Selina Concise Solutions for ICSE Class 7 Chemistry Chapter 5 Language of Chemistry

Points to Remember :

1. A chemical reaction involves the transformation of original substance into an altogether new substance(s).
This means that the starting materials lose their original properties to create a completely different product. For example, when paper burns, it turns into ash and smoke, which cannot be turned back into paper.
Teacher's Tip: If you can't get the original thing back, it is usually a chemical reaction.
Exam Tip: Always mention that "new substances" are formed when defining a chemical reaction for full marks.

2. 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.
A chemical equation serves as a shorthand way for scientists to communicate what happens during a reaction without writing long sentences. It uses universal symbols like H for Hydrogen and O for Oxygen so everyone can understand it.
Teacher's Tip: Think of a chemical equation as a recipe written in a secret code.
Exam Tip: Ensure you know the correct symbols for common elements to write accurate equations.

3. Certain necessary conditions for a chemical reaction to happen are close contact, solution form, heat, light and catalyst.
Reactants often need a little "push" or a specific environment to start interacting with each other. For instance, some substances only react when they are dissolved in water or exposed to sunlight.
Teacher's Tip: Remember the word "CLASH" - Contact, Light, Aqueous (solution), Sun (light), and Heat.
Exam Tip: If a question asks for conditions, listing any four of these will usually secure all points.

4. 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.
These are the visible clues that tell us a chemical change is taking place right before our eyes. We might see bubbles forming or feel a test tube getting hot during the process.
Teacher's Tip: Use your five senses to spot these clues, like seeing a color change or smelling a new odor.
Exam Tip: Use specific terms like "effervescence" for gas bubbles and "exothermic" for heat release to impress the examiner.

5. A complete chemical equation symbolically represents the reactants, products and their physical states.
Adding state symbols like (s) for solid or (g) for gas provides a clearer picture of the reaction's nature. It tells us exactly what form the substances are in at the start and end.
Teacher's Tip: Think of state symbols as "labels" that tell you if something is a solid, liquid, or gas.
Exam Tip: Always check if the question asks for state symbols before finalizing your equation.

6. 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.
Reactants are the starting ingredients, while products are the final result of the chemical "cooking." An arrow always points from the reactants to the products to show the direction of change.
Teacher's Tip: Remember: Reactants are on the "Left" (Starting) and Products are on the "Right" (Result).
Exam Tip: Never use an equal sign (=) between reactants and products; always use an arrow (→).

7. A chemical equation needs to be balanced to make it follow the law of the conservation of mass.
Balancing ensures that no atoms are lost or magically created during the reaction process. We must have the same number of atoms for each element on both sides of the arrow.
Teacher's Tip: Balancing is like a see-saw; both sides must weigh exactly the same.
Exam Tip: Only change the big numbers (coefficients) in front of formulas, never the small numbers (subscripts).

8. 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.
This scientific law tells us that the total weight of the reactants will always equal the total weight of the products. Even if a substance changes into a gas, its mass is still there in the atmosphere.
Teacher's Tip: Matter is like LEGO bricks; you can rebuild them into something else, but you still have the same number of bricks.
Exam Tip: Memorize the phrase "neither created nor destroyed" exactly as it is the key part of the law.

9. A chemical equation gives both qualitative and quantitative information about the reactants and products.
Qualitative information tells us "what" substances are involved, like water or oxygen. Quantitative information tells us "how much" of each substance is used or produced.
Teacher's Tip: Qualitative = Quality (what kind); Quantitative = Quantity (how many).
Exam Tip: Mention both "names" and "amounts" when explaining the information provided by an equation.

EXERCISE

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 definitions explain how we describe the magical changes in chemistry using a logical system. Balancing is the most important part because it respects the rules of nature that atoms cannot disappear.
Teacher's Tip: A balanced equation is like a perfect math sum where both sides are equal.
Exam Tip: For part (c), always mention the "Law of Conservation of Mass" to justify why balancing is required.

Question 2: State four conditions necessary for chemical reactions to take place.
Answer:
Conditions necessary for chemical reactions :
1. Close contact
2. Solution form
3. Heat
4. Light
5. Catalyst
For atoms to swap partners and react, they must be able to reach each other or have enough energy to break existing bonds. External factors like heat or light provide the energy needed to kickstart these reactions.
Teacher's Tip: Think of these as "invitations" to a chemical party; without them, the reactants might just sit there.
Exam Tip: When asked for four conditions, make sure to number them clearly to help the teacher grade faster.

Question 3: Differentiate between :
(a) Reactants and products.
(b) Chemical reaction and chemical equation.
(c) A balanced and a skeletal chemical equation.
Answer:
(a) Reactants: The substances that react with one another are called reactants. Reactants are written on the left hand side of equation.
Products: The new substances formed are called products. Products are written on the right hand side of equation.
(b) Chemical reaction: Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.
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) Balanced Equation: A balanced chemical equation is one in which the number of atoms each element on the reactant side is equal to the number of atoms of that element on the product side.
Skeletal Equation: In a skeletal equation the number of atoms on reactant side are not equal to number of atoms of product side.
Distinguishing between these terms helps you understand the difference between the actual event (the reaction) and how we write it down (the equation). A skeletal equation is like a rough draft, while a balanced equation is the final, correct version.
Teacher's Tip: Reactants are the "Input" and Products are the "Output".
Exam Tip: Use keywords like "symbolic representation" for equations and "transformation" for reactions to get full marks.

Question 4: Write word equations for the following skeletal equations:
(a) KClO₃ → KCl + O₂
(b) Zn + HCl → ZnCl₂ + H₂
(c)FeCl₂ + Cl₂ → FeCl₃
(d) CO + O₂ → CO₂
(e) Ca + O₂ → CaO
(f) Na + O₂ → Na₂O
(g) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(h) AgBr → Ag + Br₂
(i) KNO₂ → KNO₂ + O₂
Answer:
(a) 2KClO₃ → 2KCl+ 3O₂
(b) Zn + 2HCl → ZnCl₂ + H₂
(c) 2FeCl₂ + Cl₂ → 2FeCl₃
(d) 2CO + O₂ → 2CO₂
(e) 2Ca + O₂ → 2CaO
(f) 4Na + O₂ → 2Na₂O
(g) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(h) 2AgBr → 2Ag + Br₂
(i) 2KNO₃ → 2KN0₂ + O₂
Balancing these equations involves adjusting the amounts of each substance so that no atoms are lost. For example, in equation (f), we need four sodium atoms to balance the two sodium oxide molecules produced.
Teacher's Tip: Always start balancing elements that appear in only one place on each side of the arrow.
Exam Tip: Double-check your atom count for both sides once you finish balancing to ensure it is perfect.

Question 5: Balance the following chemical equations :
(a) FeS + HCl → FeCl₂ + H₂S
(b) Na₂CO₃ + HCl → NaCI + H₂O + CO₂
(c) H2 + O₂ → H₂O
(d) Na₂0 + H₂0 → NaOH
Answer:
(a) FeS + 2HCl → FeCl₂ + H₂S
(b) Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
(c) 2 H₂+ O₂ → 2 H₂O
(d) Na₂O + H2O → 2NaOH
Balancing is an essential skill where you ensure the number of atoms is consistent throughout the reaction. Notice how in (c), adding a 2 in front of H₂ and H₂O perfectly balances both hydrogen and oxygen.
Teacher's Tip: If you get stuck, try balancing the metals first, then non-metals, then Oxygen and Hydrogen last.
Exam Tip: Never leave an equation skeletal in an exam unless specifically asked; always provide the balanced version.

Question 6: What information do you get from the equation H₂ + Cl₂ → 2HCl ?
Answer:
(a) Hydrogen and chlorine molecules are the reactants.
(b) They are in gaseous form.
(c) The product is hydrogen chloride gas.
(d) Two molecules of hydrogen chloride are formed.
This single equation tells us what chemicals we are using, what they turn into, and how many molecules are involved. It provides both the names of the substances and the specific quantities produced.
Teacher's Tip: Every equation is a "data sheet" that reveals the reactants, products, and their molecular ratios.
Exam Tip: List as many points as possible (reactants, products, state, number of molecules) to get full marks on "information" questions.

Question 7: Write your observations for the following chemical reactions and name the products formed :
(a) When sugar is heated.
(b) When manganese dioxide is added to potassium chlorate and heated.
(c) When dilute acetic acid is poured on baking soda.
(d) When an aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate.
(e) When ammonium chloride is heated with sodium hydroxide.
(f) When water is added to quick lime?
Answer:
(a) Black solid mass (charcoal) is formed along with water vapours.
(b) Manganese dioxide acts as a catalyst for the decomposition of potassium chlorate into potassium chloride and oxygen at a lower temperature.
(c) Sodium acetate, CO₂ and water is formed.
(d) A white insoluble solid precipitate of silver chloride is formed along with Sodium nitrate.
(e) When solid ammonium chloride is heated with sodium hydroxide solution, a gas ammonia is evolved which is recognised by its strong pungent smell. Ammonium Chloride (solid) + Sodium hydroxide → {heated} Sodium chloride (solution) + Water + Ammonia (gas)
(f) When water is added to quick lime, a large amount of heat energy is evolved. Calcium oxide (solid) + Water → Calcium hydroxide + Heat
Observations are the physical signs like color changes, gas smells, or heat release that indicate a reaction. These reactions demonstrate how different energies like heat can break down compounds or create new ones.
Teacher's Tip: Ammonia gas always has a "stinging" smell that you won't forget once you encounter it!
Exam Tip: For observation questions, always mention the color and physical state (like "white precipitate" or "black solid") of the product.

Question 8: Write symbolic representation for the following word equations and balance them :
(a) Calcium carbonate → Calcium oxide + Carbon dioxide
(b) Carbon + Oxygen → Carbon dioxide
(c) Calcium oxide + Water → Calcium hydroxide
(d) Aluminium + Chlorine → Aluminium chloride
(e) Iron + Sulphur → Iron sulphide
Answer:
(a) CaCO₃ → CaO + CO₂
(b) C + O₂ → CO₂
(c) CaO + H₂O → Ca(OH)₂
(d) 2Al + 3Cl₂ → 2AlCl₃
(e) Fe + S → FeS
Translating word equations into symbols requires knowing the valency and formula of each substance. Balancing then ensures the equation reflects the true nature of the chemical interaction.
Teacher's Tip: Carbonate (CO₃) usually splits into Oxide (O) and Carbon dioxide (CO₂) when heated.
Exam Tip: Write the symbols clearly and leave space for the balancing coefficients in front of them.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks:
(a) The substances which undergo chemical change are called reactants.
(b) The substances formed as a result of a chemical reaction are called products.
(c) During a chemical reaction transfer of energy takes place.
(d) The basic conditions necessary for a chemical reaction is close contact.
(e) In some chemical reactions an insoluble precipitate is formed when two solutions are mixed.
Answer: [Filled in bold above]
These terms are the fundamental vocabulary of chemistry used to describe how matter interacts. Understanding what each term means allows you to visualize the experiments you perform in the lab.
Teacher's Tip: Think of a "precipitate" as a solid that "falls out" of a liquid mixture.
Exam Tip: Be careful with the spelling of technical terms like "precipitate" or "reactants."

2. Write ‘true’ or ‘false’ for the following statements :
(a) No new substance is formed during a chemical reaction : True
(b) Hydrogen sulphide has rotten egg smell : True
(c) When potassium iodide solution is added to lead acetate solution a red precipitate is formed : False
(d) A black residue is formed when sugar is heated : True
(e) When iron and sulphur are heated together a grey mass is formed which is attracted by a magnet : False
(f) A chemical equation gives only qualitative information of a chemical reaction : False
Answer: [Determined in bold above]
These statements test your factual knowledge of common chemical observations and properties. For example, knowing that iron loses its magnetic property when it chemically combines with sulphur is a key concept.
Teacher's Tip: Hydrogen Sulphide (H₂S) is famous for smelling exactly like rotten eggs!
Exam Tip: For false statements, mentally correct them to "True" to help you remember the right facts for later.

MULTIPLE CHOICE QUESTIONS

1. A chemical equation is a statement that describes a chemical change in terms of:
(a) symbols and formulae
(b) energy
(c) number of atoms
(d) colours
Answer: (a)
A chemical equation is a language of its own, using standard shorthand to explain complex changes. This system allows scientists worldwide to share their findings easily.
Teacher's Tip: Symbols are for elements, and formulae are for compounds.
Exam Tip: Choose the most complete answer; while energy and atoms are involved, the *statement* itself is made of symbols.

2. Balancing a chemical equation is based on:
(a) Law of conservation of mass
(b) Mass of reactants and products
(c) Symbols and formulae
(d) None of the above
Answer: (a)
This law is the fundamental rule of the universe that says we cannot lose or create matter. Therefore, every single atom we start with must be present at the end of the reaction.
Teacher's Tip: Balance = Conservation. They always go together.
Exam Tip: This is a very common question; memorize the link between balancing and the Law of Conservation of Mass.

3. Copper carbonate when heated, it turns :
(a) Blue
(b) Green
(c) Black
(d) Yellow
Answer: (c)
When green copper carbonate is heated, it breaks down into black copper oxide and carbon dioxide gas. This color change is a perfect visual example of a chemical reaction.
Teacher's Tip: Remember the transition: Green (Carbonate) → Black (Oxide).
Exam Tip: Color changes are high-scoring points in observational chemistry questions.

ADDITIONAL QUESTIONS

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 definitions provide the framework for studying how different substances interact at the molecular level. Balancing acts as the mathematical check to ensure our chemical "sentences" follow the laws of physics.
Teacher's Tip: Practice writing these definitions out loud to build your confidence.
Exam Tip: Underlining keywords like "symbolic representation" will help catch the teacher's eye.

2: State four conditions necessary for a chemical reaction to take place.
Answer: Close contact - For a chemical reaction to take place the reactants should be brought in close contact i.e., they should be mixed,
Solution form - Some substances react with eachother only when they are mixed in the solution form, e.g., Silver nitrate + Sodium chloride → Silver nitrate + Sodium nitrate (aq.) (aq.) (ppt.) (aq.)
Heat - Some reactants need to be heated to undergo a chemical change, e.g., Potassium chlorate →{heat} Potassium chloride + Oxygen
Light - Some reactions take place in the present of light. Carbon dioxide + Water →[sun light]{chlorophyll} Glucose + Oxygen
These conditions emphasize that molecules must collide and have energy to react. Whether it's dissolving salt in water or using a flame, these environmental factors allow the atoms to reorganize.
Teacher's Tip: Heat is the most common way to speed up a reaction in the lab.
Exam Tip: When giving examples for light, "Photosynthesis" is the best real-world example to use.

3: Write balanced chemical equations for the reactions represented by word equations in the conditions for chemical reaction.
Answer:
1. 2Na + H₂O → 2NaOH + H₂
2. AgNO₃ + NaCl → AgCl + NaNO₃
3. 2KClO₃ → 2KCl + 3O₂
4. 6CO₂ + 6H₂O → C6H₁₂O₆ + 6O₂
These equations show the precise ratios of atoms required for these common reactions to happen correctly. Notice how equation 4 (photosynthesis) involves multiple molecules of carbon dioxide and water to create one molecule of glucose.
Teacher's Tip: The '2' in 2Na means we have two separate atoms of sodium reacting.
Exam Tip: Always write the subscripts small and at the bottom, and coefficients big and in front.

4: Balance the following equations :
1. Fe + O₂ → Fe₃ O₄
2. Fe + H₂O → Fe₃O₄ + H₂
3. N₂ + O₂ → NO
4. Pb₃O₄ → PbO + O₂
Answer:
1. 3Fe + 2O₂ → Fe₃O₄
2. 3 Fe + 4H₂O → Fe₃O₄ + 4H₂
3. N₂ + O₂ → 2NO
4. 2Pb₃O₄ → 6PbO + O₂
Balancing complex formulas like Fe₃O₄ requires looking at the total number of atoms for each element across the entire equation. By adding multipliers, we ensure that the iron and oxygen atoms are perfectly accounted for on both sides.
Teacher's Tip: If you see O₄ on one side, you'll need a total of 4 Oxygen atoms on the other side too.
Exam Tip: Do a final "atom audit" (count each element one last time) before moving to the next question.

5: Balance the following equations. Also name the products formed. The first one is done for you.
 

1. NaNO₃ → NaNO₂ + O₂
Answer: 2NaNO₃ → 2NaNO₂ + O₂ (Sodium nitrite, oxygen)

2. K + Cl₂ → KCl
Answer:  2K + Cl₂ → 2KCl (Potassium chloride)

3. Ag + S → Ag₂ S
Answer: 2Ag + S → Ag₂ S (Silver sulphide)

4. Mg + O₂ → MgO
Answer: 2Mg + O₂ → 2MgO (Magnesium oxide)

5. KClO₃ → KCl + O₂
Answer: 2KClO₃ → 2KCl + 3O₂ (Potassium chloride, oxygen)

6. H₂O₂ → H₂O + O₂
Answer: 2H₂O₂ → 2H₂O + O₂ (Water, oxygen)

7. H₂ + Cl₂ → HCl
Answer: H₂ + Cl₂ → 2HCl (Hydrogen chloride)

8. Na + H₂O → NaOH + H₂
Answer: 2Na + 2H₂O → 2NaOH + H₂ (Sodium hydroxide, Hydrogen)

9. CO + O₂ → CO₂
Answer: 2CO + O₂ → 2CO₂ (Carbon dioxide)

10. Zn + HCl → ZnCl₂ + H₂
Answer: Zn + 2HCl → ZnCl₂ +H₂ (Zinc chloride, Hydrogen)

11. K₂O + H₂O → KOH
Answer: K₂O + H₂O → 2KOH (Potassium hydroxide)

12. CO₂ + C → CO
Answer: CO₂ + C → 2CO (Carbon monoxide)

13. N₂ + H₂ → NH₃
Answer: N₂ + 3H₂ → 2NH₃ (Ammonia)

14. PbCO₃ → PbO + CO₂
Answer: PbCO₃ → PbO + CO₂ (Lead (ii) oxide, Carbon dioxide)

15. FeCl₂ + Cl₂ → FeCl₃
Answer: 2FeCl₂+ Cl₂ → 2FeCl₃ (Ferric chloride)
Naming the products while balancing gives you a full understanding of the chemical transformation taking place. This exercise covers a wide variety of reactions, from simple combinations to complex decompositions.
Teacher's Tip: Any compound with just two elements usually ends in "-ide" (like Chloride or Oxide).
Exam Tip: Don't forget to name *all* products if there is more than one, like in the KClO₃ reaction.

Question 6: State what you would observe when the following substances are heated in a glass test tube: 1. Red lead 2. Copper (II) carbonate Red lead
Answer:
1. 2PbCO₃ → 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.
2. CuCO₃ → CuO + CO₂ ↑ The blue-green powder turns black and a colourless gas is evolved which extinguishes a burning flame.
Heating these minerals causes them to decompose and change their chemical identity, which is signaled by striking color changes. These experiments are classic ways to observe chemical changes in a laboratory setting.
Teacher's Tip: Carbon dioxide gas can be tested because it "blows out" a lit match or candle.
Exam Tip: Always mention that a gas is "evolved" and describe its properties (like "colourless and odourless").

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.
Each type of reaction represents a different way that atoms can shuffle around to form new bonds. Learning these patterns helps scientists predict exactly what will happen when different chemicals are mixed.
Teacher's Tip: Think of Combination as "Marriage" and Decomposition as "Divorce" for molecules.
Exam Tip: Providing the chemical equation alongside your text example will earn you higher marks.

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
By looking at the "structure" of the equation, we can categorize the reaction into one of the four main types. For example, when one element kicks another one out of a compound, it's always a displacement reaction.
Teacher's Tip: One thing becomes two = Decomposition; Two things become one = Combination.
Exam Tip: Identify if any element is "stepping in" or if they are "swapping partners" to distinguish between single and double displacement.

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 show how more "aggressive" or reactive metals can take over bonds from less reactive ones. The formation of a solid precipitate in (c) is a classic way to detect the presence of specific ions in a solution.
Teacher's Tip: The blue color of copper sulphate will fade as iron replaces the copper.
Exam Tip: Always name both products for each reaction to provide a complete answer.

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 + water → slaked lime)
Substance 'X' is commonly known as quicklime and is very reactive with water, releasing a lot of heat. The resulting slaked lime is what is traditionally brushed onto walls for whitewashing.
Teacher's Tip: "X" in chemistry riddles like this is almost always Calcium Oxide if whitewashing is mentioned.
Exam Tip: Be sure to provide both the chemical name (Calcium oxide) and the common name (Quick lime) for extra clarity.