Selina Concise Solutions for ICSE Class 8 Chemistry Chapter 5 Language of Chemistry

Language of Chemistry

Points to Remember:
• The valency of an element is the number of electrons donated or accepted by its 'atom' during chemical combination.
• There are some elements with more than one valency e.g., iron, copper, tin, lead.
• Two or more different non-metals that collectively accept or donate one or more electrons and become negatively or positively charged in the process are called radicals.
• A chemical reaction involves the transformation of original substance into an altogether new substance(s).
• A chemical reaction can be represented with the help of the symbols or the formulae of the elements and the compounds taking part in that reaction. This gives a chemical equation.
• Certain necessary conditions for a chemical reaction to happen are - close contact, solution form, heat, light and catalyst.
• Characteristics of chemical reactions are - change of colour, evolution of a gas, formation of a precipitate, change of state, change of smell and evolution/absorption of heat.
• A complete chemical equation symbolically represents the reactants, products and their physical states.
• The substances that react with each other are called reactants and they are represented on the left hand side of the equation. The substances that are formed as a result of the reaction are called products. They are represented on the right hand side of the equation.
• A chemical equation needs to be balanced to make it follow the law of the conservation of mass.
• The law of conservation of mass states that mass can be neither created nor destroyed, it can only be transformed from one form to another.
• A chemical equation gives both qualitative and quantitative information about the reactants and products.

Learning the language of chemistry is like learning the alphabet for a new language where symbols and formulas tell the story of matter. These fundamental points explain how substances interact to create something entirely new through chemical changes.
Teacher's Tip: Think of reactants as ingredients and products as the finished dish in a cooking recipe.
Exam Tip: When defining "radicals," always mention that they behave as a single unit with a specific charge.

ACTIVITY 1
Write the names and symbols of the first twenty elements that you have studied in class VI & VII.

Answer:
Name of the elements | Symbol | Valency
1. Hydrogen | H | 1
2. Helium | He | 0
3. Lithium | Li | 1
4. Beryllium | Be | 2
5. Boron | B | 3
6. Carbon | C | 4
7. Nitrogen | N | 3
8. Oxygen | O | 2
9. Fluorine | F | 1
10. Neon | Ne | 0
11. Sodium | Na | 1
12. Magnesium | Mg | 2
13. Aluminium | Al | 3
14. Silicon | Si | 4
15. Phosphorus | P | 3
16. Sulphur | S | 2
17. Chlorine | Cl | 1
18. Argon | Ar | 0
19. Potassium | K | 1
20. Calcium | Ca | 2

Memorizing these twenty elements is the first step toward understanding how the periodic table is organized. Each element has a unique symbol and valency that dictates how it bonds with other elements.
Teacher's Tip: Use a mnemonic like "Hi He Lies Because Boron Can't Not Over Feed" for the first few elements.
Exam Tip: Elements like Helium, Neon, and Argon have a valency of 0 because they are stable Noble Gases.

ACTIVITY 2
Write the molecular formulae of:
1. Copper oxide
2. Iron (III) chloride
3. Sodium hydroxide
4. Iron (II) sulphide
5. Lead (II) oxide
6. Hydrogen nitrate (nitric acid)
7. Hydrogen sulphate (sulphuric acid)
8. Calcium hydroxide
9. Magnesium carbonate
10. Ammonium carbonate

Answer:
1. Copper oxide - CuO
2. Iron (III) chloride - FeCl₃
3. Sodium hydroxide - NaOH
4. Iron (II) sulphide - FeS
5. Lead (II) oxide - PbO
6. Hydrogen nitrate (nitric acid) - HNO₃
7. Hydrogen sulphate (sulphuric acid) - H₂SO₄
8. Calcium hydroxide - Ca(OH)₂
9. Magnesium carbonate - MgCO₃
10. Ammonium carbonate - (NH₄)₂CO₃

Writing molecular formulas involves balancing the charges of the positive and negative ions or radicals involved. These formulas represent the exact ratio of atoms found in a single molecule of the compound.
Teacher's Tip: Use the "criss-cross method" where the valency of one element becomes the subscript of the other.
Exam Tip: Always use parentheses like (OH)₂ when a radical occurs more than once in a formula.

ACTIVITY 3
Write the molecular formula for each of the following compounds:
1. Sulphur trioxide
2. Iron (II) sulphide and
3. Ammonia
Find the number and names of elements present in them and calculate their molecular masses.

Answer:
1. Sulphur trioxide
1. A molecule of sulphur trioxide is represented by the formula SO₃.
2. The elements present in it are sulphur and oxygen.
3. One molecule of sulphur trioxide has one atom of sulphur and three atoms of oxygen.
4. Molecular mass of sulphur trioxide (SO₃)
= 32 + 3 x 16
= 32 + 48 = 80 amu.
2. Iron (II) sulphide
1. A molecule of iron (II) sulphide is represented by the formula FeS.
2. The elements present in it are iron and sulphur.
3. One molecule of iron (II) sulphide has one atom of iron and one atom of sulphur.
4. Molecular mass of iron (II) sulphide (FeS) = 55.5 + 32 = 87.5 amu.
3. Ammonia
1. A molecule of ammonia is represented by the formula NH₃.
2. The elements present in it are nitrogen and hydrogen.
3. One molecule of ammonia has one atom of nitrogen and three atoms of hydrogen.
4. Molecular mass of ammonia (NH₃) = 14 + 3 x 1 = 14 + 3 = 17 amu.

Molecular mass is the sum of the atomic masses of all the atoms present in a molecule. Calculating this value is essential for understanding the quantitative side of chemical reactions.
Teacher's Tip: The prefix "tri" in trioxide immediately tells you there are three oxygen atoms.
Exam Tip: Don't forget to include the unit "amu" (atomic mass unit) when stating molecular mass.

Exercise

Question 1: Define:
(a) Radical
(b) Valency
(c) Molecular formula

Answer:
(a) Radical: A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.
(b) Valency: It is the number of electrons donated or accepted by the valence shell of an atom during chemical combination.
(c) Molecular formula: It is a symbolic representation of a molecule. It shows the number of atoms of each element present in it. These atoms combine in whole number to form the molecule.

These definitions provide the foundation for naming compounds and predicting how they will bond. Understanding radicals is particularly important because they often travel together during chemical changes.
Teacher's Tip: Valency is like the "number of hands" an atom has to hold onto other atoms.
Exam Tip: For "Molecular formula," emphasize that atoms must combine in "whole numbers" as you cannot have half an atom.

Question 2: Give the symbols and valencies of following radicals:
(a) Hydroxide (b) Chloride (c) Carbonate (d) ammonium (e) Nitrate

Answer:
Radical | Symbol | Valency
(a) Hydroxide | OH^- | 1
(b) Chloride | Cl^- | 1
(c) Carbonate | CO₃^2- | 2
(d) Ammonium | NH₄⁺ | 1
(e) Nitrate | NO₃^- | 1

Radicals have specific charges that act as their valencies when forming compounds. Memorizing these common radicals will make writing complex formulas much easier.
Teacher's Tip: Note that "Ammonium" is one of the few common positive radicals, while most others are negative.
Exam Tip: Be careful with the signs; a valency of 1 for Chloride comes from its single negative charge.

Question 3: Write the molecular formula for the oxide and sulphide of following elements.
(a) Sodium (b) Calcium (c) Hydrogen

Answer:
(a) Sodium oxide Na₂O
Sodium sulphide Na₂S
(b) Calcium oxide CaO
Calcium sulphide CaS
(c) Hydrogen oxide H₂O
Hydrogen sulphide H₂S

Oxides and sulphides are formed when elements react with oxygen and sulphur respectively. The formulas change based on the valency of the metal to ensure the overall compound is neutral.
Teacher's Tip: Oxygen and Sulphur both belong to the same group and usually have a valency of 2.
Exam Tip: When both elements have the same valency, like Calcium (2) and Oxygen (2), the ratio is 1:1, resulting in CaO.

Question 4: Write the molecular formulae for the following compounds and name the elements present.
(a) Baking soda (b) Common salt (c) Sulphuric acid (d) Nitric acid

Answer:
(a) Baking soda — NaHCO₃
Elements present in Baking soda are sodium, hydrogen, oxygen and carbon.
(b) Common salt — NaCl
Element present are: Sodium and chlorine.
(c) Sulphuric acid — H₂SO₄
Element present are: Hydrogen, sulphur and oxygen.
(d) Nitric acid — HNO₃
Elements present are: Hydrogen, nitrogen and oxygen.

Household items like baking soda and salt have specific chemical names and identities. Identifying the elements within them helps us understand their properties and how they might react in the kitchen or lab.
Teacher's Tip: "Common salt" is the everyday name for the compound Sodium Chloride.
Exam Tip: If a question asks for "elements present," list every distinct type of atom found in the formula.

Question 5: The valency of aluminium is 3. Write the valency of other radicals present in the following compounds.
(a) Aluminium chloride
(b) Aluminium oxide
(c) Aluminium nitride
(d) Aluminium sulphate

Answer:
(a) Aluminium chloride — (AlCl₃) here valency of Al is 3.
Other radical – Chloride (Cl^–)
Valency of chloride = 1
(b) Aluminium oxide — (Al₂O₃)
Here valency of Al is 3
Other radical present = oxide (O^2-)
Valency of O^2- = 2
(c) Aluminium nitride — (Al N)
Here valency of aluminium = 3
Another radical = Nitride (N³-)
Valency of nitride (N^3-) = 3
(d) Aluminium sulphate — Al₂(SO₄)₃
Here valency of aluminium is 3
Another radical = Sulphate (SO₄^2-)
Valency of (SO₄^2-) = 2

By knowing the valency of one part of a compound and looking at the subscripts, you can deduce the valency of the other part. This is a great way to verify that a formula has been written correctly.
Teacher's Tip: If there is no subscript on either part, like in AlN, then their valencies must be equal.
Exam Tip: Be sure to write the full name of the radical (like "Sulphate") when identifying it in the compound.

Question 6: What is variable valency ? Give two examples of elements showing variable valency.

Answer:
Certain elements exhibit more than one valency, that means they show variable valency.
Ferrous is written as Iron (II) and Ferric is written as Iron (III).
Metal | Radicals | Valency
Iron | Ferrous [Iron (II)] | 2
Iron | Ferric [Iron (III)] | 3
Copper | Cuprous [Copper (I)] | 1
Copper | Cupric [Copper (II)] | 2

Some metals can lose different numbers of electrons depending on the conditions of the reaction. This results in the formation of different compounds with different properties using the same two elements.
Teacher's Tip: Usually, the name ending in "-ous" has the lower valency, and "-ic" has the higher valency.
Exam Tip: Use Roman numerals in brackets, like Iron (II), to specify which valency is being used in a name.

Question 7: (a) What is a chemical equation ?
(b) Why it is necessary to balance a chemical equation ?
(c) What are the limitations of a chemical equation ?

Answer:
(a) Chemical Equation- A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formulae of the substances involved in the reaction.
(b) A chemical equation needs to be balanced so as to make the number of the atoms of the reactants equal to the number of the atoms of the products.
(c)
1. It does not inform about the physical states of the reactants and the product i.e. whether they are solids, liquids and gases.
2. It does not inform about the concentration of reactants and products.
3. It does not inform about the time taken for the completion of the reaction.
4. It does not inform about the rate at which a reaction proceeds.
5. It does not inform about the heat changes during the reaction i.e. whether the heat is given out or absorbed.
6. It does not inform about the conditions such as temperature, pressure, catalyst etc. which affect the reaction.
7. It does not inform about the nature of the reaction i.e. whether it is reversible or irreversible.

A simple chemical equation is a shorthand way to describe a reaction, but it doesn't always tell the whole story. Balancing is vital because matter cannot be created or destroyed, only rearranged.
Teacher's Tip: Think of a chemical equation as a mathematical equation where the total "count" on both sides must be equal.
Exam Tip: For limitations, remember that a standard equation doesn't show "how fast" or "how hot" a reaction gets.

Question 8: What are the ways by which a chemical equation can be made more informative ?

Answer:
A chemical equation can give more informations in the following ways:
1. The physical state of the reactants and products can be indicated by putting (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous state.
2. Evolution or absorption of heat during the reaction can be denoted by adding or subtracting a heat term on the product side.
3. Temperature, pressure and catalyst can be indicated above the arrow (→ or =) separating the reactants and products.
4. Concentration of reactants and products are indicated by adding word (dil) for dilute and (conc) for concentrated before their formulae.
5. By the sign \→ or \rightleftharpoons information about irreversible and reversible reactions can be obtained.

Adding these symbols turns a basic equation into a detailed set of instructions for a chemist. It helps scientists replicate experiments exactly as they were intended.
Teacher's Tip: "aq" stands for "aqueous," which simply means the substance is dissolved in water.
Exam Tip: When asked to make an equation more informative, always include state symbols like (s), (l), and (g).

Question 9: State the law of conservation of mass.

Answer:
Law of conservation of mass: It states that mass can neither be created nor destroyed in a chemical reaction. During any change (physical or chemical), matter is neither created nor destroyed. However it may change from one form to another.

This law is a fundamental rule of the universe that says you always end up with the same amount of matter you started with. It is the reason why every chemical equation must be perfectly balanced.
Teacher's Tip: If you start with 10g of reactants, you must end with 10g of products, even if some is gas you can't see.
Exam Tip: State the law exactly: "Mass can neither be created nor destroyed."

Experimental Verification of Law of Conservation of Mass
Requirements: H-shaped tube called Landolt’s tube, Sodium chloride solution, silver nitrate solution, etc.
Procedure: A specially designed H-shaped tube is taken. Sodium chloride solution is taken in one limb of the tube and silver nitrate solution in the other limb as shown in figure. Both the limbs are now sealed and weighed. Now the tubes is averted so that the solutions can mix up together and react chemically. The reaction takes place and a white precipitate of silver chloride is obtained. AgNO₃ + NaCl \→ AgCl ↓ + NaNO₃. The tube is weighed again. The mass of the tube is found to be exactly the same as the mass obtained before inverting the tube. Thus, this experiment clearly verifies the law of conservation of mass.

The Landolt's tube experiment is a classic way to prove that mass doesn't change during a chemical reaction. Even though a new white solid is formed, the total weight remains constant because no atoms escaped the sealed tube.
Teacher's Tip: The downward arrow ↓ next to AgCl indicates that it is a precipitate (a solid that sinks).
Exam Tip: Be able to name the two solutions used: Silver Nitrate and Sodium Chloride.

Question 10: Differentiate between:
(a) Reactants and products
(b) A balanced and an unbalanced chemical equation

Answer:
(a) Reactants and products
Reactants:
1. The substances that react with one another are called reactants.
2. Reactants are written on the left hand side of equation.
Products:
1. The new substances formed are called products.
2. Products are written on the right hand side of equation.
(b) A balanced and an unbalanced chemical equation
Balanced chemical:
1. A balanced chemical reaction is the one in which the number of atoms of each element on the reactant side is equal to the number of atoms of that element on the product side.
2. Ex- H₂ + Cl₂ → HCl
Unbalanced chemical:
1. Number of elements on reactant side are not equal to the number of elements on product side.
2. Ex- H₂ + Cl₂ → 2HCl

Reactants are the starting materials while products are the end results of a chemical change. A balanced equation accurately reflects the reality that atoms are only rearranged, not lost.
Teacher's Tip: The arrow always points from the reactants toward the products.
Exam Tip: Use a simple example like the hydrogen-chlorine reaction to demonstrate balancing.

Question 11: Balance the following equations:

(a) N₂ + H₂ → NH₃
(b) H₂ + O₂ → H₂O
(c) Na₂O + H₂O → NaOH
(d) CO + O₂ → CO₂
(e) Zn + HCl → ZnCl₂ + H₂

Answer:
(a) N₂ + 3H₂→ 2NH₃
(b) 2H₂ + O₂ → 2H₂O
(c) Na₂O + H₂O → 2NaOH
(d) 2CO + O₂ → 2CO₂
(e) Zn + 2HCl → ZnCl₂ + H₂

Balancing equations requires adjusting coefficients (the large numbers in front) until the atom counts match on both sides. This ensures the equation follows the Law of Conservation of Mass.
Teacher's Tip: Never change the small subscripts when balancing; only change the large coefficients in front.
Exam Tip: Check your final work by counting every single atom on both sides one last time.

Question 12: Write balanced chemical equations for the following word equations:

(a) Iron + Chlorine → Iron (III) chloride
(b) Magnesium + dil sulphuric acid → Magnesium sulphate + water
(c) Magnesium + oxygen → Magnesium oxide
(d) Calcium oxide + water → Calcium hydroxide
(e) Sodium + chlorine → Sodium chloride

Answer:
(a) 2Fe + 3Cl₂ → 2FeCl₃
(b) Mg + H₂SO₄ → MgSO₄ + H₂
(c) 2Mg + O₂ → 2MgO
(d) CaO + H₂O → Ca(OH)₂
(e) 2Na + Cl₂ → 2NaCl

Translating word equations into chemical symbols is a key skill in the "language" of chemistry. These equations show the exact recipes for how these common chemical reactions occur.
Teacher's Tip: Remember that some gases like Chlorine (Cl₂) and Oxygen (O₂) are always diatomic (travel in pairs).
Exam Tip: Be careful with the formula for Calcium hydroxide; the hydroxide radical needs brackets: Ca(OH₂}.

Question 13: What information do you get from the following chemical equation:
Zn(s) + 2HCl (dil) → ZnCl₂ (aq) + H₂(g)

Answer:
This gives zinc chloride and hydrogen. The word equation is:
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen
Formulae for the products are ZnCl₂ and H₂

This equation tells us that solid zinc reacts with dilute acid to form a dissolved salt and hydrogen gas. The symbols in brackets like (s) and (g) provide crucial details about the state of matter.
Teacher's Tip: The number "2" in front of HCl tells us we need two molecules of acid for every one atom of zinc.
Exam Tip: Identifying the "products" means listing what is on the right side of the arrow.

ADDITIONAL QUESTIONS

Question 1: (a) Define chemical reaction.
(b) What is a chemical equation?
(c) Why do we need to balance chemical equations?

Answer:
(a) Chemical reaction: Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.
(b) Chemical equation: A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formula of the substances involved in the reaction.
(c) A chemical equation needs to be balanced so as to make the number of the atoms of the reactants equal to the number of the atoms of the products.

These core concepts connect the physical world to the symbolic world of chemistry. We balance equations not just because it's a rule, but because it represents the physical reality of how atoms behave.
Teacher's Tip: A chemical reaction is like a dance where partners switch, but no dancer leaves the room.
Exam Tip: Use the term "symbolic representation" when defining a chemical equation.

Question 2: State four conditions necessary for a chemical reaction to take place.

Answer:
1. Close contact- For a chemical reaction to take place the reactants should be brought in close contact i.e., they should be mixed.
2. Solution form- Some substances react with each other only when they are mixed in the solution form, e.g., Silver nitrate + Sodium chloride → Silver nitrate + Sodium nitrate (aq.) (aq.) (ppt.) (aq.)
3. Heat- Some reactants need to be heated to undergo a chemical change, e.g., Potassium chlorate x→{heat} Potassium chloride + Oxygen
4. Light- Some reactions take place in the presence of light, e.g., Carbon dioxide + Water x→{chlorophyll} Glucose + Oxygen (under sun light).

For molecules to react, they usually need help from their environment, like energy from heat or being dissolved in water. Without these specific conditions, many substances would just sit next to each other without ever changing.
Teacher's Tip: Think of light-driven reactions as "Photosynthesis," which is how plants make food.
Exam Tip: Be able to give one specific example for each condition, such as "heating" for Potassium chlorate.

Question 3: Write balanced chemical equations for the reactions represented by word equations in the conditions for a chemical reaction.

Answer:

(i) 2Na + H₂O → 2NaOH + H₂
(ii) AgNO₃ + NaCl → AgCl + NaNO₃
(iii) 2KClO₃ → 2KCl + 3O₂
(iv) 6CO2 + 6H₂O → C₆H₁₂O₆ + 6O₂

These equations represent the chemical reality behind the conditions mentioned earlier. For example, equation (iv) is the balanced version of photosynthesis.
Teacher's Tip: C6H12O6 is the complex formula for simple sugar, or glucose.
Exam Tip: Ensure that the number of Oxygen atoms is identical on both sides, especially in the more complex equations.

Question 4: Balance the following equations:

(i) Fe + O₂ → Fe₃O₄
(ii) Fe + H₂O → Fe₃O₄ + H₂
(iii) N₂ + O₂ → NO
(iv) Pb₃O₄ → PbO + O₂

Answer:
(i) 3Fe + 2O₂ → Fe₃O₄
(ii) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
(iii) N₂ + O₂ → 2NO
(iv) 2Pb₃O₄ → 6PbO + O₂

Balancing these equations shows that even complex minerals like Fe3O4 follow strict ratios. The large coefficients ensure that no atoms of iron or oxygen are missing after the reaction.
Teacher's Tip: When you see O₂ and O₄, you know you'll need at least a coefficient of 2 for the O₂.
Exam Tip: If the equation seems hard to balance, try starting with the element that appears in the fewest places.

Question 5: Balance the following equations. Also name the products formed.

Answer:
1. 2NaNO₃ → 2NaNO₂ + O₂ | Product: Sodium nitrite, oxygen
2. 2K + Cl₂ → 2KCl | Product: Potassium chloride
3. 2Ag + S → Ag₂S | Product: Silver sulphide
4. 2Mg + O₂ → 2MgO | Product: Magnesium oxide
5. 2KClO₃ → 2KCl + 3O₂ | Product: Potassium chloride, oxygen
6. 2H₂O₂ → 2H₂O + O₂ | Product: Water, oxygen
7. H₂ + Cl₂ → 2HCl | Product: Hydrogen chloride
8. 2Na + 2H₂O → 2NaOH + H₂ | Product: Sodium hydroxide, Hydrogen
9. 2CO + O₂ → 2CO₂ | Product: Carbon dioxide
10. Zn + 2HCl → ZnCl₂ + H₂ | Product: Zinc chloride, Hydrogen
11. 2K₂O + 2H₂O → 4KOH | Product: Potassium hydroxide
12. CO₂ + C → 2CO | Product: Carbon monoxide
13. N₂ + 3H₂ → 2NH₃ | Product: Ammonia
14. PbCO₃ → PbO + CO₂ | Product: Lead (ii) oxide, Carbon dioxide
15. 2FeCl₂ + Cl₂ → 2FeCl₃ | Product: Ferric chloride

This comprehensive list covers many common chemical reactions used in laboratories. Naming the products correctly requires identifying the metal and the new radical it has bonded with.
Teacher's Tip: Notice how NaNO₃ (Nitrate) turns into NaNO₂ (Nitrite) when it loses an oxygen.
Exam Tip: Be careful with Hydrogen Chloride (HCl) and Hydrogen Gas (H₂); they are very different products!

Question 6: State what you would observe when the following substances are heated in a glass test tube:
(i) Red lead
(ii) Copper (II) carbonate

Answer:
(i) 2Pb₃O₄ → 6PbO + O₂ ↑
The dark red lead as red powder changes to yellow colour. The yellow colour residue on further heating sticks to the tube and give reddish colour to glass. A colourless and odourless gas is evolved.
(ii) CuCO₃ → CuO + CO₂ ↑
The blue-green powder turns black and a colourless gas is evolved which extinguishes a burning flame.

Heating these compounds causes them to decompose into simpler substances with visible color changes. These observations are "chemical fingerprints" that help chemists identify what is happening inside the tube.
Teacher's Tip: Carbon dioxide's special property is that it "puts out fire," which is why it extinguishes the flame.
Exam Tip: When describing observations, always mention both the color change of the solid and the nature of the gas produced.

Question 7: Explain the following reaction with one suitable example for each.
(a) Combination reactions
(b) Decomposition reaction
(c) Displacement reaction
(d) Double decomposition reaction

Answer:
(a) Combination reactions: In this reaction two or more substances combine to form a new substance, e.g. Burning of hydrogen in air.
(b) Decomposition reaction: In this reaction a substance breaks up on heating to form two or more simpler substances, e.g. Electrolysis of water.
(c) Displacement reaction: In this reaction a more reactive element displaces a less reactive element from its compound, e.g. Reaction of iron with copper sulphate.
(d) Double decomposition reaction: In this reaction two compounds in solution state react with each other to form two new substances by exchanging their radicals, e.g. Reaction of sodium hydroxide with dilute hydrochloric acid.

Most chemical reactions can be categorized into one of these four types based on how the atoms are moving. This classification helps scientists predict the outcome of mixing different chemicals together.
Teacher's Tip: Decomposition is the opposite of combination; one builds up, the other breaks down.
Exam Tip: For "Double decomposition," remember that the key feature is the "exchange of radicals."

Question 8: Name the type of chemical reaction shown by the following equations:

(a) CaCO₃ → CaO + CO₂
(b) 2Mg + O₂ → 2MgO
(c) Fe + CuSO₄ → FeSO₄ + Cu
(d) NaOH + HCl → NaCl + H₂O
(e) Fe2O₃ + 2Al → Al₂O₃ + 2Fe

Answer:
(a) CaCO₃ → CaO + CO₂ → Decomposition reaction
(b) 2Mg + O₂ → 2MgO → Combination reaction
(c) Fe + CuSO₄ → FeSO₄ + Cu → Displacement reaction
(d) NaOH + HCl → NaCl + H₂O → Double decomposition reaction
(e) Fe₂O₃ + 2Al → Al₂O₃ + 2Fe → Displacement reaction

Being able to label a reaction type at a glance is a very useful skill for students of chemistry. It requires looking at how many reactants there are and how the parts of the formulas are rearranging.
Teacher's Tip: In reaction (e), Aluminium is "stealing" the oxygen away from Iron because it is more reactive.
Exam Tip: If you see one single reactant breaking into two, it's always a decomposition reaction.

Question 9: Write your observations and name the products when
(a) Zinc reacts with dilute hydrochloric acid.
(b) Iron nails are added to an aqueous solution of copper sulphate.
(c) An aqueous solution of barium chloride is added to dilute sulphuric acid.

Answer:
(a) When zinc pieces are added to hydrochloric acid, zinc displaces hydrogen. As a result, zinc chloride and hydrogen gas are produced.
(b) When iron pieces are added to an aqueous solution of copper sulphate iron being more reactive displaces copper from copper sulphate solution to produce ferrous sulphate.
(c) An aqueous solution of barium chloride when mixed with dilute sulphuric acid produces a white insoluble solid barium sulphate and hydrochloric acid.

These reactions involve Displacement and Double Decomposition respectively. The formation of a gas (hydrogen) or a solid precipitate (barium sulphate) makes these reactions very easy to see in a lab.
Teacher's Tip: In reaction (b), you'll see the blue solution turn light green as the copper is replaced by iron.
Exam Tip: Use the word "insoluble" when describing the formation of a precipitate like barium sulphate.

Question 10: A solution of a substance 'X' is used for white washing.
(a) Name the substance 'X' and write its formula.
(b) Write the reaction of the substance 'X' named in (a) above with water.

Answer:
(a) Calcium oxide CaO
(b) CaO + H₂O → Ca(OH)₂ (quick lime turns to slaked lime)

White washing is a common practical use of chemistry in our daily lives. The reaction of "quick lime" with water releases a lot of heat and creates the white paste used on walls.
Teacher's Tip: Calcium oxide is commonly called "quick lime," and Calcium hydroxide is "slaked lime."
Exam Tip: Make sure you know both the chemical name (Calcium oxide) and its common name for such questions.