ICSE Chemistry Class 8 Notes Chapter 8 Water
→ No life is possible without water.
→ Water is essential for cleanliness. All the the big cities like Bombay, Kolkata, Chennai are situated on the banks of oceans.
→ Water is chemically of 2 parts, hydrogen gas and oxygen [H2O].
→ Potable water is that water which is fit for drinking. It is free from germs, bacterial, transparent and free from harmful salts.
→ Pure water freezes at 0°C and boils at 100°C.
→ Water is ‘universal solvent’, it can dissolve every substance.
→ Distilled water does not contain minerals and dissolved air and is of flat taste.
→ Contaminated water contains disease causing germs, which enter the water through industrial waste decaying matter.
→ Water bom diseases are gastroenteritis, diarrhoea, typhoid and jaundice.
→ When sugar is dissolved in water sugar solution is obtained. Here sugar is solute “The substance which dissolves in solvent
(water)”. “Solvent is the liquid which dissolves the solute.” Solution is the homogeneous mixture of solute and solvent.
→ Solubility : “Maximum amount of solute that can be dissolved in l00g of solvent at specific temperature.”
→ Miscible mixture : “Mixture of two liquids which get mixed (dissolve).” e.g. alcohol and water.
→ Immiscible mixture : “A mixture of two liquids which do not get mixture (dissolve).” e.g. oil and water.
→ Solubility of gasses decreases with increase in temperature and increases with increase in pressure.
→ Dissolved air contains O2, N2 and CO2 which gives higher percentage of oxygen.
→ Dissolved mineral, salts and air gives good taste to water and provides essential minerals for living bodies for their growth, increases the metabolic activity of the system.
→ Dissolved oxygen for respiration in aquatic plants and animals: CO2for photosynthesis in aquatic plants and N2 for nourishment for submerged plants.
→ True solution : “Homogeneous mixture”, particles cannot be seen by naked eye or even by microscope. Can pass through filter paper, do not show Tyndall effect. Size of particles is less than 1 nm.
→ Colloidals : ‘Heterogeneous mixture”, size particles between 1 nm and 100 nm. Can pass through filter paper but cannot pass through a semipermeable membrane, do not settle down, not visible to naked eye but can be seen under microscope show Brownian moment.
→ Suspension : “Homogeneous mixture of undissolved particles in dispersion medium”, are of bigger size, more than 100 nm, visible to the naked eye, cannot pass through filter paper, settle down after sometime, e.g. sand, mud, red-blood corpuscles.
→ Saturated solution : “A solution, which cannot dissolve more of solute at a given temperature.”
→ Unsaturated solution is a solution which can dissolve more of solute at a given temperature.
→ Solubility can be increased or saturated solution becomes unsaturated by increasing temperature or adding more of solvent.
→ Super saturated solution is a solution which can hold more of solute at a given temperature than that present in a saturated sol.”
→ Crystals are solids having-definite regular shapes, symmetrical arrangement and sharp edges.
→ Crystallisation : The process of separation of crystal from a hot saturated solution.
→ Water of crystallisation is the number of water [molecules that combine chemically – indefinite] molecular proportion with the
concerned salt in the crystalline state.
→ Efflorescent substances are substances which lose wholly or partially their water and become powdery.
Na2CO2.10H2O (washing soda), Epsom salt MgSO3.H2O.
→ Deliquescent substances : “Absorb moisture from atmosphere, dissolve in it and form a saturated solution.” NaOH,
CaCl2.
→ Hygroscopic substances are amorphous solids or liquids, absorb moisture from air and do not change their original state. H2SO2(conc.), CaO(quick lime).
→ Drying agents : Absorb or remove moisture from other substances. Cone H2SO4, P2O5, CaO. They remove chemically combined water from compound due to their strong affinity for water.
→ Soft water : “Water that gives lather with soap.”
Hard water : “Does not give lather with ordinaiy soap.”
→ Temporary hardness is due to presence of bicarbonates of calcium or magnesium and hardness can be removed by boiling. Bicarbonate breaks into carbonate and CO2and H2O.
→ Permanent hardness is due to presence of CaSO4 or CaCl2 or MgSO4 or MgCl2. It cannot tumsoft on boiling. This hardness can be removed by adding washing soda Na2CO3.
→ Hardwater is unfit for drinking, for laundries as clothes do not get cleaned and soap is wasted, is unfit for industrial use as it leaves behind a crusty boiler scale or fur in boiler.
Check Your Progress (Page 100)
Answer the following.
Question 1.
What are the main sources of water?
Answer:
Surface water is found in the oceans, seas, rivers, lakes, streams, etc. Sources of groundwater include springs and wells.
Question 2.
Water dissolves all nitrate salts and most chloride salts. True or false?
Answer:
True.
Question 3.
Differentiate between miscible and immiscible liquids.
Answer:
Liquids that dissolve in water are called miscible liquids that do not dissolve in water are called immiscible liquids.
Question 4.
What type of solution is sugar in water – homogeneous or heterogeneous?
Answer:
A solution of sugar in water is a homogeneous mixture.
Question 5.
Can you see the particles of a suspension?
Answer:
The particles of suspensions can be seen with the naked eye.
Check Your Progress (Page 107)
Answer these following.
Question 1.
Calcium chloride is a hygroscopic substance. True or false?
Answer:
True.
Question 2.
What will happen when sodium reacts with cold water?
Answer:
Sodium react with cold water to liberate hydrogen gas and form the corresponding metal hydroxide.
Question 3.
What is the chemical formula of aluminium oxide?
Answer:
The chemical formula of aluminium oxide is Al2O3.
Question 4.
Write the reaction between magnesium oxide and water.
Answer:
Question 5.
What are the types of hardness of water?
Answer:
There are two types of hardness of water :
- Temporary hardness of water.
- Permanent hardness of water.