basic form: AX + BY → AY + BX

Examples of ionic reactions:

1. Formation of precipitate.
 
EX. NaCl (aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
EX. BaCl2 (aq) + Na2 SO4(aq) → 2NaCl(aq) + BaSO4(s)

2. Formation of a gas
EX. HCl(aq) + FeS(s) → FeCl2(aq) + H2S(g)

3. Formation of water. (If the reaction is between an acid and a base it is called a neutralization reaction.)
EX. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)4. Formation of a product which decompose
EX. CaCO3(s) + HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) 

Combustion of Hydrocarbons 

Another important type of reaction, in addition to the four types above, is that of the combustion of a hydrocarbon. When a hydrocarbon is burned with sufficient oxygen supply, the products are always carbon dioxide and water vapor. If the supply of oxygen is low or restricted, then carbon monoxide will be produced. This is why it is so dangerous to have an automobile engine running inside a closed garage or to use a charcoal grill indoors.

• EX. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
• EX. 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)

NOTE:

• Complete combustion means the higher oxidation number is attained.
• Incomplete combustion means the lower oxidation number is attained.
• The phrase “To burn” means to add oxygen unless told otherwise.

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