Maharashtra Board Class 9 Science Chapter 14 Substances in Common Use PDF Download

Substances In Common Use

We use various substances in day to day life. We have previously learnt in some detail about a few of them, their uses and constituents and the method of their preparation.

Can You Recall?

1. What are the important substances that we use in day to day life? For what purposes do we use them?

2. How are the various substances in day to day use classified from the scientific point of view?

Important Salts In Daily Life

The names of some substances in everyday use are given below. Classify them into groups like acids, bases, metals, non-metals and salts.

Substances: Table salt, soap, toothpaste, baking soda, water, curds, milk, alum, iron, sulphur, washing powder.

What Are Salts?

The ionic compounds which do not contain H+ and OH- ions and contain only one kind of cation and anion are called simple salts. For example, Na₂SO₄, K₃PO₄, CaCl₂.

Inorganic substances occur naturally in the form of salts rather than acids or bases. About 80 million tons of salts are added every year to seawater. Therefore, the sea is said to be a rich source of salts. In fact, the sea is a rich source of several salts of various elements such as chlorine, sodium, magnesium, potassium, calcium, bromine. However, we also use other salts apart from these in day to day life. Let us learn more about them.

Do You Know?

The important salts found in sea water are:

1. Sodium chloride

2. Magnesium chloride

3. Magnesium sulphate

4. Potassium chloride

5. Calcium carbonate

6. Magnesium bromide

Try This

Prepare saturated solutions of some salts and put 2-3 drops of the universal indicator in them and note your observations in the table alongside.

Teacher's Note

Salts are very important in our daily life. For example, table salt is used in every Indian kitchen every day to make food tasty.

Exam Trick

Remember: Sea water is salty because it has many salts mixed in it, just like how milk is white because of fat mixed in it.

Points to Remember

Simple salts do not have H+ or OH- ions in them.
The sea is a rich source of many different salts.
Salts can be neutral, acidic, or basic depending on which acid and base made them.
About 80 million tons of salts are added to seawater every year.

Sodium Chloride (Table Salt - NaCl)

Table salt, or common salt, which gives a salty taste to food, is the most used of all salts. Its chemical name is sodium chloride. Sodium chloride is formed by a neutralization reaction between sodium hydroxide and hydrochloric acid. We have already seen that this is a neutral salt and that the pH value of its aqueous solution is 7.

Can You Tell?

1. What is the strip shown below? What is it used for?

2. How is it determined whether a substance is acidic, basic or neutral?

3. Make a list of substances in day to day use in accordance with their pH value (1 to 14).

Do You Know?

Salt is also obtained from a certain type of rock. This salt is called rock salt. The mineral halite and Himalayan rock salt are some examples of rock salt. This salt is used to treat many diseases.

The 25% aqueous solution of salt is called saturated brine. When 1/5 of this solution is evaporated the dissolved salt gets crystallized and salt gets separated from the solution.

Properties and Uses

1. Common salt is a colourless and crystalline ionic compound. There is no water of crystallization in its crystalline structure.

2. It is a neutral salt, salty in taste.

3. This compound is used for the production of salts like Na₂CO₃, NaHCO₃.

4. When an electric current is passed through a saturated solution of sodium chloride (brine) it is electrolysed and hydrogen gas is released at the cathode while chlorine gas is released at the anode. This method is used for production of chlorine gas. In this method an important basic compound NaOH is formed in the cell.

\[2\text{NaCl} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{Cl}_2 \uparrow + \text{H}_2 \uparrow\]

5. When salt is heated to a high temperature (about 800°C), it melts. This is called the fused state of the salt.

6. When fused salt is electrolysed, chlorine gas is released at the anode and liquid sodium metal, at the cathode.

Teacher's Note

Table salt is used in every home in India to add taste to food. It also helps our body stay healthy by balancing water and salt levels.

Exam Trick

Remember: NaCl is neutral because it comes from a strong acid (HCl) and a strong base (NaOH). If both are strong, the salt is neutral.

Points to Remember

Table salt has no water of crystallization in its crystal structure.
Saturated brine is 25% salt solution in water.
Sodium chloride is used to make other important salts.
When electric current passes through salt solution, chlorine gas and hydrogen gas are released.

Sodium Bicarbonate (Baking Soda - NaHCO₃)

Your mother brings cake on your birthday or makes it at home. She also makes crisp bhaji. Have you ever asked your mother what makes the cake porous or the bhaji crisp?

Mother adds baking soda in the batter. Baking soda is a white noncrystalline powder. Its chemical name is sodium hydrogen carbonate or sodium bicarbonate and its molecular formula is NaHCO₃.

Research

What are the constituents of baking powder? Where is the baking powder used?

Properties and Uses

1. NaHCO₃ reacts with moist litmus paper and red litmus turns blue which means that it is basic in nature.

2. It is used to make bread, cake, dhokla.

3. Being basic in nature it is used to reduce acidity in the stomach.

4. NaHCO₃ is used to make the active substance CO₂ in the fire extinguisher.

5. Baking soda is used to clean an oven.

Teacher's Note

Baking soda is used in every Indian kitchen to make bread and cakes fluffy and soft. It is also used to cure acidity in the stomach.

Exam Trick

Remember: Baking soda makes things porous and light, just like how it makes your cake fluffy when you bake it.

Points to Remember

Baking soda is a white powder with no crystals.
It turns red litmus paper blue, so it is basic.
It is used in cooking to make cakes and bread rise.
It helps reduce stomach acidity because it is basic.

Bleaching Powder (Calcium Oxychloride CaOCl₂)

Try This

Take a piece of coloured cloth. Put some saturated solution of bleaching powder on a small part and observe. What change takes place in the colour of the cloth?

Tap water has a typical strong odour in the rainy season. Have you experienced it? Water in a swimming pool also has the same odour. It is the odour of the chlorine gas used to destroy the microbes in the water. Chlorine gas is a strong oxidizing agent and therefore, it has a strong disinfecting as well as bleaching action.

Chlorine is inconvenient to handle because of its gaseous state. Instead, the solid bleaching powder which has the same effect is more convenient to use. Bleaching powder undergoes slow decomposition due to the carbon dioxide in air and chlorine gas is released. Bleaching powder gets its property because of this release of chlorine gas.

Bleaching powder is obtained by the reaction of chlorine gas with slaked lime.

\[\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}\]

Find Out

1. About various types of bleaching powder available in the market.

2. What distinguishes these different types?

Properties and Uses

1. Bleaching powder is a yellowish white coloured solid substance.

2. Its chemical name is calcium oxychloride.

3. It has a strong odour of chlorine gas.

4. It is used for disinfection of drinking water at the water works and the water in the swimming pool.

5. It is used for bleaching of cloth.

6. It is used for disinfection by the road side and garbage sites.

7. Dilute sulphuric acid and dilute hydrochloric acid react rapidly with bleaching powder to release chlorine gas completely.

\[\text{CaOCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{Cl}_2 \uparrow + \text{H}_2\text{O}\]

8. Calcium oxychloride reacts slowly with carbon dioxide to form calcium carbonate and chlorine.

Teacher's Note

Bleaching powder is used in Indian villages and cities to clean water and prevent diseases. You can smell it when swimming pools are cleaned.

Exam Trick

Remember: Bleaching powder smells like chlorine because chlorine gas is slowly released from it in the air.

Points to Remember

Bleaching powder is yellowish white and has a strong chlorine smell.
It is made by mixing chlorine gas with slaked lime.
It is used to clean drinking water and swimming pools.
It kills germs and also bleaches colored cloth.

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