- Reactions of non-metals with oxygen Important Questions - Grade 9 Natural

Table of Contents
1. Section A: Multiple Choice
2. Section B: True or False
3. Section C: Match the Column
4. Section D: Fill in the Blanks
5. Section E: Short Questions
6. Section F: Scenario Question
7. Section G: Long Question
8. Answer Key
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Section A: Multiple Choice

Choose the correct answer for each question. Write only the letter (A-D) of your choice.

1. Which of the following statements best describes chemical change? [2]
   1. A change where the substance remains the same but changes shape or state
   2. A change where new substances with different properties are formed
   3. A change that can always be easily reversed
   4. A change where only the colour of a substance changes
2. Thandi heats a piece of magnesium ribbon in air until it burns with a bright white flame. What type of reaction is this? [2]
   1. Decomposition reaction
   2. Oxidation reaction
   3. Neutralisation reaction
   4. Precipitation reaction
3. Which indicator would turn red in an acidic solution? [2]
   1. Phenolphthalein
   2. Methyl orange
   3. Universal indicator in strong base
   4. Bromothymol blue in neutral solution
4. What is the chemical formula for hydrochloric acid? [2]
   1. H₂SO₄
   2. HCl
   3. HNO₃
   4. CH₃COOH

Section B: True or False

State whether each statement is TRUE or FALSE.

1. During a physical change, no new substances are formed. [1]
2. All acids have a pH value greater than 7. [1]
3. When an acid reacts with a base, the process is called neutralisation and produces salt and water. [1]
4. Rusting of iron is an example of a physical change. [1]

Section C: Match the Column

Match the items in Column A with the correct descriptions in Column B. Write only the letter of the correct match next to the number.

[4]

Section D: Fill in the Blanks

Word Bank: exothermic, endothermic, reactants, products

1. In a chemical reaction, the starting substances are called ________ and the new substances formed are called ________. [2]
2. A reaction that releases heat energy to the surroundings is called an ________ reaction. [1]
3. When heat energy is absorbed from the surroundings during a chemical reaction, the reaction is described as ________. [1]

Section E: Short Questions

1. State two observable signs that indicate a chemical reaction has taken place. [2]
2. Explain the difference between a strong acid and a weak acid. [2]
3. Why is it dangerous to mix acids with certain household cleaning products? [2]
4. Describe what happens during the thermal decomposition of copper carbonate. [3]

Section F: Scenario Question

Sipho is investigating the reaction between vinegar (acetic acid) and baking soda (sodium hydrogen carbonate). He places two tablespoons of baking soda into a beaker and then pours 50 mL of vinegar over it. Immediately, he observes vigorous fizzing and bubbling. The beaker feels cool to the touch. After a few minutes, the fizzing stops.

1. Name the gas that is produced during this reaction. [1]
2. Is this reaction exothermic or endothermic? Give a reason for your answer. [2]
3. Write a word equation for the reaction between acetic acid and sodium hydrogen carbonate. [3]
4. Explain why the fizzing stops after a few minutes. [2]

Section G: Long Question

Lerato wants to investigate the effect of concentration on the rate of reaction between hydrochloric acid and marble chips (calcium carbonate). She sets up three identical conical flasks, each containing 5 g of marble chips. She adds 50 mL of hydrochloric acid to each flask, but uses different concentrations: Flask A contains dilute acid, Flask B contains moderately concentrated acid, and Flask C contains highly concentrated acid. She measures the time taken for the fizzing to stop in each flask.

1. State the independent variable, dependent variable, and two controlled variables in this investigation. [4]
2. Predict which flask (A, B, or C) will show the fastest reaction rate. Explain your prediction using the collision theory. [3]
3. Write a balanced chemical equation for the reaction between hydrochloric acid (HCl) and calcium carbonate (CaCO₃). [3]
4. Lerato notices that the flask becomes slightly warm during the reaction. What does this observation tell you about the energy change in this reaction? Explain fully. [3]

Grand Total: [50]

Answer Key

Well done for completing the worksheet! Before you check your answers, make sure you have attempted every question to the best of your ability. Use this answer key to mark your work carefully, and take time to understand any mistakes. Remember, making mistakes is an important part of learning science!

Section A - Multiple Choice

Section A - Question 1
Answer: B - A change where new substances with different properties are formed
A chemical change always produces one or more new substances that have different properties from the original substances. For example, when wood burns, it forms ash, smoke, and gases that are completely different from wood.

Section A - Question 2
Answer: B - Oxidation reaction
When magnesium burns in air, it combines with oxygen to form magnesium oxide. Any reaction where a substance combines with oxygen is called an oxidation reaction. The bright white flame and white powder formed are evidence of this chemical change.

Section A - Question 3
Answer: B - Methyl orange
Methyl orange is an indicator that turns red in acidic solutions and yellow in basic solutions. This makes it very useful for detecting acids in the laboratory.

Section A - Question 4
Answer: B - HCl
Hydrochloric acid is a strong acid with the chemical formula HCl. It is made of hydrogen (H) and chlorine (Cl) atoms. The other options are sulfuric acid (H₂SO₄), nitric acid (HNO₃), and acetic acid (CH₃COOH).

Section B - True or False

Section B - Question 1
TRUE
This statement is correct. During physical changes like melting, freezing, boiling, or dissolving, the chemical identity of the substance stays the same - only its appearance, state, or form changes.

Section B - Question 2
FALSE
Corrected statement: All acids have a pH value less than 7.
Acids have pH values below 7, with stronger acids having lower pH values (closer to 0). Substances with pH greater than 7 are bases, not acids. A pH of exactly 7 indicates a neutral substance like pure water.

Section B - Question 3
TRUE
This statement is correct. Neutralisation is the chemical reaction between an acid and a base that produces a salt and water. For example, hydrochloric acid reacting with sodium hydroxide produces sodium chloride (salt) and water.

Section B - Question 4
FALSE
Corrected statement: Rusting of iron is an example of a chemical change.
Rusting is a chemical change because iron reacts with oxygen and water in the air to form a completely new substance called iron oxide (rust), which has different properties from iron. It cannot be easily reversed.

Section C - Match the Column

1 → C (Litmus paper → An indicator used to test for acids and bases)
Litmus paper is a common indicator that turns red in acids and blue in bases, making it useful for identifying whether a solution is acidic or basic.

2 → A (Acid → A substance with pH less than 7)
By definition, acids are substances that have a pH value below 7 and can donate hydrogen ions in solution.

3 → B (Combustion → A reaction where oxygen combines with a substance)
Combustion, or burning, is a chemical reaction in which a substance rapidly combines with oxygen, usually producing heat and light.

4 → D (Corrosion → The gradual destruction of metals by chemical reactions with the environment)
Corrosion is the slow chemical breakdown of metals when they react with substances in their environment, such as oxygen and moisture. Rusting is a common example of corrosion.

Section D - Fill in the Blanks

Section D - Question 1
reactants and products
In any chemical reaction, the reactants are the substances you start with, and the products are the new substances formed. For example, in the burning of magnesium, magnesium and oxygen are reactants, while magnesium oxide is the product.

Section D - Question 2
exothermic
An exothermic reaction releases heat energy into the surroundings, making the container or area feel warm or hot. Combustion and neutralisation are common examples of exothermic reactions.

Section D - Question 3
endothermic
An endothermic reaction absorbs heat energy from the surroundings, often making the container feel cold. Thermal decomposition and dissolving certain salts in water can be endothermic.

Section E - Short Questions

Section E - Question 1
Model Answer:
Two observable signs that a chemical reaction has occurred include: formation of a gas (bubbles or fizzing), colour change, formation of a precipitate (solid forming in a liquid), release or absorption of heat (temperature change), or the production of light. Any two of these are acceptable.
Mark breakdown: 1 mark for each correct observable sign [1 + 1 = 2]

Section E - Question 2
Model Answer:
A strong acid is one that ionises completely in water, releasing all its hydrogen ions, such as hydrochloric acid or sulfuric acid. A weak acid only partially ionises in water, releasing only some of its hydrogen ions, such as acetic acid (vinegar) or citric acid.
Mark breakdown: 1 mark for explaining strong acid ionises completely; 1 mark for explaining weak acid ionises partially [1 + 1 = 2]

Section E - Question 3
Model Answer:
It is dangerous to mix acids with certain household cleaning products because they can react to produce toxic gases (such as chlorine gas), cause violent exothermic reactions that can splatter or explode, or create harmful chemical burns. Such mixtures can cause serious injury or poisoning.
Mark breakdown: 1 mark for mentioning production of toxic/harmful gases or violent reaction; 1 mark for explaining danger to health or safety [1 + 1 = 2]

Section E - Question 4
Model Answer:
When copper carbonate is heated, it undergoes thermal decomposition. This means it breaks down into simpler substances: copper oxide (a black solid) and carbon dioxide gas. You can observe a colour change from green to black as the copper carbonate decomposes. This is an example of an endothermic reaction because heat energy must be supplied continuously for the decomposition to occur.
Mark breakdown: 1 mark for stating decomposition occurs; 1 mark for naming products (copper oxide and carbon dioxide); 1 mark for describing observable change or energy requirement [1 + 1 + 1 = 3]

Section F - Scenario Question

Section F - Question 1
Answer: Carbon dioxide (CO₂)
The gas produced during the reaction between vinegar and baking soda is carbon dioxide, which causes the fizzing and bubbling that Sipho observes.

Section F - Question 2
Answer: The reaction is endothermic.
The reason is that the beaker feels cool to the touch, which indicates that heat energy is being absorbed from the surroundings during the reaction. In an exothermic reaction, the beaker would feel warm because heat is released.
Mark breakdown: 1 mark for stating endothermic; 1 mark for correct reasoning based on temperature observation [1 + 1 = 2]

Section F - Question 3
Answer:
acetic acid + sodium hydrogen carbonate → sodium acetate + water + carbon dioxide
or
vinegar + baking soda → sodium acetate + water + carbon dioxide
Mark breakdown: 1 mark for correct reactants; 1 mark for correct products including all three (sodium acetate, water, carbon dioxide); 1 mark for correct format of word equation [1 + 1 + 1 = 3]

Section F - Question 4
Answer:
The fizzing stops after a few minutes because one or both of the reactants have been completely used up. Once all the baking soda or all the vinegar has reacted, there are no more reactants available to produce carbon dioxide gas, so the reaction stops.
Mark breakdown: 1 mark for stating reactant(s) used up; 1 mark for explaining that no more gas can be produced [1 + 1 = 2]

Section G - Long Question

Section G - Question 1
Answer:

• Independent variable: Concentration of hydrochloric acid [1]
• Dependent variable: Time taken for fizzing to stop (or rate of reaction) [1]
• Controlled variables (any two): Mass of marble chips (5 g), volume of acid (50 mL), size of marble chips, temperature of the acid, type of flask used [1 + 1]

Section G - Question 2
Answer:
Flask C (highly concentrated acid) will show the fastest reaction rate. [1]
According to collision theory, a higher concentration of acid means there are more acid particles per unit volume. This increases the frequency of collisions between acid particles and marble chips, leading to more successful collisions per unit time and therefore a faster rate of reaction. [2]
Total: [3]

Section G - Question 3
Answer:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
or in words:
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
Mark breakdown: 1 mark for correct reactants; 1 mark for correct products; 1 mark for correct balancing [1 + 1 + 1 = 3]

Section G - Question 4
Answer:
The observation that the flask becomes slightly warm tells us that the reaction is exothermic. [1]
This means that heat energy is released during the chemical reaction between hydrochloric acid and calcium carbonate. [1]
The energy released comes from the breaking and forming of chemical bonds during the reaction, where more energy is released when new bonds form in the products than is needed to break bonds in the reactants. [1]
Total: [3]

Mark Allocation Summary