Case Based Questions: Nature of Matter: Elements, Compounds, and Mixtures | Science Curiosity Class 8 - New NCERT PDF Download

Q1: Read the source below and answer the questions that follow:

During recess, a group of students plays near a school building. They kick a football, read books, and sip water from bottles while chatting under trees. One student wonders: which things here are made of matter, and which aren't? They also discuss whether a new substance could absorb pollution like carbon dioxide from the air.

(a) From the scene, classify three entities as matter and one as not matter—explain why. (1 Mark)
(b) How might discovering a compound that absorbs carbon dioxide help the environment near the playground? (1 Mark)
(c) Suggest an exciting experiment to test if playground dust is matter and part of the air. ​(1 Mark)​​​

Ans:
(a) Matter: football (solid, has mass), water (liquid, takes space), book (solid, weighs something); 
No matter: sunlight (energy, no mass).
(b) It could reduce air pollution, making breathing easier and preventing health issues like allergies from car exhaust nearby.
(c) Shake dust into a glass of water—if it settles visibly (non-uniform mixture), it proves dust is matter suspended in air.

Q2: Read the source below and answer the questions that follow:

In the home science class, students prepared flattened rice, peanuts, onions, and spices to make poha. They noticed that peanuts and onions stayed separate and crunchy, while sugar dissolved in water to form a uniform solution. Everyone enjoyed the mixture, and they discussed how food items can be classified as mixtures.

(a) What type of mixture is poha, and why is it non-uniform? (1 Mark)
(b) Give two kitchen examples of uniform mixtures like sugar water. (1 Mark)
(c) How can you separate poha components to explore pure substances excitingly? ​(1 Mark)​​​

Ans:
(a) Poha is a non-uniform mixture; components like peanuts are visible and keep their properties without chemical change.
(b) Examples: lemonade (sugar in lemon water) or saltwater (salt dissolved evenly).
(c) Use a sieve for peanuts/onions and evaporation for spices—reveals pure substances like salt that can't be split physically.

Q3: Read the source below and answer the questions that follow:

In a science lab, students blew through a straw into clear lime water. They watched as it slowly turned milky. When they blew using exhaled breath, it changed faster. They cheered, realising the bubbly reaction demonstrated gases present in the air.

(a) What gas turns lime water milky, and what compound forms? (1 Mark)
(b) Why is air a uniform mixture—list two components. (1 Mark)
(c) Design a fun extension: how to show exhaled air has more of this gas than playground air? ​(1 Mark)​​​

Ans:
(a) Carbon dioxide forms calcium carbonate (milky solid) with calcium hydroxide and water.
(b) Air is uniform—gases like nitrogen (78%) and oxygen mix evenly, invisible separately.
(c) Blow into lime water (turns milky quickly) vs. leave open to air (slower)—shows breathing adds more CO₂ from body processes.

Q4: Read the source below and answer the questions that follow:

At a picnic, Karan opened a bottle of soda water and noticed the fizz rising rapidly. He tasted the tangy drink and wondered whether the fizz was due to a single substance or a mixture. He observed as bubbles formed and thought about how substances mix.

(a) Classify soda water as a mixture type, naming its main components. (1 Mark)
(b) Why is it uniform, and how does it differ from oil and water? (1 Mark)
(c) Predict what happens if you heat soda water—explain using mixture properties. (1 Mark)​​​

Ans:
(a) It's a gas-liquid uniform mixture of carbon dioxide gas dissolved in water.
(b) Components (gas and liquid) are evenly distributed and invisible separately, unlike oil-water (non-uniform, visible layers).
(c) Gas bubbles out faster on heating, as mixtures don't chemically react; components separate physically.

Q5: Read the source below and answer the questions that follow:

In a science class, students passed electricity through water containing a drop of acid. Gas bubbles appeared in two test tubes: one made a ‘pop’ with a candle, while the other made the flame brighter. They were curious about which gases were produced and what it revealed about water.

(a) Name the two gases produced and classify them as elements or compounds. (1 Mark)
(b) Why does this show water is a compound, not a mixture? ​(1 Mark)​​​​
(c) If you did this without acid, what might happen, and why add it? (1 Mark)

Ans:
(a) Hydrogen (pop sound) and oxygen (brighter flame); both are elements.
(b) Water breaks into elements chemically in a fixed ratio (2:1 H to O), forming a new substance unlike separable mixtures.
(c) Fewer bubbles (pure water conducts poorly); acid helps electricity flow by making it conductive.

Q6: Read the source below and answer the questions that follow:

Sneha was curious during a science experiment. She heated sugar in a boiling tube and noticed it slowly changed colour—first brown, then black. Tiny droplets formed on the glass wall while a black solid remained at the bottom. When she tested this black substance, it burned just like coal.

(a) What elements make up sugar, and why is it a compound? (1 Mark)
(b) Compare this to heating sand: why doesn't sand change similarly? (1 Mark)
(c) How does this show compounds have different properties from their elements? (1 Mark)​​​

Ans:
(a) Carbon, hydrogen, oxygen; it's a compound as they combine chemically in fixed ratios (e.g., C₆H₁₂O₆).
(b) Sand (mixture of silica) doesn't decompose chemically; it just heats physically.
(c) Sugar (sweet, soluble) breaks into carbon (black, burns) and water (liquid); new properties emerge from bonding.

Q7: Read the source below and answer the questions that follow:

The teacher brought two samples to class. Sample A was a simple mixture of iron filings and yellow sulfur powder. But after heating, Sample B turned into a hard black mass. When a magnet was brought close, Sample A responded, but Sample B did not. On adding acid to Sample B, a foul smell of rotten eggs came out.

(a) What is Sample B, and why is it a compound? (1 Mark)
(b) Write the word equation for Sample B reacting with acid. (1 Mark)
(c) Why can't you separate iron from Sample B physically, unlike Sample A? ​(1 Mark)​​​

Ans:
(a) Iron sulfide; elements chemically combine in a fixed ratio, losing original properties.
(b) Iron sulfide + dilute hydrochloric acid → iron chloride + hydrogen sulfide gas.
(c) In compounds, atoms bond tightly; physical methods like magnets fail, unlike loose mixture components.

Q8: Read the source below and answer the questions that follow:

At home, Ravi observed everyday metallic objects. His spoons were made of stainless steel and never rusted. His family’s brass lamp shone golden, and bronze statues looked strong and long-lasting. Ravi learned these were all special combinations of metals, made by heating and mixing, which gave them strength and new properties.

(a) Classify these as mixtures, and name their main components. (1 Mark)
(b) Why are alloys uniform mixtures, and how do they differ from poha? (1 Mark)
(c) Design a simple test to show stainless steel isn't a pure element. ​(1 Mark)​​​

Ans:
(a) Solid-solid uniform mixtures; stainless steel (iron + chromium + nickel), brass (copper + zinc), bronze (copper + tin).
(b) Metals mix evenly at the atomic level, invisible separately; poha is non-uniform with visible chunks.
(c) Heat or add acid: pure iron rusts or dissolves, but alloy resists due to mixed properties.

Q9: Read the source below and answer the questions that follow:

During a school hike, Priya spotted a glittering rock and brought it to class. The teacher showed that it could scratch glass—it was quartz, made of silica. He explained that rocks are actually mixtures of different minerals, such as quartz and calcite. These minerals are important in making things like cement and buildings.

(a) Is quartz an element, compound, or mixture? Justify. (1 Mark)
(b) How are most minerals different from native minerals like gold? (1 Mark)
(c) If you grind the rock, what mixture type results, and why study it? ​(1 Mark)​​​

Ans:
(a) Compound (silicon + oxygen in fixed ratio); can't separate physically.
(b) Most are compounds (e.g., calcite: calcium + carbon + oxygen); native elements are pure elements like gold.
(c) Non-uniform mixture (visible grains); studying separates pure minerals for uses like building.

Q10: Read the source below and answer the questions that follow:

The teacher asked the class to imagine a strange world where water wasn’t a compound but just a mixture of hydrogen and oxygen gases. Students pictured water bubbling like soda, hydrogen catching fire, and oxygen making flames spread everywhere. They realised in such a world, water would be dangerous, not life-saving.

(a) Why is real water a compound, and what fixed ratio proves it? (1 Mark)
(b) Predict fire risk if water were a mixture—use properties of H and O. (1 mark)
(c) How does this change help solve real problems, like fire-fighting? ​(1 Mark)​​​

Ans:
(a) Hydrogen and oxygen chemically bond inseparably in a 2:1 atom ratio, forming new properties.
(b) High risk—hydrogen explodes, oxygen fuels flames; no extinguishing effect.
(c) As a compound, water's non-flammable nature safely puts out fires, unlike separable explosive gases.