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CHAPTER 2 Acids, Bases and Salts Y ou have learnt in your previous classes that the sour and bitter tastes of food are due to acids and bases, respectively, present in them. If someone in the family is suffer...

CHAPTER 2 Acids, Bases and Salts Y ou have learnt in your previous classes that the sour and bitter tastes of food are due to acids and bases, respectively, present in them. If someone in the family is suffering from a problem of acidity after overeating, which of the following would you suggest as a remedy– lemon juice, vinegar or baking soda solution? n Which property did you think of while choosing the remedy? Surely you must have used your knowledge about the ability of acids and bases to nullify each other’s effect. n Recall how we tested sour and bitter substances without tasting them. You already know that acids are sour in taste and change the colour of blue litmus to red, whereas, bases are bitter and change the colour of the red litmus to blue. Litmus is a natural indicator, turmeric is another such indicator. Have you noticed that a stain of curry on a white cloth becomes reddish-brown when soap, which is basic in nature, is scrubbed on it? It turns yellow again when the cloth is washed with plenty of water. You can also use synthetic indicators such as methyl orange and phenolphthalein to test for acids and bases. In this Chapter, we will study the reactions of acids and bases, how acids and bases cancel out each other’s effects and many more interesting things that we use and see in our day-to-day life. Do You Know? Litmus solution is a purple dye, which is extracted from lichen, a plant belonging to the division Thallophyta, and is commonly used as an indicator. When the litmus solution is neither acidic nor basic, its colour is purple. There are many other natural materials like red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a solution. These are called acid-base indicators or sometimes simply indicators. 2024-25  Q U E S T I O N ? 1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? 2. 1 UNDERSTANDING THE CHEMIC UNDERSTANDING AL PROPERTIES OF CHEMICAL ACIDS AND BASES 2.1.1 Acids and Bases in the Laboratory Activity 2.1 n Collect the following solutions from the science laboratory– hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), acetic acid (CH 3COOH), sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH). n Put a drop of each of the above solutions on a watch-glass one by one and test with a drop of the indicators shown in Table 2.1. n What change in colour did you observe with red litmus, blue litmus, phenolphthalein and methyl orange solutions for each of the solutions taken? n Tabulate your observations in Table 2.1. Table 2.1 Sample Red Blue Phenolph- Methyl solution litmus litmus -thalein orange solution solution solution solution These indicators tell us whether a substance is acidic or basic by change in colour. There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators. Let us try out some of these indicators. Activity 2.2 n Take some finely chopped onions in a plastic bag along with some strips of clean cloth. Tie up the bag tightly and leave overnight in the fridge. The cloth strips can now be used to test for acids and bases. n Take two of these cloth strips and check their odour. n Keep them on a clean surface and put a few drops of dilute HCl solution on one strip and a few drops of dilute NaOH solution on the other. 18 Science 2024-25  n Rinse both cloth strips with water and again check their odour. n Note your observations. n Now take some dilute vanilla essence and clove oil and check their odour. n Take some dilute HCl solution in one test tube and dilute NaOH solution in another. Add a few drops of dilute vanilla essence to both test tubes and shake well. Check the odour once again and record changes in odour, if any. n Similarly, test the change in the odour of clove oil with dilute HCl and dilute NaOH solutions and record your observations. Which of these – vanilla, onion and clove, can be used as olfactory indicators on the basis of your observations? Let us do some more activities to understand the chemical properties of acids and bases. 2.1.2 How do Acids and Bases React with Metals? Activity 2.3 CAUTION: This activity needs the teacher’s assistance. n Set the apparatus as shown in Fig. 2.1. n Take about 5 mL of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it. n What do you observe on the surface of zinc granules? n Pass the gas being evolved through the soap solution. n Why are bubbles formed in the soap solution? n Take a burning candle near a gas filled bubble. n What do you observe? n Repeat this Activity with some more acids like HCl, HNO3 and CH3COOH. n Are the observations in all the cases the same or different? Figure 2.1 Reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas by burning Acids, Bases and Salts 19 2024-25  Note that the metal in the above reactions displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt. Thus, the reaction of a metal with an acid can be summarised as – Acid + Metal → Salt + Hydrogen gas Can you now write the equations for the reactions you have observed? Activity 2.4 n Place a few pieces of granulated zinc metal in a test tube. n Add 2 mL of sodium hydroxide solution and warm the contents of the test tube. n Repeat the rest of the steps as in Activity 2.3 and record your observations. The reaction that takes place can be written as follows. 2NaOH(aq) + Zn(s) → Na2ZnO2(s) + H2(g) (Sodium zincate) You find again that hydrogen is formed in the reaction. However, such reactions are not possible with all metals. 2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids? Activity 2.5 n Take two test tubes, label them as A and B. n Take about 0.5 g of sodium carbonate (Na 2CO3) in test tube A and about 0.5 g of sodium hydrogencarbonate (NaHCO3) in test tube B. n Add about 2 mL of dilute HCl to both the test tubes. n What do you observe? n Pass the gas produced in each case through lime water (calcium Figure 2.2 hydroxide solution) as shown in Passing carbon dioxide gas Fig. 2.2 and record your observations. through calcium hydroxide solution The reactions occurring in the above Activity are written as – Test tube A: Na 2CO3 (s) + 2 HCl(aq) → 2NaCl(aq) + H2 O(l) + CO2 (g) Test tube B: NaHCO3 (s) + HCl(aq) → NaCl(aq) + H2O(l) + CO 2 (g) On passing the carbon dioxide gas evolved through lime water, Ca(OH)2 (aq) + CO2 (g) → CaCO3 ( s) + H2 O (l) (Lime water) (White precipitate) 20 Science 2024-25 On passing excess carbon dioxide the following reaction takes place: CaCO3 (s ) + H2 O(l)+ CO2 (g) → Ca(HCO3 )2 (aq ) (Soluble in water) Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water. Thus, the reaction can be summarised as – Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water 2.1.4 How do Acids and Bases React with each other? Activity 2.6 n Take about 2 mL of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution. n What is the colour of the solution? n Add dilute HCl solution to the above solution drop by drop. n Is there any colour change for the reaction mixture? n Why did the colour of phenolphthalein change after the addition of an acid? n Now add a few drops of NaOH to the above mixture. n Does the pink colour of phenolphthalein reappear? n Why do you think this has happened? In the above Activity, we have observed that the effect of a base is nullified by an acid and vice-versa. The reaction taking place is written as – NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as – Base + Acid → Salt + Water 2.1.5 Reaction of Metallic Oxides with Acids Activity 2.7 n Take a small amount of copper oxide in a beaker and add dilute hydrochloric acid slowly while stirring. n Note the colour of the solution. What has happened to the copper oxide? You will notice that the colour of the solution becomes blue-green and the copper oxide dissolves. The blue-green colour of the solution is due to the formation of copper(II) chloride in the reaction. The general reaction between a metal oxide and an acid can be written as – Metal oxide + Acid → Salt + Water Acids, Bases and Salts 21 2024-25  Now write and balance the equation for the above reaction. Since metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides. 2.1.6 Reaction of a Non-metallic Oxide with Base You saw the reaction between carbon dioxide and calcium hydroxide (lime water) in Activity 2.5. Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. Since this is similar to the reaction between a base and an acid, we can conclude that non- metallic oxides are acidic in nature. Q U E S T I O N S 1. Why should curd and sour substances not be kept in brass and copper ? vessels? 2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas? 3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. WHATT DO ALL ACIDS AND ALL BASES HA 2.2 WHA HAVE VE IN COMMON? In Section 2.1 we have seen that all acids have similar chemical properties. What leads to this similarity in properties? We saw in Activity 2.3 that all acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids. Let us perform an Activity to investigate whether all compounds containing hydrogen are acidic. Activity 2.8 n Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc. n Fix two nails on a cork, and place the cork in a 100 mL beaker. n Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in Fig. 2.3. n Now pour some dilute HCl in the beaker and switch on the current. n Repeat with dilute sulphuric acid. n What do you observe? n Repeat the experiment separately with Figure 2.3 glucose and alcohol solutions. What do you Acid solution in water observe now? conducts electricity n Does the bulb glow in all cases? 22 Science 2024-25  The bulb will start glowing in the case of acids, as shown in Fig. 2.3. But you will observe that glucose and alcohol solutions do not conduct electricity. Glowing of the bulb indicates that there is a flow of electric current through the solution. The electric current is carried through the acidic solution by ions. Acids contain H+ ion as cation and anion such as Cl– in HCl, NO3– in HNO3, SO2– 4 in H2SO4, CH3COO– in CH3COOH. Since the cation present in acids is H , this suggests that acids produce hydrogen ions, H+(aq), in + solution, which are responsible for their acidic properties. Repeat the same Activity using alkalis such as sodium hydroxide, calcium hydroxide, etc. What can you conclude from the results of this Activity? 2.2.1 What Happens to an Acid or a Base in a Water Solution? Do acids produce ions only in aqueous solution? Let us test this. Activity 2.9 n Take about 1g solid NaCl in a clean and dry test tube and set up the apparatus as shown in Fig. 2.4. n Add some concentrated sulphuric acid to the test tube. n What do you observe? Is there a gas coming out of the delivery tube? n Test the gas evolved successively with dry and wet blue litmus paper. n In which case does the litmus paper change colour? n On the basis of the above Activity, what do you infer about the acidic character of: (i) dry HCl gas Figure 2.4 Preparation of HCl gas (ii) HCl solution? Note to teachers: If the climate is very humid, you will have to pass the gas produced through a guard tube (drying tube) containing calcium chloride to dry the gas. This experiment suggests that hydrogen ions in HCl are produced in the presence of water. The separation of H+ ion from HCl molecules cannot occur in the absence of water. HCl + H2O → H3O+ + Cl– Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+(aq) or hydronium ion (H3O+). H+ + H2O → H3O+ We have seen that acids give H3O+ or H+(aq) ion in water. Let us see what happens when a base is dissolved in water. H2O NaOH(s)  + – → Na (aq) + OH (aq) Acids, Bases and Salts 23 2024-25  H O 2 KOH(s) → K + (aq) + OH – (aq) H 2O Mg(OH)2(s)  2+ – → Mg (aq)+2OH (aq) Bases generate hydroxide (OH–) ions in water. Bases which are soluble in water are called alkalis. Do You Know? All bases do not dissolve in water. An alkali is a base that dissolves in water. They are soapy to touch, bitter and corrosive. Never taste or touch them as they may cause harm. Which of the bases in the Table 2.1 are alkalis? Now as we have identified that all acids generate H+(aq) and all – bases generate OH (aq), we can view the neutralisation reaction as follows – Acid + Base → Salt + Water H X + M OH → MX + HOH – H+(aq) + OH (aq) → H2O(l) Let us see what is involved when water is mixed with an acid or a base. Activity 2.10 n Take 10 mL water in a beaker. n Add a few drops of concentrated H2SO4 to it and swirl the beaker slowly. n Touch the base of the beaker. n Is there a change in temperature? n Is this an exothermic or endothermic process? n Repeat the above Activity with sodium hydroxide pellets Figure 2.5 and record your observations. Warning sign displayed on containers containing concentrated acids and The process of dissolving an acid or a base in water is a highly bases exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Look out for the warning sign (shown in Fig. 2.5) on the can of concentrated sulphuric acid and on the bottle of sodium hydroxide pellets. Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH–) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted. 24 Science 2024-25