Tag: chemistry

Questions Related to chemistry

Final setting time of cement should not be more than:

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. 1 hour

  2. 2 hours

  3. 5 hours

  4. 10 hours

Show answer
Correct Option: D
Explanation:

Initial setting time of cement should be more than 45 mins. This is to allow enough time for transportation and placing of concrete before setting starts. Final setting time should be less than 375 minutes. 375 minutes is equivalent to 10.25 hours. Hence, the final setting time of cement should not be more than 10 hours.

Which of the following is not present in portland cement?

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. $Ca _3Al _2O _6$

  2. $Ca _3SiO _5$

  3. $Ca _2SiO _4$

  4. $Ca _3(PO _4) _2$

Show answer
Correct Option: D
Explanation:
  • Portland cement gets its strength from chemical reactions between the cement and water. The process is known as hydration.
  • Chemical composition of portland cements are-
  1.  $Ca _3Al _2O _6$
  2. $Ca _3SiO _5$
  3. $Ca _2SiO _4$
  4. $Na _2O$
  5. $K _2O$
  6. $CaSO _4.2H _2O$
  7. $Ca _4Al _2Fe _2O _{10}$
  • Hence option D is not present in portland cement.

When plaster of paris comes in contact with water it sets into a hard mass. The composition of the hard mass is:

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. $CaSO _4.H _2O$

  2. $CaSO _4.Ca(OH) _2$

  3. $CaSO _4.2H _2O$

  4. $CaSO _4.2Ca(OH) _2$

Show answer
Correct Option: C
Explanation:

When plaster of paris comes in contact with water it sets into a hard mass which is called gypsum. Its composition is $CaSO _4.2H _2O$

Plaster of paris is obtained by heating gypsum at 373 K.

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. True

  2. False

Show answer
Correct Option: B
Explanation:

Plaster of Paris is obtained by heating gypsum or calcium sulphate dihydrate at 373 K in a kiln.
$\displaystyle CaSO _4 \cdot 2H _2O \xrightarrow {373 K} CaSO _4 \cdot \dfrac {1}{2} H _2O$

Plaster of Paris has the formula:

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. $CaSO _4.(1/2)H _2O$

  2. $CaSO _4.H _2O$

  3. $CaSO _4.2H _2O$

  4. none of the above

Show answer
Correct Option: A
Explanation:

The composition of plaster of paris is $CaSO _4.(1/2)H _2O$ and the composition of gypsum is $CaSO _4.2H _2O$.

Hence the correct answer is option (A).

Gypsum is added to Portland cement to:

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. fasten the process of setting

  2. slow down the process of setting

  3. improve the colour of the cement

  4. increase the melting point of cement

Show answer
Correct Option: B
Explanation:

Gypsum slows down the process of setting of the cement so that it gets sufficiently hardened.

The average composition of Portland cement is:

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. $CaO : 40 - 50\%$, $SiO _2 : 30 - 40\%$, $Al _2O _3$ and $Fe _2O _3 : 10 - 20\%$

  2. $CaO : 50 - 60\%$, $SiO _2 : 20 - 25\%$, $Al _2O _3 : 5 - 10\%$, $MgO : 2 - 3\%, Fe _2O _3 : 1 - 2\%$ and $SO _3 : 1 - 2\%$

  3. $SiO _2 : 40 - 50\%$, $CaO : 30 - 40\%$, $Al _2O _3 : 10 - 20\%$

  4. $CaO : 50\%$, $SiO _2 : 50\%$

Show answer
Correct Option: B
Explanation:

Port land cement consists of raw materials mainly compounds of lime mixed with silica and alumina.

$ CaO \rightarrow 50-60$%
$ SiO _2 \rightarrow 20-25$%
$ Al _2O _3 \rightarrow 5-10$%
$ Fe _2O _3 \rightarrow 1-2$%
$ MgO \rightarrow 2-3$%
$ SO _3 \rightarrow 1-2$%
and few amount of oxides of iron, magnesium and sulphur.

Cement is formed by burning of _________ in a hot kiln.

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. limestone and clay

  2. limestone and graphite

  3. chalk and graphite

  4. sulphur and limestone

Show answer
Correct Option: B
Explanation:

Cement is formed by burning of graphite and limestone in a hot kiln. Many other materials such as shells , chalk or marl combined with shale, clay, slate blast furnace slag , silica sand and iron ore are also used in manufacture of cement.

Police crime detective makes an impression of the track of a motor vehicle tire on a mold made from:

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. quick-drying clay

  2. plaster of Paris

  3. lycopodium powder

  4. wax

Show answer
Correct Option: B
Explanation:

The mold made from plaster of paris can be conserved for a long time and thus crime detectives make use of it so that they get exact impression and can preserve it for future use.

Cement, the important building material is a mixture of oxides of several elements. Besides calcium, iron and sulphur, oxides of elements of which of the group (s) are present in the mixture?

chemistry the s-block elements (alkali and alkaline earth metals) some important compounds of calcium some important compounds of magnesium and calcium compounds of s block elements

  1. Group 2

  2. Groups 2, 13 and 14

  3. Groups 2 and 13

  4. Groups 2 and 14

Show answer
Correct Option: B
Explanation:
Cement is a mixture of calcium (present in group 2) and aluminium silicates (contains group 13 and 14 elements).