Online solved CBSE Guess questions - Class VII, Science - Acids, Bases and Salts


Class VII CBSE Science - Acids, Bases and Salts

CBSE Guess - CBSE Guide - Solved CBSE Questions

Question: How would you prepare 
(i) crystals of copper sulphate from metallic copper ?
(ii) Calcium chloride from Marble (calcium carbonate) ?
Answer: (i) Take a solution of sulphuric acid and put some copper turnings to it. The solution becomes blue due to the formation of copper sulphate, by the crystallisation of this solution and crystals of copper sulphate can be obtained:
Cu + H2SO4 -------> CuSO4 + H2
(ii) Calcium chloride can be obtained by treating marble (Calcium Carbonate) with hydrochloric acid.
CaCO3 + 2 HCL -------> CaCl2 + H2 + CO2

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Question: What is an oxide?
Answer: An oxide is the compound of an element with oxygen. For example - water is an oxide of hydrogen, Rust is an oxide of iron.

Question: Name there types of oxides with their properties.
Answer: Three types of oxides are -
(a) Acidic Oxides:
The oxides which on treatment with water form am acids are called acidic oxides. For example,
CO2 + H2 ------> H2CO3 (carbonic acid)
Non-metals burn in oxygen and form acidic oxides. Such oxides turn blue litmus red like - Carbon dioxide (CO2), Sulphur dioxide (SO2).   
(b) Basic or Metallic Oxides:
The oxides which on treatment with water form alkalines are known as basic oxides. Metallic oxides are mostly basic oxides which turn red litmus blue. For example - Na2O, MgO etc.
Na2O + H2O ----> 2NaOH
(c) Amphoteric Oxides:
The oxides which show the properties of both acidic and basic oxides are known as amphoteric oxides for example, Al2O3, SO2 etc.

Question: What is acid ? (Write definition of acids with their examples) How acids are produced?
Answer: Acids are the chemicals which contain hydrogen atom and are sour in taste.
Acids are produced when oxides of non-metals react with water. Examples of some common acids are - HCl (Hydrochloric Acid), H2SO4 (Sulphuric Acid), HNO3 (Nitric Acid).

Question: Why are mineral acids dangerous?
Answer: Mineral Acids are very strong acids and are mostly used in industries other than laboratories.  Being strong acids they are corrosive in nature. Such acids need to be handled very carefully as they can easily corrode skin, human body, metals, clothes and all coming in contact. Hence, mineral acids are dangerous. Some examples of mineral acids are - hydrochloric acid, sulphuric acid, nitric acid, etc.

Question: What are organic acids ? Name some organic acids, which are used as food ingredients.  
Answer: Acids which contain -COOH functional groups are termed as organic acids. Organic acids are weak acids and are present in animal and plant materials.
Examples of organic acids used as food ingredients - acetic acid, citric acid (C6H8O7), tartaric acid.

Question: What are the differences between Dilute and Concentrated acids ?
Answer: Dilute acid - A dilute acid solution is one which contains only small amount of solute or its solution diluted to a definite strength is called dilute acid.
Concentrated Acid - A concentrated acid is one which contains a high proportion of solute.

Question:  Answer to these questions -
(a) Name the acid present in vinegar used in kitchen
(b) Name the acid present in lemon
(c) Which is called "the king of chemicals"
(d) Name the property due to which salts absorb water from atmosphere
Answer: (a) acetic acid. (b) citric acid. (c) sulphuric acid. (d) deliquescence.  

Question: What are bases ? Give the characteristics of bases.
Answer: Bases are the hydroxide of metals, which give up hydroxide ion after dissociation in aqueous solution.
Characteristics of Bases -
  • Bitter in taste.
  • Turn red litmus to blue.
  • React with acid to form salt and water.
  • Bases are soluble in water and known as alkalies.
Question: What are Acids ? Give some characteristics of acids.
Answer: For definition of acids please refer to our post
Characteristics of Acids
  • Sour in taste.
  • Turn blue litmus to red.
  • Dilution of acid in water is an exothermic reaction (heat evolves).
  • React with metals to evolve hydrogen gas.
2HCl + 2Na -------> 2NaCl + H2
H2SO4 + Zn -------> ZnSO4 + H2
  • React with bases to form salt and water. The process called, neutralisation.
HCl + NaOH ------> NaCl + H2O
H2SO4 + Ca(OH)2 ------> CaSO4 + 2H2O
  • React with basic oxides to form salt and water.
CaO + 2HCl -----> CaCl2 + H2O
Na2O + H2SO4 -----> Na2SO4 + H2O
  • React with carbonates to form salt, water and carbon dioxide.
CaCO3 + 2 HCl -----> CaCl2 + H2O + CO2
MgCO3 + H2SO4 -----> MgSO4 + H2O + CO2

Click here for further solved questions, important "Fill in the blanks" and more from Chapter 5 Acids, Bases and Salts

 NCERT Solution, CBSE Notes on Acids, Bases and Salts - further study


CBSE to offer two versions of Maths paper for classes 9, 10, 11 and 12 from 2019 subject to NCERT and UGC approvals


The Central Board of Secondary Education (CBSE) has proposed to introduce two versions of the maths paper — one difficult and one relatively easy — for classes 9, 10, 11 and 12 from 2019 subject to NCERT and UGC approvals.

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The Central Board of Secondary Education (CBSE) is exploring the possibilities and proposals to introduce two versions of the mathematics question paper so that students who aren’t keen to take up the subject in higher studies don’t have to sit for the tougher edition in the Board Exam.

A number of school boards overseas follow this practice. For instance, the Cambridge International General Certificate of Secondary Education (IGSCE) has a core curriculum in each subject which is generally within the ability range of a large majority of the students. It also offers an extended curriculum that is designed for students who are more academically able. International Baccalaureate (IB) also provides this flexibility.

In the last week of June the governing body of CBSE approved a proposal to have two options - standard mathematics and advanced mathematics - instead of the single math paper now on offer that is a mix of standard and advanced mathematics.

Sources said the logic is that students who don’t intend to pursue the study of mathematics would be able to avoid the tag that they aren’t good at the subject if they score low marks in it at the high-school level. For instance, students who want to take up engineering and sit for the IIT Join Entrance Examination (JEE Main), JEE Advanced or any other similar exam will need to study mathematics at the higher level. Those who aspire to study medicine and sit for the National Eligibility cum Entrance Test (NEET) or any other MBBS admission tests will have the choice of studying the subject at the easier level.

Under the proposed system, the advanced mathematics paper will have more content and a higher difficulty level. According to one member, "The idea is that students of arts and commerce may take normal (standard) mathematics. Advanced mathematics, dealing with higher maths, may be pursued by pure science students. It would help them in their career as well".

The CBSE has referred the matter to a committee that will design the syllabuses for both papers. However, the board will have to seek approval from the National Council of Educational Research and Training (NCERT) before rolling out the new syllabuses.
CBSE had also introduced a separate paper - applied mathematics - for Classes XI and XII from this year under the vocational stream. This paper, which has a practical component of 20 per cent, will continue.

Last year, the NCERT had conducted a National Achievement Survey to assess the skills of Class III, V and VIII students in mathematics, environment studies and language. The survey had showed that children feared mathematics. The NCERT is currently revising its school curriculum, which is followed by over 15 school boards across the country.

The CBSE governing body member said the final decision on the alternative maths papers might be announced by the end of 2018. Another governing body member said the syllabuses for all classes would be "rationalized" so that they were in tune with the class-wise "expected learning outcome" conceived by the NCERT six months ago. It has been learnt from reliable sources that there will be no reduction in the syllabuses instead; there may be some additional content so that the students do not lag behind.

An NCERT faculty member said the council had received 33,000 suggestions from the public on the revision of the school curriculum. Most of the respondents have recommended the addition of content.

Once the proposal is approved by the CBSE internally, it will be shared with the National Council of Educational Research and Training (NCERT) which is the body responsible for framing school curriculum. To ensure that universities accept the two different levels of mathematics being offered at the high school level, the proposal will also need to be approved by the University Grants Commission (UGC).


Class 8 NCERT Mathematics Guide: Rational Numbers - properties and important points


Class 8, Mathematics - NCERT Guide

RATIONAL NUMBERS


Properties of Closure
Whole Numbers
>> Whole Numbers are closed under addition. In general, a + b is a Whole Numbers for any two Whole Numbers a and b.
>> Whole Numbers are not closed under subtraction. 5 - 7 = -2, this is not a Whole Number.
>> Whole Numbers are closed under multiplication. In general, if a and b are any two Whole Numbers, their product ab is Whole Number.
>> Whole Numbers are not closed under division..

Integers
>> Integers are closed under addition. In general, a + b is an integer for any two integers a and b.
>> Integers are closed under subtraction. For any two integers a and b, a - b is an integer.
>> Integers are closed under multiplication. For any two integers a and b, a x b is an integer.
>> Integers e not closed under division.

Rational Numbers
>> Sum of two rational numbers is again a rational number. Rational Numbers are closed under addition.
>> The difference of two rational numbers is a rational number. Rational numbers are closed under subtraction
>> Rational numbers are closed under multiplication.
>> Rational numbers are not closed under division.      

Properties of Commutativity
Whole Numbers
>> Addition is commutative.
>> Subtraction is not commutative.
>> Multiplication is commutative.
>> Division is commutative.

Integers
>> Addition is commutative.
>> Subtraction is not commutative.
>> Multiplication is commutative.
>> Division is commutative.

Rational Numbers
>> Addition is commutative.
>> Subtraction is not commutative.
>> Multiplication is commutative.
>> Division is not commutative.

Properties of Associativity
Whole Numbers
>> Addition is associative.
>> Subtraction is not associative.
>> Multiplication is associative.
>> Division is not associative.

Integers
>> Addition is associative.
>> Subtraction is not associative.
>> Multiplication is associative.
>> Division is not associative.

Rational Numbers
>> Addition is associative.
>> Subtraction is not associative.
>> Multiplication is associative.
>> Division is not associative.

Points to Remember
>> Zero is called the identity for addition of rational numbers. It is additive identity for integers and whole numbers. 
>> 1 is multiplicative identity for rational numbers.
>> The additive inverse of the rational number a / b is (- a / b) and vice-versa.
>> The multiplicative or reciprocal inverse of the rational number a / b  is c / d  if, a / b x c / d = 1
>> Rational numbers can be represented on a number line.
>> Between any two rational numbers there are countless rational numbers.

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