CBSE Class 7 Science Advanced Chemistry Water Very Short Answer Questions Set 03

Very Short Answer Questions

Question 1. State whether the following statement is true or false: Hard water is fit for dyeing clothes.
Answer:
False. Hard water is not suitable for dyeing, because the dye does not fix properly on the fabric.
In simple words: Dye does not stick well in hard water.

Teacher's Note:
1. Hardness spoils washing, dyeing and boilers.
2. Soft water is used in industries.

Question 2. State whether the following statement is true or false: Cation exchange resin is regenerated by passing HCl solution.
Answer:
True. When the cation exchanger has lost its \(\ce{H+}\) ions, passing HCl solution (or NaCl solution) restores them.
In simple words: \(\ce{HCl}\) refills the resin with hydrogen ions.

Teacher's Note:
1. The anion resin is regenerated with \(\ce{NaOH}\).
2. Regeneration makes the resin reusable.

Question 3. State whether the following statement is true or false: Salt is added to pure water to increase its electrical conductivity.
Answer:
True. Pure water is a poor conductor. Adding a salt (or an acid or alkali) supplies ions and increases the conductivity.
In simple words: Ions make water conduct.

Teacher's Note:
1. In electrolysis a little acid is added for the same reason.
2. Pure water has almost no ions.

Question 4. State whether the following statement is true or false: Two volumes of hydrogen and one volume of oxygen compose one volume of water.
Answer:
False. Two volumes of hydrogen and one volume of oxygen combine to give two volumes of water (steam): \(\ce{2H2 + O2 -> 2H2O}\).
In simple words: The product is two volumes, not one.

Teacher's Note:
1. Count the coefficients in the equation.
2. Volumes are measured at the same temperature and pressure.

Question 5. State whether the following statement is true or false: Permanent hardness is removed by Clark's method.
Answer:
False. Clark's method removes temporary hardness, which is caused by bicarbonates.
In simple words: Clark's method works only on temporary hardness.

Teacher's Note:
1. Permanent hardness needs washing soda or ion exchange.
2. Slaked lime is used in Clark's method.

Question 6. State whether the following statement is true or false: Specific heat of water is 1 cal g⁻¹ °C⁻¹.
Answer:
True. It takes 1 calorie to raise the temperature of 1 g of water by 1°C.
In simple words: This is the highest specific heat among common substances.

Teacher's Note:
1. It makes water a good coolant.
2. Use it in heat calculations.

Question 7. ______ and ______ are two types of underground water.
Answer:
Well water and spring water
In simple words: Underground water is drawn from wells or comes out as springs.

Teacher's Note:
1. Both are filtered by soil.
2. Both contain dissolved salts.

Question 8. Hydrated sodium aluminium silicate is called ______.
Answer:
permutit
In simple words: Permutit is a sodium aluminium silicate used for softening water.

Teacher's Note:
1. Formula: \(\ce{Na2Al2Si2O8.xH2O}\).
2. It is used in the permutit process.

Question 9. The exhausted permutit contains ______ and ______ ions in place of \(\ce{Na+}\) ions.
Answer:
\(\ce{Ca^2+}\) and \(\ce{Mg^2+}\)
In simple words: The sodium ions have been swapped for calcium and magnesium ions.

Teacher's Note:
1. Then the permutit stops working.
2. \(\ce{NaCl}\) solution regenerates it.

Question 10. In sedimentation process, alum ______ the process of precipitation of suspended impurities.
Answer:
hastens (speeds up)
In simple words: Alum makes the suspended particles settle faster.

Teacher's Note:
1. Some bacteria are trapped as well.
2. It is added with lime in the mixing chamber.

Question 11. Addition of ______ releases ______ which helps in killing the harmful bacteria present in water.
Answer:
bleaching powder releases chlorine
In simple words: Chlorine from bleaching powder kills germs in water.

Teacher's Note:
1. This step is called chlorination.
2. It is the last step of purification.

Question 12. Addition of acid to water, ______ the conductivity.
Answer:
increases
In simple words: Acids give ions in water, so more current can pass.

Teacher's Note:
1. Pure water conducts poorly.
2. That is why acidulated water is used in electrolysis.

Question 13. In electrolysis of water ______ gas is liberated at cathode.
Answer:
hydrogen
In simple words: The cathode is the negative electrode, and hydrogen collects there.

Teacher's Note:
1. Oxygen is liberated at the anode.
2. Hydrogen burns with a pop sound.

Question 14. The change of water into water vapour at 30°C at slow rate is called ______.
Answer:
evaporation
In simple words: Slow change to vapour below the boiling point is evaporation.

Teacher's Note:
1. The rate increases with temperature.
2. Boiling is a fast change at 100°C.

Question 15. Small water droplets associated with dust particles form larger aggregates called ______.
Answer:
micelles
In simple words: These groups of droplets and dust are seen as clouds.

Teacher's Note:
1. Micelles are formed in the water cycle.
2. When they get heavy, they fall as rain.

Question 16. Percentage of water in animals varies from ______ to ______.
Answer:
70% to 90%
In simple words: Animal bodies are mostly water.

Teacher's Note:
1. Water is the main part of living cells.
2. Plants have a high percentage too.

Teacher's Note:
1. Learn Table 5.1 of the chapter.
2. Match cause to pollutant.

Teacher's Note:
1. Link each method to its key feature.
2. Resins are used in ion exchange.

Teacher's Note:
1. Entry (a), 250 cal/g, matches nothing.
2. Recall the numbers from the properties of water.