Class 7 Science NCERT Notes – Chapter 4: The World of Metals and Non-metals (PDF, MindMap, Q&A, Quizzes)

Chapter 4 (Science): The World of Metals and Non-metals – CBSE Class 7 NCERT Science Detailed Study Notes.

1. Introduction to Metals and Non-metals

The study of materials begins with observing their properties and uses in daily life. An example from a village in Rajasthan shows students, Yashwant and Anandi, learning from an ironsmith named Sudarshan. He makes everyday items like flat pans (tawas), buckets (baltis), tongs (chimtas), and farming tools such as spades (phawras), axes (kulhadis), trowels (khurpis), and rakes (jelees). This practical application demonstrates the fundamental role of metals in society.

2. Physical Properties of Metals

Metals are distinguished by a set of common physical properties, though exceptions exist.

2.1 Hardness and Physical State

• General Rule: Most metals are hard and exist in a solid state at room temperature.
• Exceptions:
  • Sodium and Potassium: These metals are so soft they can be cut with a knife.
  • Mercury: This is the only metal found in a liquid state at room temperature and is commonly seen in thermometers.

2.2 Malleability

• Definition: The property by which materials can be beaten into thin sheets.
• Examples: Beating a piece of copper or an iron nail with a hammer will cause it to flatten. This property is used to create:
  • Thin silver foil placed on some sweets.
  • Aluminium foil used for wrapping food.
• Most Malleable Metals: Gold and silver are the most malleable metals.
• Contrast with Non-metals: Materials like a piece of coal or a lump of sulfur are not malleable. They are brittle, meaning they break into pieces when beaten. Wood is neither malleable nor brittle.

2.3 Ductility

• Definition: The property of materials by which they can be drawn into wires.
• Examples:
  • Electrical fittings use wires made of copper or aluminium.
  • Ornaments like bangles, necklaces, and earrings are often made from metal wires.
  • Stringed musical instruments such as the veena, sitar, violin, and guitar use metal wires.
  • Steel ropes (an alloy of iron and carbon) are used in suspension bridges and cranes due to their ability to support heavy loads.
• Most Ductile Metal: Gold is exceptionally ductile. A single gram of gold can be drawn into a wire approximately 2 kilometers long.
• Contrast with Non-metals: Non-metals like coal and sulfur are not ductile and cannot be drawn into wires.

2.4 Sonority

• Definition: The property of metals that enables them to produce a ringing sound when struck. Metals are described as sonorous.
• Examples:
  • A metal spoon or coin dropped on the floor produces a ringing sound.
  • The sound of a school bell is due to the sonority of the metal it is made from.
  • Ghungroos (ankle bells used in Indian classical dance) produce their characteristic sound because of this property.
• Contrast with Non-metals: Materials like coal and wood produce a dull sound when dropped and are not sonorous.

2.5 Conduction of Heat

• Definition: Conduction is the transfer of heat from one point to another within a material. Materials that transfer heat are called conductors.
• Properties of Metals: Metals are good conductors of heat. An experiment where metal and wooden spoons are placed in hot water shows that the metal spoon becomes hot to the touch, while the wooden spoon does not.
• Applications:
  • Cooking vessels are made of metal to efficiently transfer heat to the food.
  • Handles of cooking vessels are made from poor conductors like wood to prevent heat transfer to the user’s hand.
• Contrast with Non-metals: Wood is a poor conductor of heat.

2.6 Conduction of Electricity

• Definition: Materials that allow electricity to flow through them easily are good conductors of electricity. Those that do not are poor conductors.
• Properties of Metals: Metals are good conductors of electricity. In an experiment using a ‘tester’ circuit, materials like aluminium foil, an iron nail, and a copper wire allow the bulb to glow.
• Applications: Electricians use screwdrivers with handles made of poor conductors and wear rubber gloves and shoes to protect themselves from electric shock.
• Contrast with Non-metals: Materials like sulfur, coal, dry wood, stone, rubber, and nylon rope are poor conductors of electricity and do not allow the bulb in a tester circuit to glow.

3. Chemical Properties and Reactions

3.1 Reaction of Metals with Oxygen

• Formation of Oxides: When metals react with oxygen (e.g., by burning in air), they form metal oxides.
• Experiment: Burning a magnesium ribbon produces a dazzling white flame and leaves behind a white powder, which is magnesium oxide.
• Nature of Metal Oxides: Metal oxides are generally basic in nature. When magnesium oxide powder is dissolved in water, the resulting solution turns red litmus paper blue, indicating it is a base.

3.2 Reaction of Non-metals with Oxygen

• Formation of Oxides: When non-metals react with oxygen, they form non-metal oxides.
• Experiment: Burning sulfur powder in a deflagrating spoon produces a gas. When this gas is dissolved in water, the resulting solution is tested.
• Nature of Non-metal Oxides: Non-metal oxides are acidic in nature. The solution formed from the gas produced by burning sulfur turns blue litmus paper red, indicating it is an acid.

3.3 Corrosion of Metals

• Definition: Corrosion is the gradual deterioration of metal surfaces caused by reactions with air, water, or other substances in the environment.
• Examples:
  • Rusting of Iron: Iron forms a brown, flaky deposit (rust) when exposed to both air and water. This is a significant economic problem, costing enormous amounts annually for repairs and replacement.
  • Copper: A green coating forms on the surface of copper objects.
  • Silver: A black coating forms on the surface of silver objects.
• Prevention of Rusting: Rusting can be prevented by methods such as painting, oiling, greasing, and galvanisation (applying a protective layer of zinc metal on iron).
• Historical Example: The Iron Pillar of Delhi, over 1600 years old, has shown remarkable resistance to rust despite exposure to the elements, showcasing the advanced metallurgy of ancient India.

4. Elements, Non-metals, and Their Importance

4.1 Elements

• Definition: An element is a fundamental substance that cannot be broken down into simpler substances.
• Classification: Metals and non-metals are sub-categories of elements. There are currently 118 known elements, which are the basic building blocks of all matter. Some are naturally occurring, while others are artificially made.
• Distinction: Materials like plastic, glass, wood, rubber, and paper are not classified as metals or non-metals because they are not elements.

4.2 Properties of Non-metals

• Summary: Non-metals are typically soft and have a dull appearance. They are brittle (neither malleable nor ductile), non-sonorous, and poor conductors of heat and electricity.
• Special Case: Phosphorus is a non-metal that is so reactive it catches fire when exposed to atmospheric air and is therefore stored in water.

4.3 Importance of Non-metals in Life

While metals are highly visible, non-metals are essential for life and industry.

• Oxygen: Crucial for breathing and survival.
• Carbon: The fundamental building block of all life forms (proteins, fats, carbohydrates).
• Nitrogen: An essential nutrient for plant growth, used in manufacturing fertilisers.
• Chlorine: Commonly used for water purification.
• Iodine: A solution of iodine is used as an antiseptic on wounds.

5. Metals and Society

Metals and their alloys (mixtures of metals, or a metal and a non-metal) are vital for daily use (utensils, tools) and modern technologies. Special metals are used in critical sectors like atomic energy (zirconium) and aerospace (titanium). In India, recycling metals like iron and aluminium is an important practice for sustainability and waste minimization.

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Quiz and Q&A Section

Part 1: Short-Answer Questions (2-3 sentences each)

1. What is malleability, and which two metals are the most malleable?
2. Explain the property of ductility and provide an example of a highly ductile metal.
3. Why are cooking pans made of metal but their handles often made of wood?
4. Describe the property of sonority and give an everyday example.
5. What happens when a magnesium ribbon is burned in the air? What is the nature of the product?
6. How are non-metals like sulfur different from metals in terms of malleability?
7. Name one metal that is liquid at room temperature and two metals that are soft enough to be cut with a knife.
8. What is corrosion? Give two examples besides the rusting of iron.
9. Why does an electrician wear rubber gloves while working?
10. What are the two essential conditions required for iron to rust?
11. How is the oxide of a non-metal, like sulfur, different from the oxide of a metal in terms of its chemical nature?
12. Define an element and explain how metals and non-metals relate to this definition.
13. List three practical uses of metal wires based on the property of ductility.
14. What is galvanisation and why is it used?
15. Explain why phosphorus is stored in water.
16. Give two examples of how non-metals are essential for life.
17. What is an alloy? Give an example of an alloy mentioned in the text.
18. Describe the Iron Pillar of Delhi and explain what makes it fascinating.
19. What is the difference in the sound produced when a metal coin and a piece of coal are dropped? Which property does this demonstrate?
20. Why do Harappan artifacts contain copper and gold but very little iron?
21. Name three non-metals and one important use for each.
22. How did the use of iron contribute to the progress of civilisation in India?
23. What are the general physical characteristics of non-metals?
24. Explain what happens when the gas from burning sulfur is dissolved in water and tested with litmus paper.
25. List four items an ironsmith might make for daily use or farming.

Part 2: Multiple-Choice Questions (MCQs)

1. Which property of metals allows them to be drawn into wires?
   • a) Malleability b) Sonority c) Ductility d) Conduction
2. The oxide of which of the following would turn red litmus paper blue?
   • a) Sulfur b) Carbon c) Magnesium d) Phosphorus
3. Which of the following is a liquid at room temperature?
   • a) Sodium b) Iron c) Mercury d) Gold
4. The green coating on a copper object is a form of:
   • a) Galvanisation b) Sonority c) Ductility d) Corrosion
5. Which of the following materials is a poor conductor of electricity?
   • a) Copper wire b) Iron nail c) Aluminium foil d) A lump of sulfur
6. One gram of which metal can be drawn into a 2 km long wire?
   • a) Silver b) Gold c) Copper d) Aluminium
7. The property of producing a ringing sound is called:
   • a) Malleability b) Ductility c) Sonority d) Brittleness
8. Oxides of non-metals are generally:
   • a) Acidic in nature b) Basic in nature c) Neutral in nature d) Both acidic and basic
9. Which of the following metals is soft enough to be cut with a knife?
   • a) Iron b) Copper c) Potassium d) Aluminium
10. Rusting of iron requires the presence of:
    • a) Oxygen only b) Water only c) Both oxygen and water d) Oil and grease
11. A substance that breaks into pieces when beaten is described as:
    • a) Ductile b) Malleable c) Sonorous d) Brittle
12. The Iron Pillar of Delhi is famous for its:
    • a) Extreme height b) Gold content c) Resistance to rust d) Use in construction
13. Which non-metal is applied to wounds as an antiseptic?
    • a) Chlorine b) Iodine c) Sulfur d) Carbon
14. Steel, used in suspension bridges, is an alloy primarily of:
    • a) Copper and zinc b) Iron and carbon c) Gold and silver d) Aluminium and magnesium
15. What is the fundamental substance that cannot be broken down into simpler substances?
    • a) An alloy b) A compound c) An element d) A mixture
16. Which non-metal is essential for breathing?
    • a) Nitrogen b) Carbon c) Oxygen d) Hydrogen
17. Galvanisation is a process of coating iron with a layer of:
    • a) Copper b) Zinc c) Paint d) Silver
18. What is the building block of all life forms, and a key component of proteins, fats, and carbohydrates?
    • a) Oxygen b) Nitrogen c) Carbon d) Phosphorus
19. A material that allows heat to pass through it is called a:
    • a) Conductor b) Insulator c) Non-metal d) Brittle substance
20. Which two metals are mentioned as being the most malleable?
    • a) Iron and Copper b) Aluminium and Zinc c) Gold and Silver d) Sodium and Potassium

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Answer Keys

Answer Key for Short-Answer Questions

1. Malleability is the property that allows materials to be beaten into thin sheets. The two most malleable metals mentioned are gold and silver.
2. Ductility is the property that allows materials to be drawn into wires. Gold is an extremely ductile metal, as one gram can be stretched into a 2-kilometer-long wire.
3. Cooking pans are made of metal because metals are good conductors of heat, which allows for efficient cooking. Their handles are often made of wood because wood is a poor conductor of heat, preventing the handle from becoming too hot to touch.
4. Sonority is the property of metals to produce a ringing sound when struck. A common example is the sound of a school bell ringing or a metal coin making a ringing sound when dropped.
5. When a magnesium ribbon is burned, it reacts with oxygen in the air with a dazzling white flame. It changes into a white powder called magnesium oxide, which is basic in nature.
6. Non-metals like sulfur are brittle, meaning they break into pieces when beaten with a hammer. This is the opposite of metals, which are malleable and flatten into sheets.
7. Mercury is a metal that is liquid at room temperature. Sodium and potassium are metals soft enough to be cut with a knife.
8. Corrosion is the gradual deterioration of a metal’s surface due to substances like air and water. Two examples are the formation of a green coating on copper surfaces and a black coating on silver objects.
9. An electrician wears rubber gloves because rubber is a poor conductor of electricity. The gloves prevent electricity from flowing through the electrician’s body, protecting them from electric shock.
10. The two essential conditions for iron to rust are the presence of both air (oxygen) and water. Iron will not rust if one of these is absent.
11. The oxide of a non-metal like sulfur is acidic in nature, turning blue litmus paper red. The oxide of a metal is basic in nature, turning red litmus paper blue.
12. An element is a basic substance that cannot be broken down into simpler substances. Metals and non-metals are sub-categories of elements, representing the fundamental building blocks of matter.
13. Three practical uses of metal wires are in electrical fittings (copper, aluminium), making ornaments (gold, silver), and in stringed musical instruments (steel, etc.).
14. Galvanisation is the process of applying a protective layer of zinc metal onto iron. It is used to prevent the iron from rusting.
15. Phosphorus is a highly reactive non-metal that spontaneously catches fire when exposed to atmospheric air. It is stored in water to prevent this reaction.
16. The non-metal oxygen is essential for breathing and survival. The non-metal carbon is the fundamental building block of all life forms, making up proteins, fats, and carbohydrates.
17. An alloy is a mixture of two or more metals, or a metal and a non-metal. Steel is an example mentioned, which is a mixture of iron (metal) and carbon (non-metal).
18. The Iron Pillar of Delhi, made over 1600 years ago, is fascinating because it has barely any rust despite centuries of exposure to wind and rain. This demonstrates the advanced metallurgical skills of ancient India.
19. A metal coin produces a ringing sound, while a piece of coal produces a dull sound. This demonstrates the property of sonority, which metals possess.
20. The Harappans knew how to use metals like copper and gold. However, the use of iron only gained prominence much later in history, so it is not commonly found in their artifacts.
21. Oxygen is used for breathing, nitrogen is used in fertilisers to help plants grow, and chlorine is used for water purification.
22. The use of iron contributed to the progress of civilisation by enabling the creation of stronger agricultural tools, like ploughs. These superior tools improved farming efficiency.
23. Non-metals are generally soft, dull in appearance, and brittle. They are neither malleable nor ductile, are not sonorous, and are poor conductors of heat and electricity.
24. When the gas from burning sulfur (a non-metal oxide) is dissolved in water, it forms an acidic solution. This solution will turn blue litmus paper red, confirming its acidic nature.
25. An ironsmith might make flat pans (tawas), buckets (baltis), tongs (chimtas), and farming tools like spades (phawras) and axes (kulhadis).

Answer Key for Multiple-Choice Questions (MCQs)

1. c) Ductility
2. c) Magnesium
3. c) Mercury
4. d) Corrosion
5. d) A lump of sulfur
6. b) Gold
7. c) Sonority
8. a) Acidic in nature
9. c) Potassium
10. c) Both oxygen and water
11. d) Brittle
12. c) Resistance to rust
13. b) Iodine
14. b) Iron and carbon
15. c) An element
16. c) Oxygen
17. b) Zinc
18. c) Carbon
19. a) Conductor
20. c) Gold and Silver

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Essay Questions with Model Answers

1. Compare and contrast the physical properties of metals and non-metals using at least four distinct properties. Provide specific examples for each property.

Answer: Metals and non-metals can be distinguished by several key physical properties. Firstly, in terms of their response to physical force, metals are malleable (can be beaten into thin sheets like aluminum foil) and ductile (can be drawn into wires like copper electrical wires). In contrast, non-metals like sulfur and coal are brittle and will shatter if struck. Secondly, metals are sonorous, meaning they produce a ringing sound when struck, like a school bell. Non-metals produce a dull sound. Thirdly, metals are generally good conductors of heat and electricity. This is why cooking pots are made of metal, while non-metals like wood and rubber are poor conductors, making them useful for handles and electrical insulation. Finally, most metals are hard and solid (e.g., iron), with exceptions like liquid mercury and soft sodium, while non-metals are often soft or gaseous and dull in appearance.

2. Describe the chemical nature of the oxides formed by metals and non-metals. Detail the experiments that can be used to demonstrate these properties.

Answer: Metals and non-metals react with oxygen to form oxides with opposite chemical natures. Metal oxides are typically basic. This can be demonstrated by burning a magnesium ribbon, which reacts with oxygen to form a white powder, magnesium oxide. When this powder is dissolved in water, the resulting solution will turn red litmus paper blue, confirming its basic nature. Conversely, non-metal oxides are typically acidic. This is shown by burning sulfur powder in a deflagrating spoon. The gas produced is collected and dissolved in water. This solution will turn blue litmus paper red, demonstrating that it is acidic. These experiments show a fundamental chemical difference: metals form basic oxides, while non-metals form acidic oxides.

3. What is corrosion? Discuss the process of rusting of iron in detail, including the necessary conditions and methods of prevention.

Answer: Corrosion is the gradual deterioration of a metal’s surface caused by its reaction with substances in the environment, such as air and water. The most common example of corrosion is the rusting of iron. For iron to rust, it must be exposed to both air (specifically oxygen) and water simultaneously. An experiment with three nails—one in dry air, one in boiled water sealed with oil, and one in contact with both air and water—demonstrates that only the nail exposed to both will rust. Rusting is a major problem that leads to significant financial costs for replacing damaged structures. It can be prevented by several methods, including painting, oiling, or greasing the iron surface to create a barrier against air and water. Another effective method is galvanisation, which involves applying a protective coating of zinc metal over the iron.

4. Explain why non-metals, despite being less visually prominent than metals, are essential for life and modern society. Provide at least four examples.

Answer: Although metals are widely recognized for their strength and conductivity, non-metals are fundamentally essential to life and modern society. Firstly, oxygen, a non-metal, is indispensable for respiration in almost all living organisms; without it, life as we know it would not exist. Secondly, carbon is the foundational building block of all organic life, forming the key structure of proteins, fats, and carbohydrates necessary for growth and energy. Thirdly, nitrogen is a critical nutrient for plant growth, and its use in fertilisers is vital for modern agriculture and food production. Finally, other non-metals have crucial societal roles; for instance, chlorine is used globally for water purification to make it safe for consumption, and an iodine solution serves as a common antiseptic for treating wounds.

5. Define “element” and explain the classification of materials into metals and non-metals. Why are substances like wood, plastic, and glass not included in this classification?

Answer: An element is a pure substance that cannot be broken down into simpler substances by chemical means. All matter is composed of these basic building blocks, of which 118 are currently known. Metals and non-metals are the two primary sub-categories of elements, differentiated by their distinct physical and chemical properties. For example, metals are typically lustrous, malleable, ductile, and good conductors, while non-metals are dull, brittle, and poor conductors. Substances like wood, plastic, and glass are not classified as metals or non-metals because they are not elements. They are compounds or mixtures, meaning they are made up of two or more different elements chemically bonded together. Therefore, they do not fit the fundamental definition of an element.

6. Discuss the historical significance of iron in the development of civilisation in India, contrasting its use with that of earlier metals like copper and gold.

Answer: The discovery and use of metals have been pivotal in human history. The Harappan civilisation, for example, knew how to use metals like copper and gold to make objects ranging from utensils to jewellery. However, evidence of iron use by Harappans is scarce because its widespread application came much later. The subsequent adoption of iron marked a significant advancement for civilisation in India. Due to its superior strength compared to earlier materials, iron enabled the creation of much more effective agricultural tools, such as iron ploughs. This innovation likely boosted agricultural productivity, supporting larger populations and contributing significantly to societal progress.

7. Describe the property of ductility in detail, providing diverse examples of its application in everyday life and heavy industry.

Answer: Ductility is the physical property of a material, primarily metals, that allows it to be drawn into thin wires without breaking. This property is crucial for numerous applications. In everyday life, the ductility of copper and aluminium makes them ideal for producing electrical wires used in all household and commercial fittings. It is also used in creating fine wires of gold and silver for making jewellery like necklaces and bangles. In music, the strings of instruments like the sitar, guitar, and violin are metal wires whose properties are determined by ductility. In heavy industry, the principle is applied to create steel ropes, which are alloys of iron and carbon. These strong, ductile ropes are essential for constructing suspension bridges and for use in cranes that lift extremely heavy objects.

8. Using the examples from the text, explain the concepts of heat and electrical conductivity. Why is understanding these properties important for safety and practical applications?

Answer: Conductivity refers to a material’s ability to transfer heat or electricity. Metals are good conductors of both. An experiment with a metal spoon and a wooden spoon in hot water demonstrates heat conductivity; the metal spoon gets hot while the wood does not, showing metal is a good conductor. Similarly, an experiment with a simple electrical circuit shows that objects like an iron nail and copper wire allow a bulb to glow, proving they are good electrical conductors, whereas sulfur and wood do not. Understanding these properties is vital for safety and practical use. For safety, electricians use tools with rubber or plastic handles and wear rubber gloves because these are poor conductors (insulators) that prevent electric shock. For practical applications, cooking pots are made of metal to conduct heat effectively, while electrical wires are made of conductive metals to transmit power efficiently.

9. What makes the Iron Pillar of Delhi a “wonder of ancient Indian metallurgy”? Situate this artifact in its historical context and explain its scientific significance.

Answer: The Iron Pillar of Delhi is considered a marvel of ancient Indian metallurgy due to its incredible resistance to corrosion. The pillar was made during the time of Chandragupta II, over 1600 years ago. It stands about 8 meters high and weighs over 6000 kilograms. What is scientifically significant is that despite being exposed to the elements—including wind, rain, and intense weather—for more than a millennium and a half, the iron pillar has barely rusted. This achievement is remarkable because iron is highly susceptible to rusting, a problem that costs modern societies enormous amounts of money to manage. The pillar’s longevity suggests that ancient Indian metallurgists possessed an advanced understanding of iron processing and alloy creation that produced a highly pure, rust-resistant iron, a feat that remains impressive to modern scientists.

10. Imagine you are designing a new electrical device. Based on the text, which materials would you choose for the wiring, and which for the outer casing? Justify your choices based on the properties of metals and non-metals.

Answer: Based on the properties described in the text, for the internal wiring of an electrical device, I would choose a metal like copper or aluminium. These materials are chosen because metals are excellent conductors of electricity, allowing electrical current to flow through them easily and efficiently power the device. Furthermore, they are ductile, meaning they can be drawn into the thin, flexible wires needed for complex circuitry. For the outer casing of the device, I would choose a material that is a poor conductor of electricity, such as plastic or rubber, which are mentioned as non-elements that do not conduct electricity. The primary reason is safety; a non-conductive casing insulates the user from the electrical components inside, preventing the risk of electric shock. This combination of a conductive metal for function and a non-conductive material for safety is fundamental to the design of most electrical appliances.

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Glossary of Key Terms

• Alloys: Mixtures of two or more metals, or a metal and a non-metal. Example: Steel (iron and carbon).
• Brittle: A property of a material that causes it to break or shatter into pieces when beaten or struck. Example: Coal, sulfur.
• Conduction: The transfer of heat from one point to another within a material.
• Conductors (of heat/electricity): Materials that allow heat or electricity to pass through them easily. Metals are good conductors.
• Corrosion: The gradual deterioration of a metal’s surface due to its reaction with substances in the environment like air and water. Examples: Rust on iron, green coating on copper.
• Ductility: The property of a material that allows it to be drawn into thin wires.
• Element: A fundamental substance that cannot be broken down into simpler substances. Metals and non-metals are categories of elements.
• Galvanisation: The process of applying a protective layer of zinc metal onto iron to prevent it from rusting.
• Malleability: The property of a material that allows it to be beaten into thin sheets.
• Metals: A category of elements that are generally hard, lustrous, malleable, ductile, sonorous, and good conductors of heat and electricity. Their oxides are typically basic.
• Non-metals: A category of elements that are generally soft, dull, brittle, non-sonorous, and poor conductors of heat and electricity. Their oxides are typically acidic.
• Poor Conductors (of heat/electricity): Materials that do not allow heat or electricity to pass through them easily. Also known as insulators. Examples: Wood, rubber, sulfur.
• Sonority: The property of metals that enables them to produce a ringing sound when struck.
• Sonorous: A term used to describe materials that possess the property of sonority.