edited.pdf

CHAPTER 3 Metals and Non-metals I n Class IX you have learnt about various elements. You have seen that elements can be classified as metals or non-metals on the basis of their properties. n Think of some uses of metals and non-metals in your daily life....

CHAPTER 3 Metals and Non-metals I n Class IX you have learnt about various elements. You have seen that elements can be classified as metals or non-metals on the basis of their properties. n Think of some uses of metals and non-metals in your daily life. n What properties did you think of while categorising elements as metals or non-metals? n How are these properties related to the uses of these elements? Let us look at some of these properties in detail. 3.1 PHYSIC AL PROPERTIES 3.1.1 Metals The easiest way to start grouping substances is by comparing their physical properties. Let us study this with the help of the following activities. For performing Activities 3.1 to 3.6, collect the samples of following metals – iron, copper, aluminium, magnesium, sodium, lead, zinc and any other metal that is easily available. Activity 3.1 n Take samples of iron, copper, aluminium and magnesium. Note the appearance of each sample. n Clean the surface of each sample by rubbing them with sand paper and note their appearance again. Metals, in their pure state, have a shining surface. This property is called metallic lustre. Activity 3.2 n Take small pieces of iron, copper, aluminium, and magnesium. Try to cut these metals with a sharp knife and note your observations. n Hold a piece of sodium metal with a pair of tongs. CAUTION: Always handle sodium metal with care. Dry it by pressing between the folds of a filter paper. n Put it on a watch-glass and try to cut it with a knife. n What do you observe? 2024-25  You will find that metals are generally hard. The hardness varies from metal to metal. Activity 3.3 n Take pieces of iron, zinc, lead and copper. n Place any one metal on a block of iron and strike it four or five times with a hammer. What do you observe? n Repeat with other metals. n Record the change in the shape of these metals. You will find that some metals can be beaten into thin sheets. This property is called malleability. Did you know that gold and silver are the most malleable metals? Activity 3.4 n List the metals whose wires you have seen in daily life. The ability of metals to be drawn into thin wires is called ductility. Gold is the most ductile metal. You will be surprised to know that a wire of about 2 km length can be drawn from one gram of gold. It is because of their malleability and ductility that metals can be given different shapes according to our needs. Can you name some metals that are used for making cooking vessels? Do you know why these metals are used for making vessels? Let us do the following Activity to find out the answer. Activity 3.5 n Take an aluminium or copper wire. Clamp this wire on a stand, as shown in Fig. 3.1. n Fix a pin to the free end of the wire using wax. n Heat the wire with a spirit lamp, candle or a burner near the place where it is clamped. n What do you observe after some time? n Note your observations. Does the metal wire melt? Figure 3.1 Metals are good conductors of heat. The above activity shows that metals are good conductors of heat and have high melting points. The best conductors of heat are silver and copper. Lead and mercury are comparatively poor conductors of heat. Do metals also conduct electricity? Let us find out. 38 Science 2024-25  Activity 3.6 n Set up an electric circuit as shown in Fig. 3.2. n Place the metal to be tested in the circuit between terminals A and B as shown. n Does the bulb glow? What does this indicate? You must have seen that the wires that carry current in your homes have a coating of polyvinylchloride (PVC) or a rubber-like material. Why are electric wires coated Figure 3.2 with such substances? Metals are good What happens when metals strike a hard surface? Do they produce conductors of electricity. a sound? The metals that produce a sound on striking a hard surface are said to be sonorous. Can you now say why school bells are made of metals? 3.1.2 Non-metals In the previous Class you have learnt that there are very few non-metals as compared to metals. Some of the examples of non-metals are carbon, sulphur, iodine, oxygen, hydrogen, etc. The non-metals are either solids or gases except bromine which is a liquid. Do non-metals also have physical properties similar to that of metals? Let us find out. Activity 3.7 n Collect samples of carbon (coal or graphite), sulphur and iodine. n Carry out the Activities 3.1 to 3.4 and 3.6 with these non-metals and record your observations. Compile your observations regarding metals and non-metals in Table 3.1. Table 3.1 Element Symbol Type of Hardness Malleability Ductility Conducts Sonority surface Electricity On the bases of the observations recorded in Table 3.1, discuss the general physical properties of metals and non-metals in the class. You must have concluded that we cannot group elements according to their physical properties alone, as there are many exceptions. For example – (i) All metals except mercury exist as solids at room temperature. In Activity 3.5, you have observed that metals have high melting Metals and Non-metals 39 2024-25  points but gallium and caesium have very low melting points. These two metals will melt if you keep them on your palm. (ii) Iodine is a non-metal but it is lustrous. (iii) Carbon is a non-metal that can exist in different forms. Each form is called an allotrope. Diamond, an allotrope of carbon, is the hardest natural substance known and has a very high melting and boiling point. Graphite, another allotrope of carbon, is a conductor of electricity. (iv) Alkali metals (lithium, sodium, potassium) are so soft that they can be cut with a knife. They have low densities and low melting points. Elements can be more clearly classified as metals and non-metals on the basis of their chemical properties. Activity 3.8 n Take a magnesium ribbon and some sulphur powder. n Burn the magnesium ribbon. Collect the ashes formed and dissolve them in water. n Test the resultant solution with both red and blue litmus paper. n Is the product formed on burning magnesium acidic or basic? n Now burn sulphur powder. Place a test tube over the burning sulphur to collect the fumes produced. n Add some water to the above test tube and shake. n Test this solution with blue and red litmus paper. n Is the product formed on burning sulphur acidic or basic? n Can you write equations for these reactions? Most non-metals produce acidic oxides when dissolve in water. On the other hand, most metals, give rise to basic oxides. You will be learning more about these metal oxides in the next section. Q U E S T I O N S 1. Give an example of a metal which ? (i) is a liquid at room temperature. (ii) can be easily cut with a knife. (iii) is the best conductor of heat. (iv) is a poor conductor of heat. 2. Explain the meanings of malleable and ductile. 3.2 CHEMIC AL PROPERTIES OF MET CHEMICAL METALS ALS We will learn about the chemical properties of metals in the following Sections 3.2.1 to 3.2.4. For this, collect the samples of following metals – aluminium, copper, iron, lead, magnesium, zinc and sodium. 40 Science 2024-25 3.2.1 What happens when Metals are burnt in Air? You have seen in Activity 3.8 that magnesium burns in air with a dazzling white flame. Do all metals react in the same manner? Let us check by performing the following Activity. Activity 3.9 CAUTION: The following activity needs the teacher’s assistance. It would be better if students wear eye protection. n Hold any of the samples taken above with a pair of tongs and try burning over a flame. Repeat with the other metal samples. n Collect the product if formed. n Let the products and the metal surface cool down. n Which metals burn easily? n What flame colour did you observe when the metal burnt? n How does the metal surface appear after burning? n Arrange the metals in the decreasing order of their reactivity towards oxygen. n Are the products soluble in water? Almost all metals combine with oxygen to form metal oxides. Metal + Oxygen → Metal oxide For example, when copper is heated in air, it combines with oxygen to form copper(II) oxide, a black oxide. 2Cu + O2 → 2CuO (Copper) (Copper(II) oxide) Similarly, aluminium forms aluminium oxide. 4Al + 3O2 → 2Al2O3 (Aluminium) (Aluminium oxide) Recall from Chapter 2, how copper oxide reacts with hydrochloric acid. We have learnt that metal oxides are basic in nature. But some metal oxides, such as aluminium oxide, zinc oxide show both acidic as well as basic behaviour. Such metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Aluminium oxide reacts in the following manner with acids and bases – Al2O3 + 6HCl → 2AlCl3 + 3H2O Al2O3 + 2NaOH → 2NaAlO2 + H2O (Sodium aluminate) Most metal oxides are insoluble in water but some of these dissolve in water to form alkalis. Sodium oxide and potassium oxide dissolve in water to produce alkalis as follows – Na2O(s) + H2O(l) → 2NaOH(aq) K2O(s) + H2O(l) → 2KOH(aq) Metals and Non-metals 41 2024-25  We have observed in Activity 3.9 that all metals do not react with oxygen at the same rate. Different metals show different reactivities towards oxygen. Metals such as potassium and sodium react so vigorously that they catch fire if kept in the open. Hence, to protect them and to prevent accidental fires, they are kept immersed in kerosene oil. At ordinary temperature, the surfaces of metals such as magnesium, aluminium, zinc, lead, etc., are covered with a thin layer of oxide. The protective oxide layer prevents the metal from further oxidation. Iron does not burn on heating but iron filings burn vigorously when sprinkled in the flame of the burner. Copper does not burn, but the hot metal is coated with a black coloured layer of copper(II) oxide. Silver and gold do not react with oxygen even at high temperatures. Do You Know? Anodising is a process of forming a thick oxide layer of aluminium. Aluminium develops a thin oxide layer when exposed to air. This aluminium oxide coat makes it resistant to further corrosion. The resistance can be improved further by making the oxide layer thicker. During anodising, a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid. The oxygen gas evolved at the anode reacts with aluminium to make a thicker protective oxide layer. This oxide layer can be dyed easily to give aluminium articles an attractive finish. After performing Activity 3.9, you must have observed that sodium is the most reactive of the samples of metals taken here. The reaction of magnesium is less vigorous implying that it is not as reactive as sodium. But burning in oxygen does not help us to decide about the reactivity of zinc, iron, copper or lead. Let us see some more reactions to arrive at a conclusion about the order of reactivity of these metals. 3.2.2 What happens when Metals react with Water? Activity 3.10 CAUTION: This Activity needs the teacher’s assistance. n Collect the samples of the same metals as in Activity 3.9. n Put small pieces of the samples separately in beakers half-filled with cold water. n Which metals reacted with cold water? Arrange them in the increasing order of their reactivity with cold water. n Did any metal produce fire on water? n Does any metal start floating after some time? n Put the metals that did not react with cold water in beakers half-filled with hot water. n For the metals that did not react with hot water, arrange the apparatus as shown in Fig. 3.3 and observe their reaction with steam. n Which metals did not react even with steam? n Arrange the metals in the decreasing order of reactivity with water. 42 Science 2024-25  Figure 3.3 Action of steam on a metal Metals react with water and produce a metal oxide and hydrogen gas. Metal oxides that are soluble in water dissolve in it to further form metal hydroxide. But all metals do not react with water. Metal + Water → Metal oxide + Hydrogen Metal oxide + Water → Metal hydroxide Metals like potassium and sodium react violently with cold water. In case of sodium and potassium, the reaction is so violent and exothermic that the evolved hydrogen immediately catches fire. 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) + heat energy 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) + heat energy The reaction of calcium with water is less violent. The heat evolved is not sufficient for the hydrogen to catch fire. Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g) Calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of the metal. Magnesium does not react with cold water. It reacts with hot water to form magnesium hydroxide and hydrogen. It also starts floating due to the bubbles of hydrogen gas sticking to its surface. Metals like aluminium, iron and zinc do not react either with cold or hot water. But they react with steam to form the metal oxide and hydrogen. 2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g) 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) Metals such as lead, copper, silver and gold do not react with water at all. 3.2.3 What happens when Metals react with Acids? You have already learnt that metals react with acids to give a salt and hydrogen gas. Metals and Non-metals 43 2024-25  Metal + Dilute acid → Salt + Hydrogen But do all metals react in the same manner? Let us find out. Activity 3.11 n Collect all the metal samples except sodium and potassium again. If the samples are tarnished, rub them clean with sand paper. CAUTION: Do not take sodium and potassium as they react vigorously even with cold water. n Put the samples separately in test tubes containing dilute hydrochloric acid. n Suspend thermometers in the test tubes, so that their bulbs are dipped in the acid. n Observe the rate of formation of bubbles carefully. n Which metals reacted vigorously with dilute hydrochloric acid? n With which metal did you record the highest temperature? n Arrange the metals in the decreasing order of reactivity with dilute acids. Write equations for the reactions of magnesium, aluminium, zinc and iron with dilute hydrochloric acid. Hydrogen gas is not evolved when a metal reacts with nitric acid. It is because HNO3 is a strong oxidising agent. It oxidises the H2 produced to water and itself gets reduced to any of the nitrogen oxides (N2O, NO, NO2). But magnesium (Mg) and manganese (Mn) react with very dilute HNO3 to evolve H2 gas. You must have observed in Activity 3.11, that the rate of formation of bubbles was the fastest in the case of magnesium. The reaction was also the most exothermic in this case. The reactivity decreases in the order Mg > Al > Zn > Fe. In the case of copper, no bubbles were seen and the temperature also remained unchanged. This shows that copper does not react with dilute HCl. Do You Know? Aqua regia, (Latin for ‘royal water’) is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio of 3:1. It can dissolve gold, even though neither of these acids can do so alone. Aqua regia is a highly corrosive, fuming liquid. It is one of the few reagents that is able to dissolve gold and platinum. 3.2.4 How do Metals react with Solutions of other Metal Salts? Activity 3.12 n Take a clean wire of copper and an iron nail. n Put the copper wire in a solution of iron sulphate and the iron nail in a solution of copper sulphate taken in test tubes (Fig. 3.4). n Record your observations after 20 minutes. 44 Science 2024-25  n In which test tube did you find that a reaction has occurred? n On what basis can you say that a reaction has actually taken place? n Can you correlate your observations for the Activities 3.9, 3.10 and 3.11? n Write a balanced chemical equation for the reaction that has taken place. n Name the type of reaction. Reactive metals can displace less reactive metals from their compounds in solution or molten form. We have seen in the previous sections that all metals are not equally reactive. We checked the reactivity of various metals with oxygen, water and acids. But all metals do not react with these reagents. So we were not able to put all the metal samples we had collected in decreasing order of their reactivity. Displacement reactions studied in Chapter 1 give better evidence about the reactivity of metals. It is simple and easy if metal A displaces Figure 3.4 metal B from its solution, it is more reactive than B. Reaction of metals with salt solutions Metal A + Salt solution of B → Salt solution of A + Metal B Which metal, copper or iron, is more reactive according to your observations in Activity 3.12? 3.2.5 The Reactivity Series The reactivity series is a list of metals arranged in the order of their decreasing activities. After performing displacement experiments (Activities 1.9 and 3.12), the following series, (Table 3.2) known as the reactivity or activity series has been developed. Table 3.2 Activity series : Relative reactivities of metals K Potassium Most reactive Na Sodium Ca Calcium Mg Magnesium Al Aluminium Zn Zinc Reactivity decreases Fe Iron Pb Lead [H] [Hydrogen] Cu Copper Hg Mercury Ag Silver Au Gold Least reactive Metals and Non-metals 45 2024-25  Q U E S T I O N S 1. Why is sodium kept immersed in kerosene oil? ? 2. Write equations for the reactions of (i) iron with steam (ii) calcium and potassium with water 3. Samples of four metals A, B, C and D were taken and added to the following solution one by one. The results obtained have been tabulated as follows. Metal Iron(II) sulphate Copper(II) sulphate Zinc sulphate Silver nitrate A No reaction Displacement B Displacement No reaction C No reaction No reaction No reaction Displacement D No reaction No reaction No reaction No reaction Use the Table above to answer the following questions about metals A, B, C and D. (i) Which is the most reactive metal? (ii) What would you observe if B is added to a solution of Copper(II) sulphate? (iii) Arrange the metals A, B, C and D in the order of decreasing reactivity. 4. Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction when iron reacts with dilute H2SO4. 5. What would you observe when zinc is added to a solution of iron(II) sulphate? Write the chemical reaction that takes place. 3.3 HOW DO METALS AND NON-MET METALS NON-METALS CT? REACT? ALS REA In the above activities, you saw the reactions of metals with a number of reagents. Why do metals react in this manner? Let us recall what we learnt about the electronic configuration of elements in Class IX. We learnt that noble gases, which have a completely filled valence shell, show little chemical activity. We, therefore, explain the reactivity of elements as a tendency to attain a completely filled valence shell. Let us have a look at the electronic configuration of noble gases and some metals and non-metals. We can see from Table 3.3 that a sodium atom has one electron in its outermost shell. If it loses the electron from its M shell then its L shell now becomes the outermost shell and that has a stable octet. The nucleus of this atom still has 11 protons but the number of electrons has become 10, so there is a net positive charge giving us a sodium cation Na+. On the other hand chlorine has seven electrons in its outermost shell 46 Science 2024-25