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CHEMISTRY TEXTBOOK FOR CLASS VIII
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Chapter 1 METALS
Unit outcomes: After learning this chapter you should be able to Understand about Metals, properties of metals and reactivity series Apply the knowledge of different properties of metals in daily life situations. Construct knowledge about reactions based on reactivity series.
Contents 1.1 Metals 1.2 Properties of metals 1.3 Reactivity series of metals
1.4 Reactions of metals Summary Exercise Extended activities
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1.1 METALS
Fig 1.1
What are Metals? Metals are substances that form naturally below the surface of the Earth. Most metals are lustrous or shiny. Metals are inorganic, which means they are made of substances that were never alive. Metals are natural compounds of earth’s crust, in which they are generally found in the form of metal ores, associated both with each other and with many other elements. They are also naturally present in the rocks washed by surface water and groundwater and in atmospheric dust.
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Examples of Metals
Fig 1.2
Metal is very strong and durable and therefore is used to make many things. These are used for making automobiles, satellites, cooking utensils, etc. Most metals are hard but some are not. Sodium and potassium are such metals that can be cut by knife whereas mercury is a liquid metal at room temperature. Iron is solid in nature.
1.2 PROPERTIES OF METALS Physical Properties of Metals
Ductility is the ability of the material to be stretched into a wire. This ability allows metals to be drawn into wires and coupled with their durability, find applications as cable wires and for soldering purposes. Because Metals can be drawn into wires we can say that metals are ductile.
Fig 1.3 Example of ductility
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Malleability is the property of substances which allows them to be beaten into flat sheets. Aluminium sheets are used in the manufacturing of Aircraft because of their lightweight and strength. Other metals sheets are used in automobile industries, for making utensils, etc. Therefore, metals are malleable.
Fig 1.4 malleability
Usually, all the metals have a shiny appearance but these metals can also be polished to have a shiny appearance.
Fig 1.5
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All the metals are good conductors of heat and electricity. Cooking utensils and irons are made up of metals as they are good conductors of heat.
Fig 1.6 Application of electrical and heat conductivity of metals
Metals are sonorous because it produces a deep or ringing sound when struck with another hard object.
Fig 1.7 Sonorous metal
List out situations from daily life in which the properties of metals are made use of.
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Chemical properties of Metals 1. Reaction with water: Only highly reactive metals react with water and not all the metals. For example, Sodium reacts vigorously with water and oxygen and gives a large amount of heat in the process. This is why sodium is stored in kerosene so that it does not come in contact with moisture or oxygen. 2. Reaction with acids: Hydrogen gas is produced when metals react with acids. For example, when zinc reacts with hydrochloric acid it produces zinc chloride and hydrogen gas. 3. Reaction with bases: Not all the metals react with bases and when they do react, they produce metal salts and hydrogen gas. When zinc reacts with strong sodium hydroxide it gives sodium zincate and hydrogen gas. 4. Reaction with oxygen: Metal oxides are produced when metals burn in the presence of oxygen. These metal oxides are basic in nature. For example: When magnesium strip is burned in the presence of oxygen it forms magnesium oxide and when magnesium oxide dissolves in water it forms magnesium hydroxide. 5.
1.3 REACTIVITY SERIES What is the Reactivity Series? The reactivity series of metals, also known as the activity series, refers to the arrangement of metals in the descending order of their reactivities. The data provided by the reactivity series can be used to predict whether a metal can displace another in a single displacement reaction. It can also be used to obtain information on the reactivity of metals towards water and acids.
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A chart of the reactivity series of common metals is provided below.
Fig 1.8 Reactivity series
Metals tend to readily lose electrons and form cations. Most of them react with atmospheric oxygen to form metal oxides. However, different metals have different reactivities towards oxygen (unreactive metals such as gold and platinum do not readily form oxides when exposed to air).
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Salient Features The metals at the top of the reactivity series are powerful reducing agents since they are easily oxidized. These metals tarnish/corrode very easily. The reducing ability of the metals grows weaker while traversing down the series. The electro positivity of the elements also reduces while moving down the reactivity series of metals. All metals that are found above hydrogen in the activity series liberate H2 gas upon reacting with dilute HCl or dilute H2SO4. Metals that are placed higher on the reactivity series have the ability to displace metals that are placed lower from their salt solutions. Higher ranking metals require greater amounts of energy for their isolation from ores and other compounds. Another important feature of the activity series is that while travelling down the series, the electron-donating ability of the metals reduces.
The metal reactivity sequence, also known as the set of operations, refers to the organization of metals in the ascending order of their reactivities. Metals tend to lose electrons and form cations quickly. Some lead to the forming of metal oxides with ambient oxygen. Non-metal properties have a relatively low boiling point, and other non-metals are gases. Likewise, non-metals are poor heat conductors, and solid non-metals are dull and brittle. Many non-metals are strongly reactive, while others are in no way reactive. It depends on the number of electrons in the outer amount of energy. Long Tabular Form of the Reactivity Series The reactivities of metals are tabulated below (in the descending order) along with their corresponding ions. Note that the metals in Red react with cold water, those in Orange cannot react with cold
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water but can react with acids, and those in Blue only react with some strong oxidizing acids.
Fig 1.9 Reactivity series
Despite being a non-metal, hydrogen is often included in the reactivity series since it helps compare the reactivities of the metals. The metals placed above hydrogen in the series can displace it from acids such as HCl and H2SO4 (since they are more reactive).
Important uses of Reactivity Series Apart from providing insight into the properties and reactivities of the metals, the reactivity series has several other important applications.
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For example, the outcome of the reactions between metals and water, metals and acids, and single displacement reactions between metals can be predicted with the help of the activity series.
Put a piece of sodium in water and observe the products formed.
1.4 REACTIONS OF METALS Reaction Between Metals and Water Calcium and the metals that are more reactive than calcium in the reactivity series can react with cold water to form the corresponding hydroxide while liberating hydrogen gas. For example, the reaction between potassium and water yields potassium hydroxide and H2 gas, as described by the chemical equation provided below. 2K + 2H2O → 2KOH + H2 Therefore, the reactivity series of metals can be used to predict the reactions between metals and water.
Reaction Between Metals and Acids Lead and the metals ranking above lead on the activity series form salts when reacted with hydrochloric acid or sulphuric acid. These reactions also involve the liberation of hydrogen gas. The reaction between zinc and sulphuric acid is an example of such a reaction. Here, zinc sulphate and H2 gas are formed as products. The chemical equation is:
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Zn + H2SO4 → ZnSO4 + H2 Thus, the reactions between metals and some acids can be predicted with the help of the reactivity series.
Single Displacement Reactions Between Metals The ions of low ranking metals are readily reduced by high ranking metals on the reactivity series. Therefore, low ranking metals are easily displaced by high ranking metals in the single displacement reactions between them. A great example of such a reaction is the displacement of copper from copper sulfate by zinc. The chemical equation for this reaction is given by: Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s) This concept has several practical applications in the extraction of metals. For example, titanium is extracted from titanium tetrachloride via a single displacement reaction with magnesium. Thus, the reactivity series of metals can also be used to predict the outcome of single displacement reactions.
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SUMMARY Elements can be classified as metals and non-metals. Metals are lustrous, malleable, ductile and are good conductors of heat and electricity. They are solids at room temperature, except mercury which is a liquid. Metals can form positive ions by losing electrons to non-metals. Metals combine with oxygen to form basic oxides. Different metals have different reactivities with water and dilute acids. A list of common metals arranged in order of their decreasing reactivity is known as an activity series. Metals above hydrogen in the Activity series can displace hydrogen from dilute acids. A more reactive metal displaces a less reactive metal from its salt solution. Metals occur in nature as free elements or in the form of their
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EXERCISE 1. You are given a hammer, a battery, a bulb, wires and a switch. (a) How could you use them to distinguish between samples of metals and non-metals? (b) Assess the usefulness of these tests in distinguishing between metals and non-metals 2. Name two metals which will displace hydrogen from dilute acids, and two metals which will not. 3. Match the following
4. Which statement about the reactivity series is correct? Metals at the bottom of the reactivity series lose electrons more easily than metals at the top of the series Metals at the top of the reactivity series react less vigorously than metals at the bottom of the series Metals at the top of the reactivity series lose electrons more easily than metals at the bottom of the series 5. What are the products of the reaction of sodium with water? Sodium oxide and hydrogen Sodium hydroxide and water Sodium hydroxide and hydrogen
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6. Which metal has the highest ductility? a) Silver b) Gold c) Platinum d) Aluminium 7. Explain electrical conductivity of metals.
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Prepared by Arun Eappen Joseph 18220386001 Physical Science St Thomas Training college