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

Short Answer Type Questions

Q1. Why does lime water become cloudy when air is passed through it?
Ans: Lime water becomes cloudy because carbon dioxide in the air reacts with it to form calcium carbonate, a white insoluble substance. This shows that air contains carbon dioxide.

Q2. How can a magnet be used to separate iron from a mixture with sulfur?
Ans: A magnet attracts iron because it is magnetic, while sulfur is not, allowing the two to be separated easily. This method works due to the different properties of iron and sulfur.

Q3. What happens when sugar is heated strongly in a test tube?
Ans: Sugar turns brown, then black, forming carbon and releasing water droplets. This shows that sugar is made of carbon, hydrogen, and oxygen.

Q4. Why is stainless steel called a uniform mixture?
Ans: Stainless steel is a uniform mixture because it combines iron, nickel, chromium, and carbon in a way that they blend evenly. The components cannot be seen separately with the naked eye.

Q5. How does breaking water into gases show it is not a mixture?
Ans: Breaking water into hydrogen and oxygen gases using electricity shows it is a compound, as these gases are chemically combined in a fixed ratio. The gases have different properties from water.

Q6. Why is a sprout salad considered a non-uniform mixture?
Ans: A sprout salad is a non-uniform mixture because its components, like green gram and tomato, are visible and not evenly mixed. Each ingredient keeps its own properties.

Q7. Why can’t common salt be split into sodium and chlorine using physical methods?
Ans: Common salt is a compound with sodium and chlorine chemically bonded in a fixed ratio. Physical methods, like heating or dissolving, cannot separate them.

Q8. What are native minerals, and how do they differ from other minerals?
Ans: Native minerals are pure elements, like gold or sulfur, found in nature. Unlike most minerals, which are compounds of multiple elements, they contain only one type of atom.

Q9. How do dust particles in the air affect its makeup?
Ans: Dust particles in the air are pollutants, not part of its natural components, and can be seen when light shines through. They vary depending on the place and time.

Q10. Why does bronze have different properties from copper and tin?
Ans: Bronze, a mixture of copper and tin, is stronger and more durable than either metal alone. Its components blend evenly but retain their individual traits.

Q11. How does oxygen in the air differ from nitrogen in its role?
Ans: Oxygen in the air helps with burning and is needed for living things, while nitrogen does not support burning. This makes their roles in air different.

Q12. How is aluminium used in everyday items?
Ans: Aluminium, a strong and lightweight metal, is used to make vehicles and buildings. Its properties make it useful for many practical purposes.

Long Answer Type Questions

Q1. How are mixtures different from compounds, using lemonade and common salt as examples?
Ans: Mixtures, like lemonade, contain substances such as sugar, water, and lemon juice that keep their own properties and can be separated physically, like by filtering or evaporating. Each component in lemonade is visible or can be tasted separately, and they mix without chemical changes. On the other hand, common salt is a compound where sodium and chlorine are chemically bonded in a fixed 1:1 ratio, creating a new substance with unique properties, like being safe to eat, unlike its elements. Compounds cannot be separated by physical methods, only by chemical processes. Lemonade’s variable composition contrasts with common salt’s fixed structure. This shows the clear difference between mixtures and compounds.

Q2. Why is air classified as a mixture and not a compound, and what gases does it contain?
Ans: Air is a mixture because its gases, like nitrogen, oxygen, and carbon dioxide, keep their individual properties and can be separated without chemical changes. Nitrogen, about 78% of air, does not help with burning, while oxygen, around 21%, supports burning and life. Carbon dioxide, a small part, can react with lime water to form a cloudy substance. Air also has water vapour and sometimes dust, which are not fixed in amount. The varying proportions and lack of chemical bonding make air a mixture, not a compound. These gases work together but remain distinct in air.

Q3. What new substance forms when iron and sulfur are heated together, and how can this be tested?
Ans: Heating iron and sulfur creates a black substance called iron sulfide, which is a compound with new properties. In the original mixture, iron is magnetic and sulfur is not, but iron sulfide is not attracted by a magnet, showing a change. Adding dilute hydrochloric acid to the mixture produces a gas with no smell, while the same acid on iron sulfide produces a gas with a rotten egg smell. These tests show that iron and sulfur have chemically combined to form a new substance. The compound cannot be separated back into iron and sulfur using physical methods. This proves a chemical change has occurred.

Q4. What are elements, and how do they differ from compounds in their makeup and properties?
Ans: Elements are substances made of one type of atom, like oxygen or iron, and cannot be broken down into simpler substances. Compounds, like water, are made when elements, such as hydrogen and oxygen, bond chemically in a fixed ratio, creating a new substance with different properties. For example, oxygen supports burning, but water puts out fires. Elements exist as single atoms or molecules, like O₂, while compounds have multiple atom types, like water’s 2:1 hydrogen-to-oxygen ratio. Compounds need chemical processes to separate, unlike elements. This makes elements the building blocks and compounds new creations.

Q5. How do minerals connect to elements and compounds, and what are their everyday uses?
Ans: Minerals are natural solids with a fixed makeup, often compounds like quartz, made of multiple elements, or rarely pure elements like gold. Most minerals, like calcite in cement, are compounds with elements bonded together. Native minerals, like sulfur, are single elements found in nature. These minerals are used to make things like cement for buildings or talcum powder for cosmetics. Their fixed composition helps us extract and use them in daily life. Knowing whether a mineral is an element or compound guides its practical use.

Q6. How does a pure substance differ from a mixture, using baking soda and fruit juice as examples?
Ans: A pure substance, like baking soda, has only one type of particle and a consistent makeup, making it a compound that cannot be separated physically. Baking soda has a fixed chemical structure with specific properties. In contrast, fruit juice is a mixture of water, sugar, and fruit extracts, where each part keeps its own properties and can be separated by physical methods like filtering. Fruit juice’s components can vary, making it non-uniform or uniform depending on mixing. Pure substances have fixed traits, while mixtures depend on their components’ ratios. This difference affects how we use them.

Q7. How can we test for carbon dioxide in exhaled air compared to regular air?
Ans: Exhaled air has more carbon dioxide than regular air because of breathing, and both can be tested by passing them through lime water. In regular air, carbon dioxide makes lime water slightly cloudy by forming calcium carbonate, a white substance. Exhaled air turns lime water cloudy faster due to its higher carbon dioxide content. The test shows carbon dioxide is present in both but more in exhaled air. This method helps compare the amount of carbon dioxide in different air samples. It confirms carbon dioxide’s role in air’s makeup.

Q8. Why are alloys like bronze useful, and how do they differ from pure metals?
Ans: Alloys like bronze, made of copper and tin, are useful because they are stronger and last longer than pure copper or tin, making them good for tools and decorations. In bronze, the metals mix evenly, creating a uniform mixture with improved properties, but each metal keeps its own traits. Pure metals, like copper, are softer and less durable. Alloys have no fixed ratio, unlike compounds, allowing flexibility in their makeup. This makes bronze ideal for practical uses where strength is needed. The blend of metals enhances its everyday value.