1. Acids, Bases and Salts

Transcription

1 Eupheus Learning 1. Acids, Bases and Salts Study Material for Grade 7 Chemistry

2 Overview Acids and Bases Identification Neutralization Salts Abhay s mother boiled an egg for him. Since it was very hot she poured some water from a bottle in a bowl and put the egg in it to cool. After a while when she came to pick the egg, she was surprised to find that the shell of the egg was gone! Wonder what happened, she checked the smell of the water and realized that it was actually vinegar! Vinegar is a type of chemical called an acid. There are different ways in which substances can be classified such as solids, liquids, gases or elements, compounds, and mixtures. Similarly, acids and bases is another important classification that we are going to study in this chapter. In this chapter we will learn: What are acids and bases Different types of acids and bases. Ways of identifying them. A process called neutralization. The concept of salts How are acids and bases used in daily life. For thousands of years people have known that vinegar, lemon juice, and many other food items taste sour. However, it was not until a few 100 years ago it was discovered why these items taste sour. It is because they all contain a substance that is now known as acid. The word acid comes from the Latin word 'acere' which mean sour.

3 Acids are present in many of the common substances such as all sour tasting food items, some oils such as olive oil and even in our stomach! Strong acids like hydrochloric acid are present in our stomach that helps us digest food. Overview Acids and Bases Identification Acids are used to make many useful products such as detergents, paints, and dyes. They are also used to remove rust from iron and steel objects. Some acids such as ethanoic acid and citric acid are used as food preservatives. Neutralization Salts An equally important class of compounds is the bases. Bases, like acids, are common in our daily lives. For example, they are found in soaps, bleaches, floor cleaners, window cleaners, detergents, oven cleaners, talcum powder, toothpastes and medicine for indigestion. Soluble bases are called alkalis. Thus, an alkali is a base which is soluble in water, but not all bases are alkalis. There are other bases which are insoluble. Some common bases are used in the making of medicines, household cleaners, paper, water purification and in the construction of buildings. The base potassium hydroxide is used in alkaline batteries.

4 Overview Acids and Bases Identification Taste sour Acids Change blue litmus to red colour Solutions in water conduct electric current Corrode Metals Acids are Oxides Bases Bitter taste Change red litmus to blue colour Soapy feel Corrosive to the skin Bases are Hydroxides Neutralization Acids are classified into two types organic and inorganic, depending on whether they have carbon as one of the elements. For example, Amino Acids are organic acids and are one of the fundamental building blocks for proteins in our body. Salts Inorganic acids are also called mineral acids and are further subdivided into strong and weak acids. An acid or a base is called strong if they dissociate in water. Dissociate means the property of the substance to break into ions in water. Acids break into Hydrogen ions and Bases break into Hydroxide ions. Examples (Acids): Organic Acids Citric acid (lemons and oranges) Tannic acid (Tea, Tamarind) Formic acid (Ant stings) Lactic acid (Sour milk) Malic acid (Apples) Inorganic Acids Hydrochloric acid (HCl) Nitric acid (HNO3) Sulphuric acid (H2SO4) Carbonic acid (H2CO3) Phosphoric acid (H3PO4)

5 Bases are also classified as strong or weak bases. Examples of bases: Overview Caustic soda (NaOH) Ammonium Hydroxide (NH4OH) Caustic Potash (KOH) LIME WATER (Ca(OH)2) Acids and Bases An ion is a charged atom or molecule. It is charged because the number of electrons does not equal the number of protons in the atom or molecule. If the atom has more electrons than protons, it is a negative ion or ANION. If it has more protons than electrons, it is a positive ion or CATION. Identification Neutralization Salts When a strong acid is diluted with water it becomes a strong acid with low concentration but not a weak acid! We have seen there are strong acids and weak acids. But can we measure how strong or weak an acid or base is? A measure called ph is used for this purpose. It is a measure of the acidity or alkalinity of a substance similar to how temperature is a measure of the degree of hotness or coolness of a substance.

6 Overview It is a numerical scale from 0 to 14. The following figure shows different substances and their ph value. The lower the ph value, the more acidic the substance. Neutral solutions such as water have a ph of 7. Acids and Bases Identification Neutralization Salts Some of the commonly known food items such as lime juice, Cola drinks, coffee are acidic with a ph value below 5 and many of the fruits and green leafy vegetables are alkaline with a ph value higher than 6. You can view the ph value of different food items at this website. Importance of ph Substances in the body have different ph values. Acidic conditions in the stomach (ph ~ 1.5) and alkaline conditions in the small intestine (ph ~ 8.4) are needed for good digestion.

7 Fresh foods quickly go bad mainly due to microorganisms such as bacteria, present in the food. Acids can be used to preserve foods because microorganisms do not grow well in solutions of low ph. The ph of the soil is important for optimal plant growth. Most plants grow best when the soil has a ph of about 5.6, that is, soil that is weakly acidic. Overview Our hair, when smooth, strong and healthy is weakly acidic with a ph of about 5. Acids and Bases Having vinegar on your chips is not dangerous, but drinking lab acid like hydrochloric acid could be fatal. So, if we cannot taste all substances, how can we know if the substance is an acid or a base? For this, we use substances called Indicators. An indicator is a compound that will change colour in the presence of an acid or base. Acid rain describes any form of precipitation with high levels of nitric and sulfuric acids. It can also occur in the form of snow, fog, and tiny bits of dry material that settle to Earth. Rotting vegetation and erupting volcanoes release some chemicals that can cause acid rain, but most acid rain falls because of human activities. The biggest culprit is the burning of fossil fuels by coal-burning power plants, factories, and automobiles. Identification Neutralization Salts Natural Indicators: Many flowers, fruits, and vegetables are natural indicators for acids and bases, and we may find them in our kitchen or garden! For example, turmeric is an excellent indicator.

8 Overview Try this out: You can use a card paper that is covered with turmeric paste to make different decoration pieces. Turmeric paste turns red when it interacts with bases. But doesn t have any change when interacts with acids. You can use a cotton swab dipped in soap water to draw patterns on the turmeric covered card to get coloured designs! Acids and Bases Identification Red cabbage is another well-known indicator. You can view this video to understand how to use red cabbage as an indicator. Litmus as an Indicator Neutralization Litmus is a mixture of dyes obtained from lichens. It is the oldest forms of ph indicator, used to test materials for acidity. Salts Blue litmus paper turns red under acidic conditions and red litmus paper turns blue. Neutral litmus paper is purple. Some other commonly available indicators: Beets: when beet comes in contact with a basic solution, its colour changes from red to purple Red cabbage: Red cabbage juice turns to greenish yellow in basic solution. Curry Powder: Curry powder changes from yellow to red in basic medium. China Rose: A solution of china rose turns green in a basic solution and bright pink or magenta in an acidic solution. Cherries: Cherries change their colour from red to blue or purple in basic solution.

9 Synthetic indicators are those which are synthesized or prepared in laboratories. Examples: phenolphthalein, methyl orange, etc.? Overview Neutralization occurs when a base is reacted with an acid. When an acid and a base react with each other, the characteristic properties of both are destroyed, then water and salt are formed. This is called neutralization. Acids and Bases General formula for acid base reaction: Acid + Base = H2O + Salt. For example, HCl + NaOH H2O + NaCl Salt means any ionic compound formed from an acid/base reaction. Identification Neutralization Salts Fig 4: Salt farming in Vietnam Neutralisation is used in various fields as they are applied in agriculture, industries, health and even at home. For example,

10 Overview Acids and Bases Agriculture: It is used to treat acidic soil. Lime fertilisers such as powdered lime, CaO, limestone, CaCO3 or ashes of burnt wood are added to the soil to neutralise its acidity. Similarly, basic soil can be treated by compost as acidic gas of compost neutralises the alkalis in basic soils. Industries: effluents from the electroplating industry contain acids such as sulphuric acid. It is treated by adding lime to neutralise it before it is disposed into rivers and streams. Identification At home: Toothpastes and baking soda are some more commonly used examples of neutralization. Neutralization Health: Gelusil is a common antacid that uses bases to neutralize excessive acid in stomachs. Insect bites: bee stings or ant bites are acidic in nature and alkaline pastes can be used to neutralize the impact. Salts A salt is produced when an acid reacts with a base. The non-metallic ions of the acid and the metal ions of the base form the salt. Acid + Base -> Salt + Water Salts can be of several different colours. Usually, salts are odourless. They are neutral. Salts have ph = 7. Some common examples of Salts are Toothpaste, Kitchen Salt. Other uses of salts are as fertilisers in agriculture, as food preservatives, photography, etc. In U.S. only 6% salt is used in food; another 17% is used for deicing streets and highways in the winter months. Black salt is made in India by mixing salt water with harad seeds. After aviation fuel is purified, salt is mixed with it to remove all traces of water before it can be used.

11 Properties of Salt Most of the salts are soluble in water. (Except for some like calcium carbonate) Solution of salt in water acts as good conductors of electricity. Some of the salts are white crystals whereas some of them are coloured. (For example, copper sulphate is blue and ferrous sulphate is green) Two or more salts that crystallise to form a single substance are referred to as mixed salts. For example, Alum is a mixed salt. Overview Acids and Bases Identification Uses Without a sprinkle of salt, would you like to have your daily food? From a spot on our dining tables to their use in industries, salt has an important role to play. Let us see how a few salts help us today. Name of the Salt Common Name Uses Sodium Chloride Table Salt/Common Salt Major ingredient in cooking and acts as a food preservative Sodium Carbonate Washing Soda Used as a detergent Neutralization Salts Sodium Bicarbonate Baking Soda Used as a baking agent, in medicines, and as a fire extinguisher Calcium Carbonate Limestone Used in construction of buildings, purification of iron and also as a dietary supplement Potassium Aluminium Sulphate Alum Used in purification of water and in the textile industry

12 Overview Acids and Bases Identification Acid: Substances with ph of less than 7. Base: Substances with ph of greater than 7. Neutralisation: Reaction between acid and base to form water. Indicator: Solutions of substances that show different colours in acidic, basic and neutral solutions. Salt: A neutral substance formed along with water when an acid reacts with a base. Aqueous: Solution in which solvent is water. Dissociate: Split into separate smaller atoms/ions. Ion: An atom having positive or negative charge. Neutral Substances having a ph of 7. Neutralization Salts

13 Worksheet WS1 Introduction 1. The classification of matter is given in the following flowchart. Give two examples of each in the below table. Elements Compounds Homogenous Mixture Heterogeneous Mixture

14 WS2 Examples of Acids & Bases Household substances Whether Acid or Base Vinegar Washing Soda Lemon juice Milk Egg white Orange juice Tomato Baking Soda WS3 Acids and Bases 1. How would you usually identify an acidic food substance? 2. How would you usually identify a base present in your house?

15 3. Name the following acids. (Research on some questions for accuracy) a) The world s strongest acid (Research) b) The chemical name for Vitamin C (present in lemons and oranges) c) Powerful acid that can dissolve gold and the name literally means Royal Water (Research) d) Acid present in ant sting e) Acid present in our stomach f) Building block of proteins g) Acid present in apples 4. Write the names of acids present in these.

16 5. Two acids dissociate in the following manner in water. Identify them as strong and weak. Example 1 Example 2 6. See the following symbol. Why is the word danger associated with acid? 7. Sulphuric acid is used in the manufacture of wide variety of products name them.

17 8. Name the acids used in food preservation. WS4 ph Scale 1. What is a ph scale? 2. Fill in the blanks. solutions such as water have a ph of 7. Acids have a ph less than. The the ph number of an acid the stronger the acid is. Therefore, an acid like hydrochloric acid with a ph of 1 is a lot more than vinegar with a ph of 3. Alkalis have a ph than Mark Acids and Bases on the diagram given below.

18 WS5 Use of Indicators 1. Activity to perform (Turmeric Indicator) Take a tablespoonful of turmeric powder. Add a little water and make a paste. Make turmeric paper by depositing turmeric paste on blotting paper/filter paper and drying it. Cut thin strips of the yellow paper obtained. Put a drop of soap solution on the strip of turmeric paper. What do you observe? Perform the same activity with the following substances: Tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water, Baking soda and Ammonia based cleaner. Turmeric gives brownish red colour in basic medium and yellow in acidic medium. Note your observations in the following table. Solution Colour of turmeric indicator Nature of Solution Acidic / Basic

19 2. Can we use tongue to distinguish all kinds of substances? Why? 3. Complete the following table. Indicator Original Colour Acid Base Red Litmus Blue Litmus Turmeric China Rose Beetroot

20 4. The figures below show two situations. The left shows the colour and ph before using an indicator and the right picture shows the colour and ph after using the indicator. Name the possible synthetic indicators. WS6 What is Neutralization? 1. Neutralization experiment that can be done at home: Combine baking soda and lemon juice. The citric acid of the lemon juice is neutralized by the sodium bicarbonate of the baking soda in a bubbling reaction that creates salty water. a) Complete the following reaction. Acid + Base = b) What is this reaction called? c) Give one example of the above reaction. d) Write 2 properties of the resultant substances formed.

21 WS7 Applications of Acids and Bases 1. Give reasons for the following: a) We use antacids to get relief from acidity in stomach. b) Basic soil can be treated by using a compost of rotting vegetables or leaves. c) Using toothpaste helps prevent tooth decay. d) Calamine solution is put on the ant sting.

22 WS8 Synthesize 1. Differentiate between acids, bases, and salts. Property Acids Bases Salts

23 2. Classify the following substances as acids, bases or neutral. Acids Bases Salts 3. Which of the following statements is correct about an aqueous solution of an acid and of a base? a.higher the ph, stronger the acid c.lower the ph, stronger the base b. Higher the ph, weaker the acid d.lower the ph, weaker the base 4. The ph of the gastric juices released during digestion is a. less than 7 b. more than 7 c. equal to 7 d. equal to 0 5. Which of the following phenomena occur, when a small amount of acid is added to water? a. Ionisation b. Neutralisation c. Dilution d. Salt formation

24 6. When some alkali is added to a solution of an acid, will the ph value of the solution increase or decrease? 7. When some acid is added to a solution of an alkali, will the ph value of the solution increase or decrease? 8. Why will there be some acids on our teeth after a meal? 9. Antacids contain weak alkalis. Why do we use weak alkalis and not strong alkalis to neutralize the excess acid in our stomach?

25 10. Rohit was playing in the ground when he was bit by ants and was in a lot of pain. His mother checked at home but did not find the required medicine. She quickly brought some toothpaste and applied it to the bite. Why do you think she did this? Do you think it would help Rohit? 11. Answer the following briefly: a) Mention the importance of ph in everyday life. b) Name few natural and synthetic indicators. c) What are indicators?

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