Living Science 2020 2021 for Class 7 Science Chapter 7 - Acids, Bases And Salts

Answer:

In our daily diet, we usually have lactic acid from curd and citric acid from oranges, lemons to enjoy the sour taste.

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Acids are corrosive in nature. They can react with metals and corrode them, that's why acids are not stored in metal containers.

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Phenolphthalein and methyl orange are the two substances which have a different colour in acidic and basic solutions. Phenolphthalein shows colourless in acidic and pink in basic solution. Methyl orange shows red in acidic and pink in basic solution. They both are commonly known as the indicator.

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When acids react with bases, their acidic nature is cancelled out. This type of reaction is called a neutralization reaction. The reaction of acids and bases produces a neutral substance known as 'salt'.

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Acids obtained from living things are known as organic acids, while the acids obtained from minerals are known as mineral acids. Example of organic acids: citric acid, lactic acid, amino acid, etc. Example of mineral acids: hydrochloric acid, sulphuric acid, etc.

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A base is any substance that neutralizes the acid. It may or may not be soluble in water. Those bases which are soluble in water are called alkalis. Thus, an alkali is a water-soluble base. For example sodium hydroxide, potassium hydroxide, etc.
Therefore, it is said that all alkalis are bases but all bases are not alkalis.

Answer:

Due to the overuse of chemical fertilizers, the soil becomes acidic due to which growth of plants decreases. To maintain the neutrality of the soil, the farmer should use either quick lime or slaked lime. Quick lime and slaked lime are common bases, which can neutralize the acidity of the soil.

Answer:

Caustic soda(sodium hydroxide) is used in the manufacture of soap.

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Our stomach releases a hydrochloric acid for digestion of food. But sometimes excess of hydrochloric acid is released which causes indigestion and pain in the stomach. The excess of hydrochloric acid in our stomach is known as acidity. Acidity can be removed by using antacids like "magnesium hydroxide", which neutralizes the acid in our stomach. 

Answer:

If we replace the hydrogen of an acid with metal, it forms a salt. e.g. HCl is an acid, if we replace hydrogen of this acid with a metal like sodium(Na), it will form NaCl. NaCl is a salt of sodium metal, which is a neutral compound. It can be prepared by neutralizing HCl with a base like NaOH.

Answer:

When an acid reacts with a base, it neutralizes the base and forms salt and water as the products. The reaction involved is known as neutralization reaction. 

Example: ${\mathrm{NH}}_4\mathrm{OH}+\mathrm{HCl}\to {\mathrm{NH}}_4\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$
 

Answer:

(a) Nitrates are obtained from nitric acid.

(b) Carbonates are obtained from carbonic acid.

(c) Chlorides are obtained from hydrochloric acid.

(d) Sulphates are obtained from sulphuric acid.

Answer:

Baking soda is basic in nature because it is made up of a strong base(NaOH) and weak acid(H₂CO₃). The chemical name of baking soda is sodium bicarbonate and its formula is NaHCO₃.

Answer:

(d) digene
Digene is not an acid–base indicator but an antacid (base). It is used as a medicine to reduce acidity of the stomach by neutralising it.

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(a) nitric acid
Nitric acid is highly corrosive in nature. It can burn our skin severely.

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(c) copper carbonate
Copper carbonate is not a base but an ionic salt.

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(d) copper
Copper is a less reactive metal; therefore, it cannot displace and liberate hydrogen from acids.

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(c) baking soda
Baking soda is the common name for sodium bicarbonate.

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(a) blue litmus red
Lime juice contains citric acid; therefore, it will turn a blue litmus paper red.

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d) none of the above
Sodium chloride is a salt, and salts are neutral in nature. Thus it will induce no change to the acid-base indicators.

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(d) have all these properties
Acids have a sour taste and are extremely corrosive in nature. They also dissolve easily in water.

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(b) are soluble in water
All alkalis are water soluble bases.

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Salt is a mixture of acid and base, so among the given options sodium chloride is a salt.
While Hydrochloric acid is an acid and sodium hydroxide is a base.

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If a liquid turns blue litmus red, then it is an acid.

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Methyl orange has red colour in an acidic solution.

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False.
When water is added to a strong acid, the reaction will evolve heat which is extremely dangerous. Hence, to dilute a strong acid, the acid should be slowly added to the water followed by regular stirring.

Answer:

Sulphuric acid is a mineral acid.

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Milk contains lactic acid.

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Carbon dioxide is released when sodium carbonate reacts with dilute hydrochloric acid.
               
${\mathrm{NaCO}}_{3 }+ \mathrm{dilHCl} \underset{}{\to } \mathrm{NaCl} + {\mathrm{H}}_2\mathrm{O} + {\mathrm{CO}}_2$

Answer:

The reaction between an acid and a base which results into salt and water is called the neutralization reaction.

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Sulphuric acid is used to manufacture superphosphate. It is done by reacting sulphuric acid with powdered rock phosphate.

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An alkali is a base that is soluble in water.

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Ammonia is a base which has no metal atom in its molecule. As an alternative of metal, ammonia has the ammonium radical $\left({\mathrm{NH}}_4^{+}\right)$ in it.

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Sodium hydroxide is a strong base, and not an acid.

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Turmeric turns pale yellow in an acidic solution.

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Sodium hydroxide is the base which is used to manufacture soaps.

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Magnesium hydroxide is the base (antacid) which can be used as a medicine to cure stomach pain due to indigestion. It is also called the milk of magnesia.

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True
When a metal reacts with an acid, it replaces and liberates hydrogen from the acid to form salts.

Example: $\mathrm{Magnesium} + \mathrm{Sulphuric} \mathrm{acid} \to \mathrm{Magnesium} \mathrm{sulphate} + \mathrm{Hydrogen} \mathrm{gas}$

Answer:

Alkali is a base; therefore, it will turn a red litmus paper blue.

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Acid-base indicators are special substances which are used to detect the acidity or basicity of other substances. The change in colour of the  indicators help us in understanding which substance is acidic and which one is basic.

Example: Litmus paper is a natural acid-base indicator.

Answer:

Acidic substance: Lemon juice is an acidic substance, as it contains citric acid.
Basic substance: Baking soda is basic in nature, as it contains sodium bicarbonate.
Neutral substance: Common salt is a neutral substance, as it is made of sodium chloride.

Answer:

Zinc will react with dilute sulphuric acid to form zinc sulphate salt and hydrogen gas. The reaction is given below:

              $\mathrm{Zn} + {\mathrm{H}}_2{\mathrm{SO}}_{4 }\underset{}{\to } {\mathrm{ZnSO}}_4 + {\mathrm{H}}_2$
      

Answer:

Metal carbonates react with dilute acids to form salt, water and carbon dioxide gas. Thus, carbon dioxide can be prepared by reacting any metal carbonate with any dilute acid. 
Example: Carbon dioxide gas is liberated when sodium carbonate reacts with dilute hydrochloric acid.
                             ${\mathrm{Na}}_2{\mathrm{CO}}_3 + 2\mathrm{HCl} \underset{}{\to 2}\mathrm{NaCl} + {\mathrm{H}}_2\mathrm{O} + {\mathrm{CO}}_2$

Answer:

Examples for strong acids: Sulphuric acid and nitric acid.
Examples for weak acids: Citric acid (present in citrus fruits) and acetic acid (vinegar).

Answer:

While biting, insects inject formic acid into the skin. This causes irritation to the skin, and sometimes results into rashes. To get relief from this, a base like baking soda is applied over the bitten area. As bases neutralize acids, the baking soda (being a base) will neutralize the injected formic acid, thereby reducing the irritation. Thus, a base should be applied to the skin to get instant relief from an ant bite.

Answer:

(a) Sulphuric acid (${\mathrm{H}}_2{\mathrm{SO}}_4$):
1. It is used to manufacture fertilizers such as ammonium sulphate and superphosphate.
2. It is also used to manufacture detergents, paints, drugs, plastics, paper, etc.
 
(b) Hydrochloric acid (HCl): 
1. It is widely used in the oil industries to dissolve the rocks containing oil.
2. It is also used to manufacture aqua regia. The aqua regia comprises of 3 parts of hydrochloric acid and 1 part of nitric acid. It is mainly used to dissolve noble metals such as gold and platinum.
 
(c) Nitric acid (${\mathrm{HNO}}_3$):
1. It is used to manufacture fertilizers such as ammonium nitrate.
2. It is also used to manufacture explosives such as TNT (trinitrotoluene) and nitroglycerine.

Answer:

Neutralization reaction is the process in which an acid reacts with a base to produce salt and water while evolving a good amount of heat. In this reaction, both the acid and base loose their properties to produce a new substance which is neutral in nature, i.e., the salt formed will neither be acidic nor basic.
$\mathrm{Acid} + \mathrm{Base} \to \mathrm{Salt} + \mathrm{Water} \left[\mathrm{with} \mathrm{evolution} \mathrm{of} \mathrm{heat}\right]$

The preparation of common salt (sodium chloride) involves the neutralization reaction between hydrochloric acid and sodium hydroxide (base). The acid and base reacts to produce sodium chloride (salt), water and heat.

The reaction is represented by the following equation:
                      $\mathrm{HCl} + \mathrm{NaOH} \underset{}{\to } \mathrm{NaCl}+ {\mathrm{H}}_2\mathrm{O}$

Answer:

Acids can be classified into mineral acids and organic acids.
Most of the mineral acids are strong and corrosive in nature. They are extremely dangerous to handle. Strong acids (highly concentrated due to less amount of water in them) can easily melt paper, wool, wood and cloth. If they come in contact with our skin, they cause severe burns to it (carbonic acid is an exception in this case). Since srong acids can corrode even metals like iron, aluminium, etc., they are stored in glass containers. Hence, we should be highly cautious while handling strong acids.

Examples: Hydrochloric acid, nitric acid, sulphuric acid, etc.

Organic acids are naturally occurring weak acids. They are non-corrosive in nature, and are safe to handle even in their concentrated state.

Examples: Acetic acid (present in vinegar), Lactic acid (present in milk), Citric acid (present in citrus fruits), etc.

Answer:

Bases are substances that have a soapy feel and bitter taste. Bases help in neutralizing the acidity by forming salt and water.
Like acids, bases can also be strong and weak. Sodium hydroxide and potassium hydroxide are strong bases which can burn our skin; but magnesium hydroxide and copper hydroxides are weak bases, and safe to handle even in their concentrated state.

Some of the properties of bases are listed below:
1. Most of the bases are metal hydroxides.
2. They are bitter in taste.
3. They are soapy in touch.
4. They turn red litmus blue.
5. Their reaction with acids gives salt and water.

Answer:

Two important uses of each base are stated below:

(a) Calcium hydroxide [$\mathrm{Ca}(\mathrm{OH}{)}_2$]:
1. It is used to whitewash buildings; it is also used as an alternative to cement in low-budgeted construction.
2. It is also used to neutralise the acidity of soil. Because of the over usage of chemical fertilisers, soil becomes acidic and hence largely affects the growth of crops. Thus, calcium hydroxide is added to the soil to neutralise its acidity, thereby ensuring the healthy growth of the crops.
 
(b) Ammonium hydroxide (${\mathrm{NH}}_4\mathrm{OH}$):
1. It is widely used to produce fertiliser like ammonium nitrate.
2. It is also used to produce nylons, plastics, dyes, etc.
 
(c) Sodium hydroxide (NaOH):
1. It is extensively used in the production of soaps.
2. It is used to produce various medicines, rayon, paper, etc.

Answer:

Salts which are neutral substances can be prepared from the following methods:

1. Neutralization Reaction – It is the reaction between an acid and a base to produce salt, water and heat.

Example: Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride (common salt), water and heat.

The reaction is represented by the following equation:

                         $\mathrm{HCl} +\mathrm{NaOH} \underset{}{\to \mathrm{NaCl} + {\mathrm{H}}_2\mathrm{O}}$

2. Reaction of metals with acids - Metals react with acids to replace hydrogen from the acids, thereby producing salts and liberating hydrogen gas.

Example: Zinc reacts with dilute sulphuric acid to form zinc sulphate salt liberating hydrogen gas.

The reaction is given below:

             $\mathrm{Zn} + {\mathrm{dilH}}_2{\mathrm{SO}}_4 \underset{}{\to } {\mathrm{ZnSO}}_4 + {\mathrm{H}}_2\mathrm{O}$

Answer:

The sodium salts of acids are called soaps. A soap is prepared by boiling vegetable oil or fats from animals along with sodium hydroxide (NaOH).

We can prepare a soap in the laboratory by following the below mentioned procedure.

Required materials:

20ml of castor oil (we can also use coconut oil or any other oil)
Half teaspoon of sodium hydroxide pellets
1 teaspoon of common salt (sodium chloride), and
Glass rod for stirring

Procedure :

• Take the 20ml castor oil in a beaker.
• Mix ½ teaspoon of sodium hydroxide pellets with about 20ml of water to prepare the sodium hydroxide solution.
• Now, mix the oil with the sodium hydroxide solution.
• Heat the mixture and allow it to boil for about 5 to 10 minutes by constantly stirring it with the glass rod.

• The soap formed can be separated from the mixture by adding a tea spoon of common salt (sodium chloride) in the beaker.
• After cooling, the solid soap gets separated from the mixture by floating on the top. Thus, a soap is prepared in the laboratory.

Answer:

The name of a salt is derived on the basis of the reactants involved in its preparation. The salts are named after the metal supplied by the base and the radical supplied by the acid.

Example 1: Sodium chloride (common salt) is formed from hydrochloric acid and sodium hydroxide. The name ‘sodium’ is supplied by the metal of the sodium hydroxide, and the name ‘chloride’ is supplied by the acid, which is a negative radical.
  $\mathrm{HCl} + \mathrm{NaOH} \underset{}{\to } \mathrm{NaCl} + {\mathrm{H}}_2\mathrm{O}$
 
Example 2: Ammonium sulphate (salt) is formed from sulphuric acid and ammonium hydroxide. The name ‘ammonium’ is supplied by the ammonium radical of the base, while the name ‘sulphate’ is supplied by the negative radical of sulphuric acid.
 ${\mathrm{H}}_2{\mathrm{SO}}_{4 }+ {\mathrm{NH}}_4\mathrm{OH} \underset{}{\to {\left( {\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_{4 }+ {\mathrm{H}}_2\mathrm{O}}$

Answer:

Turmeric is a natural acid-base indicator. It turns red in a basic solution and pale yellow in an acidic solution.
Therefore, the turmeric stain will turn red after coming in contact with the soap, indicating the basic nature of the soap (generally soaps are bases – containing sodium hydroxide).
Turmeric contains a type of yellow pigment which is responsible for the staining the clothes.
These stains can be removed using the following methods:
1. Fresh stains can be removed easily by immediately washing them with a soap. However, if there is a delay in washing, the stains will be hard to remove in a single wash. In this case, the stains need to be washed repeatedly using a soap.
2. By applying lemon juice or vinegar on these stains along with a dish soap can help remove the turmeric stains in a better way. 
3. By soaking the stained part in the soda water (baking soda + water) for half an hour, will help in removing the turmeric stains easily.

Answer:

Weak acids are completely different from dilute acids. Weak acids are non-corrosive in nature even in their highly concentrated state. Generally, all organic acids like citric acid, lactic acid, oxalic acid, etc., are weak acids. However, dilute acids are also non-corrosive in nature but are formed by mixing concentrated acids with a large amount of water. For example, dilute hydrochloric acid, dilute sulphuric acid, etc. Dilute acids can include both strong and weak acids.

Answer:

Acids are extremely corrosive in nature; therefore, they can cause extensive damage to the living organisms present in water. Also, if the wastes are allowed to flow into the water bodies without treatment, they can pollute the water, making it unfit for drinking. Hence, the wastes should be neutralised by bases before releasing them into water bodies. The base calcium hydroxide ($\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$) is widely used to treat wastes, containing acids, from the industries.

Answer:

Indicators have different colours in acids and bases. For eg., Methyl orange turns red in acidic solution and yellow in basic solution.
A weak acid produces a very few numbers of hydrogen ions as compared to strong acid. Due to the low availability of hydrogen ions, the reaction between the indicator and the weak acid does not occur properly, so colour change is weak and slow. But in case of a strong acid, the concentration of hydrogen ions is much higher, hence the indicator solution changes its colour quickly to strong red color.