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(Page No - 66)
What colour do the following indicators turn when added to a base or alkali (such as sodium hydroxide)?
(a) methyl orange
(c) red cabbage extract
Red Cabbage ExtractSolution:
What colours do the following indicators turn when added to an acid (such as hydrochloric acid)?
(b) methyl orange
Name an indicator that is red in acid solution but turns blue in basic solution.
Name an indicator that is pink in alkaline solution but turns colourless in acidic solution.
When a solution is added to a cloth strip treated with onion extract, then the smell of onion cannot be detected. State whether the given solution contains an acid or a base.
When a solution is added to vanilla extract, then the characteristic smell of vanilla cannot be detected. State whether the given solution is an acid or a base.
When Hydrochloric acid reacts with an active metal (like zinc), we observe that gas filled bubbles are formed on the surface of the metal. Pass the gas formed through soap solution. Then, bring a burning candle near the gas filled soap bubble. If the gas present in bubble burns with a ‘pop’ sound, then its hydrogen gas.
How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal ?
Carbon dioxide (CO2) gas is evolved during the reaction. We pass this gas through lime water which turns milky because of the CO2 passing through it. If we keep on passing the gas through the milky lime water, it would become clear again.
Name the gas evolved when dilute HCl reacts with sodium hydrogencarbonate. How is it recognised ?
(HCl) and Sulphuric acid (H2SO4) are strong acids.
Acetic acid (CH3COOH)
and Citric acid (C6H8O7) are weak acids.
Give the names and formulae of two strong acids and two weak acids.
(a) Citric acid – Lemon.
(b) Oxalic acid – Tomatoes.
(c) Lactic acid – Sour milk or curd.
(d) Tartaric acid – Tamarind.
Name one natural source of each of the following acids :
(a) Citric acid
(b) Oxalic acid
(c) Lactic acid
(d) Tartaric acid
Ant sting and Nettle leaf sting.
Name one animal and one plant whose stings contain formic acid (or methanoic acid).
On diluting an acid,the concentration of hydronium ions (H3O+) in it decreases.
How is the concentration of hydronium ions (H3O+) affected when the solution of an acid is diluted ?
On diluting an acid,the concentration of hydronium ions (H3O+) in it decreases.
Write word equations and then balanced equations for the reactions taking place when :
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
(Page No - 67)
Complete and balance the following chemical equations :
(a) Zn (s) + HCl (aq) ——- >
(b) Na2C03 (s) + HCl (aq) ———>
(c) NaHC03 (s) + HCl (aq) ——–>
(d) NaOH (aq) + HCl (aq) ———>
(e) CuO(s) + HCl (aq) ———->
Fill in the blanks in the following sentences :
(a) Acids have a…………….. taste and they turn…………… litmus to……………….
(b) Substances do not show their acidic properties without……………….
(c) Acids produce…………… ions on dissolving in water.
(d) Those substances whose smell (or odour) changes in acidic or basic solutions are called
(e) Onion and vanilla extract are…………..
(a) Sour; blue; bed.
(a) What is an indicator ? Name three common indicators.
(b) Name the acid-base indicator extracted from lichen.
(c) What colour does the turmeric paper turn when put in an alkaline solution ?
(a) An indicator is a ‘dye’ that changes colour when it is put in an acid or a base. The three most common indicators are: Litmus, Methyl orange and Phenolphthalein.
What is an olfactory indicator ? Name two olfactory indicators. What is the effect of adding sodium hydroxide solution to these olfactory indicators ?
whose smell (or odour) changes in acidic or basic solutions are called olfactory indicators. Onion and vanilla extracts are olfactory indicators. When a basic solution like sodium hydroxide solution is added to a cloth
strip treated with onions (or onion extract), then the onion smell cannot be detected.
(a) What happens when an acid reacts with a metal ? Give chemical equation of the reaction involved.
(b) Which gas is usually liberated when an acid reacts with a metal ? How will you test for the presence of this gas ?
(a) When an acid reacts with a metal, then a salt and hydrogen gas are formed.
(b) Hydrogen gas is liberated when an acid reacts with a metal. When reaction between an acid and a metal occurs, we observe formation of gas bubbles. When these gas bubbles are passed through soap solution, gas filled soap bubbles rise into the air. When a burning candle is brought near a gas-filled soap bubble, the gas present in the soap-bubble burns with a ‘pop’ sound. Only hydrogen gas burns making a ‘pop’ sound. This shows that hydrogen gas is evolved in the process.
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid ?
When a concentrated acid is added to water for preparing a dilute acid, then the heat is evolved gradually and easily absorbed by the large amount of water (to which the acid is being added) however if water is added to concentrated acid, then large amount of heat is evolved at once. This heat changes some of the water to steam explosively which can splash the acid on our face or clothes and causeacid burns. Even the glass container may break due to the excessive heating.
What happens when an acid reacts with a metal hydrogencarbonate ? Write equation of the reaction which takes place.
When an acid reacts with a metal hydrogen carbonate, then a salt, carbon dioxide gas and water are formed.
(a) What happens when dilute hydrochloric acid is added to sodium carbonate ? Write a balanced chemical
equation of the reaction involved.
(b) Which gas is liberated when dilute hydrochloric acid reacts with sodium carbonate ? How will you test for the presence of this gas ?
(a) When dilute hydrochloric acid reacts with sodium carbonate, then sodium chloride, carbon dioxide and water are formed.
(b) CO2 gas is liberated during the reaction.
When carbon dioxide gas formed in the form of brisk effervescence is passed through lime water, it turns the lime water milky. If excess of carbon dioxide gas is passed through the milky lime water, the solution becomes clear again. This confirms the presence of carbon dioxide gas.
What happens when an acid reacts with a base ? Explain by taking the example of hydrochloric acid and sodium hydroxide. Give equation of the chemical reaction which takes place. What is the special name of such a reaction ?
When an acid reacts with a base, then a salt and water are formed. When hydrochloric acid reacts with sodium hydroxide solution, then a neutralisation reaction takes place to form sodium chloride and water.
Such a reaction is termed as neutralisation reaction.
What happens when an acid reacts with a metal oxide ? Explain with the help of an example. Write a balanced equation for the reaction involved.
Acids react with metal oxides to form salt and water.
For example: Copper (II) Oxide, a metal oxide reacts with dilute hydrochloric acid to form copper chloride and water
(a) What are organic acids and mineral acids ?
(b) Give two examples each of organic acids and mineral acids.
(c) State some of the uses of mineral acids in industry.
(a) Organic acids are acids present in plant materials and animals. These are naturally occuring acids.
A mineral acid (or inorganic acid) is an acid derived from one or more minerals of the earth.
(b) Organic acids: Citric acid, lactic acid;
Mineral acids: Hydrochloric acid, sulphuric acid.
(c) Uses of mineral acids in industry:
What is meant by strong acids and weak acids ? Classify the following into strong acids and weak acids :
HCl, CH3COOH, H2SO4, HNO3, H2CO3, H2SO3
A strong acid is one that completely ionises in water to form a large amount of hydrogen ions whereas a weak acid only partially ionises in water and thus produces a small amount of hydrogen ions.
HCl, H2SO4, HNO3 are strong acids; CH3COOH, H2CO3, H2SO3 are weak acids.
Why do HCl, H2SO4, HNO3, etc., show acidic character in aqueous solutions while solutions of compounds like C6H12O6 (glucose) and C2H5OH (alcohol) do not show acidic character ?
The acidic character of a substance is due to the presence of hydrogen ions [H+(aq) ions] in its aqueous solution. HCl, H2SO4 etc show acidic properties because they produce hydrogen ions when dissolved in water. The solution of compounds like alcohol and glucose do not show acidic character because they do not ionize in water to produce hydrogen ions or any other ions in solution.
What is a neutralisation reaction ? Explain with an example. Give the chemical equation of the reaction which takes place.
The reaction between an acid and a base to form salt and water is called a neutralisation reaction. When hydrochloric acid reacts with sodium hydroxide solution, then a neutralisation reaction takes place to form sodium chloride and water.
Why should curd and other sour foodstuffs (like lemon juice, etc.) not be kept in metal containers (such as copper and brass vessels) ?
Curd and other sour substances contains acids which can react with the metals of brass and copper vessels to form toxic (poisonous) metal compounds which can cause food poisoning and damage our health.
(a) What is produced if an acid is added to a base ?
(b) Why does dry HCl gas not change the colour of dry litmus paper ?
(c) What colour does phenolphthalein indicator turn when added to an alkali (such as sodium hydroxide) ?
(a) Salt and water.
(b) Because dry HCl gas has no hydrogen ions (H+ ions) in it which can impart acidic properties to it.
(a) Why do acids not show acidic behaviour in the absence of water ?
(b) Why does an aqueous solution of an acid conduct electricity ?
(c) Why does distilled water not conduct electricity whereas rain water does ?
(a) The acidic behavior of an acid is due to the presence of hydrogen ions [H+ (aq) ions] which are produced only when acids are dissolved in water. In the absence of water, acids do not produce hydrogen ions and hence do not show acidic behavior.
(b) The aqueous solution of an acid conducts electricity due to the presence of charged particles called ‘ions’ in it. These ions carry electric current.
(c) Distilled water does not conduct electricity because it does not contain any ionic compounds dissolved in it whereas rain water does.
Reason: When rain water falls on earth through the atmosphere, it dissolves an acidic gas ‘carbon dioxide’ from the air and forms carbonic acid (H2CO3). The carbonic acid provides some hydrogen and carbonate ions to the rain water. Due to the presence of these ions, rain water conducts electricity.