Science textbook solutions for Class 7 science Chapter 14 Elements, Compounds and Mixtures


Important points to remember :
  1. Many kinds of matter  found in nature are in pure form, that is, they contain only one constituent. In scientific language, matter made of  only one constituent is called ‘substance’, for example,  gold, diamond, water, chalk. 
  2. Other kinds of matter are made of two or more substances. They are called  ‘mixtures’. 
  3. A  substance whose molecules  are  made  of one  or more  atoms  which are exactly  alike,  is called  an  element. 
  4. scientists have discovered 118 elements.  Of these, 92 elements  occur in nature, while the remaining are man-made. Hydrogen, oxygen, nitrogen, carbon, iron, mercury, copper  are a few of the  important  natural  elements. 
  5. Democritus named the small  particles  of elements ‘atom’ because in the Greek language  atomos  means indivisible. 
  6. In 1803, John Dalton proposed his theory stating that atoms cannot  be created  or divided into  smaller  particles  or destroyed. 
  7. Oxygen occurs  in  nature  in  the gaseous state. Two atoms of oxygen  are joined to form a molecule of oxygen, which has an independent existence. 
  8. The substance formed by a chemical combination of two or more elements is a compound. 
  9. The properties of  the original metal can be modified by mixing one or more elements in it. Such a mixture of metals  is called  an alloy. Brass, steel, twenty-two  carat  gold  are a few examples  of alloys. 
  10. Water is a compound. One molecule of water is made of two atoms of hydrogen and one atom of oxygen.

Question 1: Who are my companions ?
Group 'A'                  Group 'B'
1. Stainless steel     a) Non-metal
2. Silver                    b) compound
3. Bhajini​ mixture  c) Mixture
   for milling
4. Salt                        d) Element
5. Coal                       e) Alloy
6. Hydrogen            f) Metal

Answer :
1. Stainless steel     -   e) Alloy
2. Silver                    -   f) Metal
3. Bhajini​ mixture  - c) Mixture
   for milling
4. Salt                        -  b) compound
5. Coal                       - a) Non-metal
6. Hydrogen            - d) Element


Question 2: Write the names of elements from the following symbols :
Zn , Cd, Xe, Br, Ti, Cu, Fe, Si, Ir, Pt.
Answer :
Zn- Zinc
Cd- Cadmium
Xe- Xenon
Br- Barium
Ti- Titanium
Cu- Copper
Fe- Iron
Si- Silicon
Ir- Iridium
Pt- Platinum


Question 3: What are the molecular formulae of the following compounds ?
Hydrochloric acid, sulphuric acid, sodium chloride , glucose, methane.
Answer :
Hydrochloric acid- HCl
Sulphuric acid- H2SO4
Sodium chloride- NaCl
Glucose- C6H12O6
Methane- CH4


Question 4: Give scientific reasons.
a) Buttermilk is churned to get butter. 
Answer : Churned buttermilk is the fluid that remains when the complete fat is removed after churning butter. The churned buttermilk is the watery substance and is the end product of butter making. Nowadays it is used as condensed or dried so that it can be used in the frozen and baking industries.

b) In chromatography, the ingredients of a mixture rise up to a limited height water rises up to upper end of the paper .
Answer : These properties are different for different substances and are mutuall opposite to each other. It is the reason why all the components of the mixture rise upto a limited height when water rises up to the upper end of the paper.

c) A wet cloth is wrapped around a water storage container in summer.
Answer : Wet. cloth is wrapped around a water storage container in summer because in summer water container gets hot and water too so to keep water cool we're cloths are wrapped to reduce the amount of warming the water.


 Question 5: Explain the difference .
a) Metals and non-metals
Answer  : Metals are good conductors of heat and electricity but non-metals are poor conductors of heat and electricity. Metals are ductile in nature i.e. it can be drawn into thin wires. But non-metals are not ductile. Metals are malleable but non-metals are not malleable.

b) Mixture and compounds
Answer : Mixture is composed of molecules of different types. A compound can only be separated through chemical means. A compound is a pure substance that contains 2 or more elements chemically combined together while a mixture are formed when two substances are added together without chemical bonds being formed.

c) Atoms and molecules
Answer : Elements, which are composed of a single type of atom, contain different numbers of protons. Molecules are formed when two or more atoms bond together. If molecules contain atoms of different elements, that substance is known as a compound.

d) Separation by distillation and by separating funnel 
Answer : Answer: A mixture of two immiscible liquids can be separated by using separating funnel , and the separation of immiscible liquids depends on their density. Distillation is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation.


Question 6. Write answers to the following questions in your own words. 

a) How are the  components of mixtures separated by simple  methods ? 
Answer : Components of a solution composed of a non-volatile solid solute and a liquid solvent can be separated by distillation. Mixtures of liquids with reasonably different boiling points can also be separated by distillation. Solutions with several components can be separated by paper or thin-layer chromatography.
Below are some common separation methods:

  1. Paper Chromatography. This method is often used in the food industry. 
  2. Filtration. This is a more common method of separating an insoluble solid from a liquid.
  3. Evaporation. 
  4. Simple distillation.
  5. Fractional distillation.


b) Which elements  (metals and nonmetals), compounds and mixtures do we use in our day-to-day life ? 
Answer : Examples of compounds we use in our daily life are- salt (NaCl), sugar (C12H22O11), baking powder (NaHCO3), vinegar (CH3COOH), ethanol etc. Examples of elements we use in our daily life are- aluminium, copper, iron, silver, gold etc.

c)  In everyday life,  where and for what purpose do we use centrifugation ? 
Answer : Uses include separating blood components, isolating DNA, and purifying chemical samples. Medium-size centrifuges are common in daily life, mainly to quickly separate liquids from solids. Washing machines use centrifugation during the spin cycle to separate water from laundry, for example.

d) Where are the methods of separation by distillation and by separating funnel used ? Why ? 
Answer : The mixture is heated and the liquid with the lower boiling point forms a vapour first, then condenses in the condenser and is collected, leaving the second liquid or solid in the distilling flask. Distillation is used to separate miscible liquids (mix together completely).

e) Which precaution  will you take while using the methods of distillation and separation by separating funnel ?
Answer : The funnel should be tightly fixed to the stand in an errect position. The liquid in the funnel should be left undisturbed to allow it to separate. The stopper should be removed carefully without disturbing the setup.


# let's recall 
1. How many different states of matter are there ? Name them. 
Answer : The five phases of matter. There are four natural states of matter: Solids, liquids, gases and plasma. The fifth state is the man-made Bose-Einstein condensates. In a solid, particles are packed tightly together so they don't move much.

2. What brings about a change of state of matter ? 
Answer : Adding or removing energy from matter causes a physical change as matter moves from one state to another. For example, adding thermal energy (heat) to liquid water causes it to become steam or vapor (a gas). And removing energy from liquid water causes it to become ice (a solid). Most liquids contract as they freeze.

3. What are the properties of matter ?
Answer :  The properties of matter include any traits that can be measured, such as an object's density, color, mass, volume, length, malleability, melting point, hardness, odor, temperature, and more.

4. Do all substances have the same properties ?
Answer : Every sample of a given substance has the same properties because a substance has a fixed, uniform composition. The properties of a mixture can vary because the composition of a mixture is not fixed.


# Use your brain power
1. Which of the following are mixtures -water, sherbet, iron, steel, coal, air, salt, copper, brass, soil.
Answer : water, sherbet, air, soil are the mixture.

2. Which  elements  are present in air ? 
Answer : The molecules of two different elements, nitrogen and oxygen, make up about 99 percent of the air. The rest includes small amounts of argon and carbon dioxide. (Other gases such as neon, helium, and methane are present in trace amounts.) Oxygen is the life-giving element in the air.

3. Is carbon dioxide  an element ? 
Answer NO, A molecule of the compound carbon dioxide contains one atom of the element carbon and two atoms of the element oxygen. Each oxygen atom shares a double bond with the carbon atom. Carbon is the sixth element in the periodic table and occurs in pure form as coal and diamonds.

4. Are the  atoms  of different  elements similar  or dissimilar ?
Answer : Atoms of different elements are similar. Because they all are made of electrons, protons, and neutrons. Only difference is There numbers are different.


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