Materials: Metals And Non-Metals - Class 8 Science

What are Metals and Non-Metals?

At a fundamental level, materials are broadly classified into metals and non-metals based on their atomic structure and the way they form chemical bonds. Metals are typically elements that readily lose electrons to form positive ions (cations). They tend to be strong, shiny, and good conductors of heat and electricity. Think of gold, silver, iron, and copper. These are all examples of metals that we encounter daily, each with specific applications stemming from their properties.

Non-metals, on the other hand, are elements that tend to gain or share electrons. They are generally poor conductors of heat and electricity, often brittle in solid form, and do not possess the characteristic lustre of metals. Examples include carbon (like in pencil lead), oxygen (the air we breathe), sulphur, and phosphorus. While metals are often associated with strength and conductivity, non-metals play crucial roles in biological processes and industrial applications, like the oxygen for respiration or carbon in organic compounds. Understanding this basic distinction is the first step towards appreciating their diverse applications.

Distinguishing Physical Properties

Metals and non-metals exhibit a wide range of physical properties that help us differentiate them easily. Let's look at some key characteristics:

1. Lustre (Shine): Metals are generally lustrous, meaning they have a shiny surface. Think of polished gold or silver. Non-metals, with a few exceptions like iodine and graphite, usually lack lustre and appear dull.
2. Hardness: Most metals are hard solids (e.g., iron, copper), though some, like sodium and potassium, are soft and can be cut with a knife. Mercury is a liquid metal. Non-metals are typically soft (like sulphur) or exist as gases (oxygen, nitrogen) or liquids (bromine). Carbon, in its diamond form, is the hardest known natural substance.
3. Malleability: This is the property that allows a substance to be beaten into thin sheets without breaking. Most metals are highly malleable (e.g., aluminium foil, gold leaves). Non-metals are generally non-malleable and will break into pieces when hammered; they are brittle.
4. Ductility: This refers to the ability of a material to be drawn into thin wires. Metals are ductile (e.g., copper wires, gold wires). Non-metals are non-ductile.
5. Conductivity (Heat & Electricity): Metals are excellent conductors of heat and electricity. This is why cooking utensils are made of metal and electric wires use copper. Non-metals are generally poor conductors of heat and electricity (insulators). Graphite, an allotrope of carbon, is an exception, being a good conductor of electricity.
6. Sonority: Metals are sonorous; they produce a ringing sound when struck (e.g., school bell). Non-metals are non-sonorous.
7. State at Room Temperature: Most metals are solids at room temperature (except mercury, which is a liquid). Non-metals can be solids (carbon, sulphur), liquids (bromine), or gases (oxygen, nitrogen) at room temperature.

Chemical Properties and Reactions

1. Reaction with Oxygen — When metals react with oxygen, they form metal oxides, which are generally basic in nature. For example, magnesium burns in air to form magnesium oxide, which turns red litmus blue. 2Mg + O₂ → 2MgO. Non-metals react with oxygen to form non-metal oxides, which are usually acidic in nature (e.g., sulphur dioxide turns blue litmus red) or neutral. C + O₂ → CO₂ (acidic), S + O₂ → SO₂ (acidic).
2. Reaction with Water — Some metals, like sodium and potassium, react vigorously with cold water to produce hydrogen gas and metal hydroxides. This reaction is highly exothermic. 2Na + 2H₂O → 2NaOH + H₂ + Heat. Other metals, like iron, react slowly with steam. Non-metals generally do not react with water or steam. However, some non-metals like phosphorus are stored in water to prevent reaction with atmospheric oxygen.
3. Reaction with Acids — Metals usually react with acids to produce hydrogen gas and metal salts. The reactivity varies; more reactive metals like zinc and magnesium react vigorously, while less reactive ones like copper might not react with dilute acids. Zn + 2HCl → ZnCl₂ + H₂. Non-metals generally do not react with dilute acids.
4. Reaction with Bases — Some metals, like aluminium and zinc, react with strong bases to produce hydrogen gas. 2Al + 2NaOH + 6H₂O → 2NaAlO₂ + 3H₂. Non-metals react with bases in specific conditions, but it's not a general property like with metals. For instance, chlorine reacts with sodium hydroxide.
5. Displacement Reactions — A more reactive metal can displace a less reactive metal from its salt solution. For example, if an iron nail is placed in copper sulphate solution, iron displaces copper because iron is more reactive. Fe + CuSO₄ → FeSO₄ + Cu. Similarly, a more reactive non-metal can displace a less reactive non-metal from its salt solution. This concept helps determine the relative reactivity of elements.

Uses of Metals and Non-Metals in Daily Life

Understanding the properties of metals and non-metals allows us to use them effectively in countless ways. Metals are indispensable in modern society. Iron is used in constructing buildings, bridges, and vehicles due to its strength. Copper and aluminium are used in electrical wiring because of their excellent conductivity. Gold and silver are used in jewellery due to their lustre and malleability. Aluminium is also used for packaging (foils) and making aeroplane parts because it is lightweight yet strong.

Non-metals, while less obvious in structural applications, are equally vital. Oxygen, a gas, is essential for respiration and combustion. Nitrogen is used in fertilisers to enhance plant growth. Chlorine is used as a disinfectant in water purification. Carbon, in the form of graphite, is used as a lubricant and in pencil leads, while diamonds are used in cutting tools due to their extreme hardness. Sulphur is used in making sulphuric acid, which is crucial for many industries. Phosphorus is a key component in fireworks and matches. The diverse properties of metals and non-metals make them fundamental to our technological advancements and natural processes.

Exam Tip: Remembering Exceptions and Reactivity Series

When studying metals and non-metals, remember that there are always exceptions to general rules. For example, mercury is a liquid metal, graphite is a non-metal that conducts electricity, and iodine is a lustrous non-metal. Pay close attention to these exceptions as they are often asked in exams.

Also, thoroughly understand the concept of the reactivity series for metals. This series helps predict whether a metal will displace another metal from its salt solution. The more reactive metal will always displace the less reactive one. For non-metals, remember that halogens (Fluorine, Chlorine, Bromine, Iodine) also follow a reactivity trend, where a more reactive halogen can displace a less reactive one from its salt solution (e.g., chlorine displaces bromine from potassium bromide solution).

Practice Questions with Solutions

• Q: Give reasons for the following: a) Aluminium foils are used to wrap food items. b) Immersion rods for heating liquids are made up of metallic substances. c) Copper cannot displace zinc from its salt solution. d) Sodium and Potassium are stored in kerosene.
• A: Step 1: Analyze each statement based on the physical and chemical properties of metals and non-metals. Step 2: For (a), consider malleability and non-reactivity. Aluminium is highly malleable and can be beaten into thin foils. It also resists corrosion (formation of a protective oxide layer), making it safe for food contact. Step 3: For (b), consider thermal conductivity. Metallic substances are excellent conductors of heat, allowing them to quickly transfer heat to the liquid being heated. Step 4: For (c), consider the reactivity series. Copper is less reactive than zinc. A less reactive metal cannot displace a more reactive metal from its salt solution. Step 5: For (d), consider reactivity with air and water. Sodium and potassium are highly reactive metals. They react vigorously with oxygen in the air and with water, producing heat and hydrogen gas. Storing them in kerosene prevents contact with air and moisture. Final answer: a) Aluminium is highly malleable and corrosion-resistant, making it suitable for food packaging. b) Metals are good conductors of heat, efficiently transferring heat to liquids. c) Copper is less reactive than zinc; thus, it cannot displace zinc from its salt solution. d) Sodium and potassium are highly reactive metals that react violently with air and water. Kerosene prevents their contact with air and moisture.
• Q: State two physical properties on the basis of which metals can be distinguished from non-metals. Give an example of a metal and a non-metal for each property.
• A: Step 1: Identify physical properties that show clear differences between metals and non-metals. Step 2: Property 1: Lustre. Metals are lustrous (e.g., Gold). Non-metals are generally non-lustrous/dull (e.g., Sulphur). Step 3: Property 2: Malleability. Metals are malleable (e.g., Iron can be hammered into sheets). Non-metals are non-malleable or brittle (e.g., Carbon in solid form breaks when hammered). Step 4: Other possible properties include: Conductivity (Metals - Copper, Non-metals - Wood), Ductility (Metals - Silver, Non-metals - Phosphorus), Sonority (Metals - Aluminium, Non-metals - Coal). Final answer: 1. Lustre: Metals generally have a shiny surface (e.g., Gold). Non-metals are dull (e.g., Sulphur). 2. Malleability: Metals can be beaten into thin sheets (e.g., Aluminium). Non-metals are brittle and break when hammered (e.g., Sulphur).
• Q: What happens when dilute sulphuric acid is poured on a copper plate? Write the chemical equation if a reaction occurs.
• A: Step 1: Recall the reactivity of copper with acids. Step 2: Copper is a relatively less reactive metal compared to hydrogen. It does not react with dilute non-oxidising acids like dilute sulphuric acid to produce hydrogen gas. Step 3: Conclude that no reaction will occur under normal conditions. Final answer: No reaction occurs when dilute sulphuric acid is poured on a copper plate because copper is less reactive than hydrogen and cannot displace hydrogen from dilute sulphuric acid.

Frequently Asked Questions