Cls 10 Sci Chp 2 Solsn

  Class 10 Science  

  Chapter 2 Acids, Bases and  Salts  

Intext Questions 

Page No: 18

1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Ans:

If the colour of red litmus does not change then it is acid. If the colour of red litmus changes to blue then it is base. If there is slight change in the colour of red litmus (such as purple) then it is distilled water.

Page number 22

1. Why should curd and sour substances not be kept in brass and copper vessels

Ans:

Curd and sour food substances contain acids; these acidic substances combine with metal. This reaction turns food to poison which damage people’s health.

2. Which gas is usually liberated when an acid reacts with a metal ? Illustrate with an example. How will you test for the presence of this gas ?

Ans:

Hydrogen gas is usually liberated when an acid reacts with metal.

Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.

H2SO4 + Zn → ZnSO4 + H2 ↑

We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

Ans:

CaCO3 (s) + 2HCl (Aq) → CaCl2( Aq) + CO2 (g) + H2O (l)

Page number – 25

1. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans:

H+ ions in aqueous solution are responsible for acidic character. HCl, HNO3, etc. give H+ ions in water while alcohol and glucose do not give H+ ion in water. Therefore, alcohol and glucose do not show acidic character.

2. Why does an aqueous solution of an acid conduct electricity?

Ans:

The aqueous solution of an acid conducts electricity due to the presence of charged particles called ions in it.

3. Why does dry HCl gas not change the colour of the dry litmus paper?

Ans:

Dry HCl gas does not give H+ ions and therefore does not change the colour of dry litmus paper.

4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans:

because if water is added to concentrated acid, it release huge amount of heat which may result in explosion and can cause acid burns on face, clothes and body parts.

5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Ans:

There is a fixed number of hydronium ions per volume of the solution. On dilution, the number of hydronium ions per volume decreases and concentration decreases.

6. How is the concentration of hydroxide ions (OH–) affected when excess base is dissolved in a solution of sodium hydroxide ?

Ans:

The concentration of hydroxide ions (OH−) would increase when excess base is dissolved in a solution of sodium hydroxide.

Page number – 28

1. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?

Ans:

A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Since solution A has more hydrogen ion concentration, solution A is acidic and solution B is basic.

2. What effect does the concentration of H+ (aq) ions have on the nature of the solution ?

Ans:

More the concentration of H+ ions, higher the acidic nature of the solution.

3. Do basic solutions also have H+ (aq) ions ? If yes, then why are these basic ?

Ans:

Yes, H+ ions are far less in number than OH– ions that is responsible for their basic nature.

4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate) ?

Ans:

If the soil is too acidic (having low pH) then it is treated with materials like quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).

Page Number: 33

1. What is the common name of the compound CaOCl2 ?

Ans: Bleaching powder.

2. Name the substance which on treatment with chlorine yields bleaching powder.

Ans: Slaked lime Ca (OH)2.

3. Name the sodium compound which is used for softening hard water.

Ans: Sodium carbonate.

4. What will happen if a solution of sodium hydrogen carbonate is heated. Give the equation of the reaction involved ?

Ans:

Solution of sodium hydrogen carbonate on heating gives sodium carbonate and carbon dioxide gas is evolved.

5. Write an equation to show the reaction between plaster of Paris and water.

Ans:

Exercise questions

1. A solution turns red litmus blue, its pH is likely to be

a) 1      (b) 4       (c) 5       (d) 10

Ans: (d) 10

2. A solution reacts with crushed-egg shells to give a gas that turns lime water milky. The solution contains

(a) NaCl   (b) HCl    (c) LiCl    (d) KCl

Ans:

(b) HCl

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HC1. If we take 20 mL of the same solution of NaOH, the amount of HC1 solution (the same solution as before) required to neutralise it will be

(a) 4 mL   (b) 8 mL    (c) 12 mL   (d) 16 mL

Ans:  (d) 16 mL

4. Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic    (b) Analgesic

(c) Antacid       (d) Antiseptic

Ans:  (c) Antacid

5. Write word equations and then balanced equations for the reaction taking place when

(a) Dilute sulphuric acid reacts with zinc granules.

(b) Dilute hydrochloric acid reacts with magnesium ribbon.

(c) Dilute sulphuric acid reacts with aluminium powder.

(d) Dilute hydrochloric acid reacts with iron filings.

Ans:

a) Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)

b) Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)

c) 2Al (s) + 3H2SO4 (aq) → Al2 (SO4)3 (aq) + 3H2 (g)

d) 2Fe (s) + 6HCl (aq) → 2FeCl3 (aq) + 3H2 (g)

6. Compounds such as alcohol and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.

Ans:

> Take solutions of alcohols and glucose.

> Fix two nails on a cork, and place the cork in 100 mL beaker.

> Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in the given Figure.

> Now pour alcohol in the beaker and switch on the current.

> The bulb does not glow.

> Repeat the experiment with glucose. The bulb does not glow in this case also.

> This means no ions or H+ ions are present in the solution.

> This shows that alcohols and glucose are not acids.

7. Why does distilled water not conduct electricity, whereas rainwater does ?

Ans:

> Distilled water does not contain any ionic compounds in it.

> Rainwater has dissolved acidic gas such as carbon dioxide from the air and that forms carbonic acid. presence of acids, rainwater can conduct electricity.

8. Why do acids not show acidic behaviour in the absence of water ?

Ans:

The acid produces hydrogen ions only in the presence of water.

9. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9 respectively. Which solution is

(a) Neutral

(b) Strongly alkaline

(c) Strongly acidic

(d) Weakly acidic

(e) Weakly alkaline

Arrange the pH in increasing order of hydrogen ion concentration.

Ans:

(a) D  (b) C  (c) B  (d) A  (e) E

Increasing order of hydrogen ion concentration

11 < 9 < 7 < 4 < 1

i. e., C < E < D < A < B

10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why ?

Ans:

The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.

11. fresh milk has a pH of 6. How do you think the pH will change as it turns into curd ? Explain your answer.

Ans:

pH of milk falls below 6 as it turns into curd due to the formation of lactic acid during this process. Lactic acid present in it reduces its pH value.

12. A milkman adds a very small amount of baking soda to fresh milk.

(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline ?

(b) Why does this milk take a long time to set as curd ?

Ans: 

a) because in alkaline condition, milk does not set as curd easily.

b) The alkaline milk takes a longer time to set into curd because the lactic acid being formed has to first neutralise the alkali present in it.

13. Plaster of Paris should be stored in a moisture proof container. Explain why?

Ans:

Because it absorbs water from moisture and turn into hard substance (Gypsum).

14. What is a neutralisation reaction ? Give two examples.

Ans:

The reaction between an acid and a base to form salt and water is called a neutralisation reaction.

Examples:

Cls 10 Sci Chp 1 slun

Class 10 Science

Chapter 1 Chemical Reactions and Equations

Questions and Answers 

Page Number: 6

1. Why should a magnesium ribbon be cleaned before burning in air ?

Ans: Because Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide (MgO) layer and it prevent further reactions with Oxygen, therefore necessary to clean the ribbon to remove the layer of MgO.

2. Write a balanced equation for the following chemical reactions.

i) Hydrogen + Chloride -> Hydrogen chloride

ii) Barium chloride + Aluminium sulphate   -> Barium sulphate + Aluminium chloride

iii) Sodium + Water -> Sodium hydroxide + Hydrogen

Ans:

i) H2 + Cl2 → 2HCl

ii) 3BaCl2 + Al2(SO4)3 →3BaSO4 + 2AlCl3 

iii) 2Na + 2H2O → 2NaOH + H2

3. Write a balanced chemical equation with state symbols for the following reactions

i) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and solution of Sodium chloride.

ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution to produce Sodium chloride solution and water.

Ans:

i) BaCl2 + Na2SO4 → BaSO4 + 2NaCl

ii) NaOH + HCl → NaCl + H2O

Page number – 10

1. A solution of a substance ‘X’ is used for whitewashing.

(i) Name the substance ‘X’ and write its formula.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Ans:

i) The substance ‘X’ which is used in whitewashing is quick lime or Calcium Oxide and its formula is CaO.

ii) CaO + H2O → Ca(OH)2

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas

Ans:

Water contains two parts of hydrogen and one part oxygen. Therefore, during the electrolysis of water the amount of hydrogen gas collected in one of the test tubes is double than  oxygen.

Page number – 13

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Ans:

When an iron nail dipped in the copper sulphate solution, iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes to the formation of light green solution of iron sulphate. The reaction is:

Fe + CuSO4 → FeSO4 + Cu

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Ans:

Sodium hydroxide and hydrochloric acid react to form sodium chloride and water.

NaOH + HCl -> NaCl + H2O

3. Identify the substances that are oxidized and that are reduced in the following equation.

i) 4Na(s) + O2(g) → 2Na2O(s)

ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Ans:

i) Oxidized substance : Na, 

  Reduced substance: O2

ii) Oxidized substance : H2

    Reduced substance: CuO

Exercise Questions 

1. Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) → 2Pb(s) + CO2(g)

(a) Lead is getting reduced

(b) Carbon Dioxide is getting oxidised

(c) Carbon is getting oxidised

(d) Lead oxide is getting reduced

       (i) (a) and (b)

       (ii) (a) and (c)

       (iii) (a), (b) and (c)

       (iv) all

Ans: (i) (a) and (b)

2. Fe2O3 + 2Al → Al2O3 + 2Fe

The above reaction is an example of a

1. Combination reaction.
2. Double displacement reaction.
3. Decomposition reaction.
4. Displacement reaction.

Ans: 4. Displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

1. Hydrogen gas and Iron chloride are produced.
2. Chlorine gas and Iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

Ans: 1. Hydrogen gas and Iron chloride are produced.

4. What is a balanced chemical equation? Why should a chemical equation be balanced?

Ans: A balanced equation is the number of different atoms in reactant and product sides are equal. Balancing chemical equation is necessary for follow the Law of Conservation of mass.

5. Translate the following statements into chemical equations and balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(c) Barium chloride reacts with aluminium sulphate to give Aluminium chloride and a precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and Hydrogen gas.

Ans:

(a) 3H2 (g) + N2 (g) → 2NH3(g)

(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)

(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)

(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

6. Balance the following chemical equations.

(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O

(b) NaOH + H2SO4 → Na2SO4 + H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

(d) BaCl2 + H2SO4 → BaSO4 + HCl

Ans:

(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

7. Write the balanced chemical equations for the following reactions :

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Ans:

(a) Ca (OH)2 + CO2 → CaCO3 + H2O

(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag

(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu

(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

8. Write the balanced chemical equation for the following and identify the type of reaction in each case :

(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium

(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)

(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)

(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Ans:

(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s)

Type : Double displacement reaction

(b) ZnCO3 (s) → ZnO (s) + CO2 (g)

Type : Decomposition reaction

(c) H2 (g) + Cl2 (g) → 2HCl(g)

Type : Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)

Type : Displacement reaction

9. What does one mean by exothermic and endothermic reactions ? Give examples.

Ans:

Exothermic reactions : The reactions in which heat is evolved are known as exothermic reactions.

Ex:

(i) C (s) + O2 (g) → CO2 (g) + Heat

(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat

Endothermic reactions : The reaction in which heat is absorbed are known as endothermic reactions.

Ex:

(i) C (s) + 2S (s) → CS2 (l) – Heat

(ii) N2 (g) + O2 (g) → 2NO(g) – Heat

10. Why is respiration considered an exothermic reaction ? Explain.

Ans:

Because in respiration oxidation of glucose takes place which produces large amount of heat energy.

C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy

11. Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.

Ans:

In a decomposition reaction, a single compound breaks down to produce two or more simpler substances.

For example:

in a combination reaction, two or more substances simply combine to form a new substance.

For example:

12. Write one equation each for decomposition reactions in which energy is supplied in the form of heat, light or electricity.

Ans:

13. What is the difference between displacement and double displacement reactions? Write relevant equations for the above.

Ans:

In displacement reactions, a more reactive metal displaces a less reactive metal from its solution.

For example,

Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)

In double displacement reactions, two reactants in solution exchange their ions.

For example,

AgNO3(aq) + NaCl (aq) → AgCl(s) + NaNO3 (aq)

14. In the refining of Silver, the recovery of silver from Silver nitrate solution involves displacement reaction by Copper metal. Write down the reaction involved.

Ans:

Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

15. What do you mean by a precipitation reaction? Explain by giving examples.

Ans:

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

For Example:

Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2NaCl (aq)

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

16. Explain the following in terms of gain of oxygen with two examples each.

(a) Oxidation

(b) Reduction

Ans:

(a) Oxidation : The addition of oxygen to a substance is called oxidation.

Example :

4Na(s) + O2(g) → 2Na2O(s)

H2S + O2 → H2O + SO2

(b) Reduction : The removal of oxygen from a substance is called reduction.

Example :

CuO(s) + H2(g) → Cu(s) + H2O(l)

2HgO → 2Hg + O2

17. A shiny brown coloured element ‘X’ on heating in the air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Ans:

Element ‘X’ is copper (Cu).

The black coloured compound is copper oxide (CuO). The reaction involved is

2Cu(s) + O2(g) → 2CuO(s)

18. Why do we apply paint on iron articles?

Ans:

Paint does not allow iron articles to come in contact with air, water and saves iron articles rusting.

19) Oil and Fat containing food items are flushed with Nitrogen. Why?

Ans:

Because nitrogen acts as an antioxidant and it prevents them from oxidation.

20. Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Ans:

The gradual destruction of pure metals by the action of air, moisture or a chemical (such as an acid) on their surface is called corrosion.

For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

4Fe + 3O2 + nH2O → 2Fe2O3.nH2O

Rancidity is the process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long.

Cls 9 Sci Chp 2 is matter around us pure? Notes

Classs 9 Science

Chapter 2 is matter around us pure?

Notes 

Introduction

Anything which occupies space and has mass is called matter. Matter can be divided in two categories.

(i) Pure Substance: It consists of single types of particles which are same in their chemical nature.

(ii) Mixtures: Mixture consists of two or more particles.

Mixture and its types

Mixture consists of more than one kind of pure substances which can be separated by physical method.

Mixtures are of two types

(i) Homogeneous mixture

(ii) Heterogeneous mixture

(i) Homogeneous mixture: A mixture is said to be homogeneous if all the components of the mixture are uniformly mixed and there are no boundaries of separation between them.

Ex: Sugar in water, etc.

(ii) Heterogeneous mixtures: A mixture is said to be heterogeneous if all the components of the mixture are not uniformly mixed and there are visible boundaries of separation between them.

Ex: Water and sand, Air etc.

Solution and its properties

A solution is a homogeneous mixture of two or more substances.

Ex: Lemonade, soda water etc.

A solution has two components:

(i) Solvent   (ii) Solute

(i) Solvent: The component of the solution that dissolves the other component in it (usually the component present in larger amount) is called the solvent.

(ii) Solute: The component of the solution that is dissolved in the solvent (usually present in lesser quantity) is called the solute.

Properties of Solution:

1. A solution is a homogeneous mixture.

2. The particles of a solution are smaller than 1 nm (10-9) in diameter which cannot be seen by naked eyes.

3. They do not scatter a beam of light passing through the solution that is they don’t show tyndall effect. So, the path of light is not visible in a solution.

4. The solute particles cannot be separated from the mixture by the process of filtration.

5. The solution is stable and solute particles do not settle down when left undisturbed.

Concentration of a solution

(i) Saturated solution: When no more amount of solute can be dissolved in a solution at a given temperature, it is called a saturated solution.

(ii) Unsaturated solution: When more amount of solute can be dissolved in a solution at a given temperature, it is called a saturated solution.

(iii) Solubility: The amount of the solute present in the saturated solution at the given temperature iscalled its solubility.

The concentration of a solution is the amount of solute present in a given amount (mass or volume) of solution. Also, the amount of solute dissolved in a given mass or volume of solvent is called concentration of solution.

Concentration of solution = Amount of solute/Amount of solvent

or

Amount of solute/Amount of solution (Here, amount means mass or volume).

Two methods of finding concentration of solution:

(i) Mass by mass percentage of a solution = (Mass of solute/Mass of solution) ×100

(ii) Mass by volume percentage of a solution = (Mass of solute/Volume of solution) ×100

Suspension and its properties

A suspension is a heterogeneous mixture in which the the solute particles do not dissolve but remain suspended throughout the bulk of the medium.

Ex: Chalk in water, smoke in the air

Properties of Suspension :

1. It is a heterogeneous mixture.

2. Particles of a suspension are visible to the naked eye.

3. Size of the particles is greater than 100 nm.

4. It is unstable mixture. Solute settles down at the bottom over period of time.

5. If the solution is passed through filter paper, solute and solvent gets separated.

6. It scatters light when light is passed through the solution i.e. it shows Tyndall effect.

Colloidal solution and its properties

Colloid solution is heterogeneous mixture in which the size of particles lies between the true solutions and suspensions.

• Colloidal particles can easily scatter a beam of visible light. This phenomenon is called Tyndall effect.

Properties of colloidal solution:

1. The particles of colloid can’t be seen by naked eyes individually.

2. It is a heterogeneous mixture and thus solute and solvent can’t be separated by filter paper.

3. Size of particles is smaller than suspensions but greater than solutions (1 nm to 100 nm).

4. It is a stable mixture. Particles do not settle down at the bottom over a period of time.

5. They do not settle down when left undisturbed which means colloid is quite stable.

Some common examples of colloids (in the table)

Separation of the components of mixtures

Different methods of separation are used to get individual components from mixture. Heterogeneous mixtures can be separated into their respective constituents by simple physical methods like handpicking, sieving, filtration etc.

Obtaining coloured components from blue/black ink

Process of evaporation is used to obtain coloured components from blue/black ink. The process of evaporation is used to separate a substance which is dissolved in water.

• It is based on the fact that liquid vaporises easily than the solid.

• Helps in separating volatile substances from non-volatile substances.

Steps of obtaining coloured components from blue/black ink:

• Fill half a beaker with water.

• Put a watch glass on the mouth of the beaker.

• Put few drops of ink on the watch glass.

• Now start heating the beaker. We do not want to heat the ink directly. You will see that evaporation is taking place from the watch glass.

• Continue heating as the evaporation goes on and stop heating when you do not see any further change on the watch glass.

Separation of cream from milk

• The process of centrifugation is used to separate the cream from milk. It is a method of separating the suspended particles of substance from a liquid.

• This process is carried out by the machine called centrifuge.

• Sometimes, the solid particles in a liquid are very small and pass through a filter paper. For such particles the filtration technique cannot be used.

• The mixture is rotated rapidly so that the heavier particles in the mixtures settle down to the bottom.

• The basic principle of centrifugation is that the denser particles are forced to the bottom and the liquid being lighter remains at the top.

Steps of separating cream from milk:

• Take some full-cream milk in a test tube.

• Centrifuge it by using a centrifuging machine for two minutes.

Application of centrifugation:

• Used in diagnostic laboratories for blood and urine tests.

• Used in dairies and home to separate butter from cream.

• Used in washing machines to squeeze out water from wet clothes.

Separating two immiscible liquids

• The separation of separating two immiscible liquid is carried out by the use of funnel. 

• The basic principle involve is the difference between the densities of two liquids form two separate layers.

Steps of separating kerosene oil and water:

• Pour the mixture of kerosene oil and water in a separating funnel.

• Let it stand undisturbed for sometime so that separate layers of oil and water are formed.

• Open the stopcock of the separating funnel and pour out the lower layer of water carefully.

• Close the stopcock of the separating funnel as the oil reaches the stop-cock.

Application of funnel:

To separate mixture of oil and water.

• In the extraction of iron from its ore, the lighter slag is removed from the top by this method to leave the molten iron at the bottom in the furnace.

Sublimation

• This process is used to separate mixtures that contain a sublimable volatile component from a non-sublimable impurity.

• Sublimation is process where a substance directly changes from solid to gaseous state on heating.

• Ammonium chloride, camphor, naphthalene and anthracene are some examples which can sublime.

Chromatography

• Used to separate those solutes which dissolve in the same solvent.

• Used for sepration of colours.

• The colours which are more soluble in water rises faster and get colours get separated into layers.

Applications

To separate

• colours in a dye

• pigments from natural colours

• drugs from blood.

Distillation

• Used for separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points.

• Mixture of acetone and water is separated by this method.

Fractional distillation

• Fractional distillation is used to separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K.

• Air is a homogeneous mixture and can be separated into its components by fractional distillation.

Below is diagram which shows the steps of separation of air:

• The air is compressed by increasing the pressure and is then cooled by decreasing the temperature to get liquid air.

• The liquid air is warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.

It is used to separate a gas from the air.

Crystallization

• Used to remove impurities from solid and purify it.

• It separates a pure solid from mixture in the form of crystals.

• This process is used in purification of salt from sea water, separation of crystals of alum from impure samples.

• It is better method than evaporation because:

(i) Solids decompose or some, like sugar, may get charred on heating to dryness.

(ii) Some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.

Physical and Chemical changes

• The process which brings about changes in physical properties and no new substances are formed are physical changes. The common physical changes are changes in colour, hardness, rigidity, fluidity, density, melting point, boiling point etc.

• The process in which new substances are formed and chemical properties of substances get changed are chemical changes. Some chemical properties are odour, inflammability etc.

Types of pure substances

The pure substance is divided in two types on the basis of their chemical composition:

(i) Elements  (ii) Compounds 

(i)  Elements

• According to Antoine Laurent Lavoisier, element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions.

• It is divided in three types which are metals, non-metals and metalloids.

Properties of Metals

(i) They have a lustre (shine).

(ii) They have silvery-grey or golden-yellow colour.

(iii) They conduct heat and electricity.

(iv) They are ductile (can be drawn into wires).(v) They are malleable (can be hammered into thin sheets).

(vi) They are sonorous (make a ringing sound when hit).

• Examples of metals are gold, silver, copper, iron, sodium, potassium etc.

• Mercury is the only metal that is liquid at room temperature.

Properties of non-metals

(i) They display a variety of colours.(ii) They are poor conductors of heat and electricity.

(iii) They are not lustrous, sonorous or malleable.

• Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.

Metalloids: Elements having intermediate properties between those of metals and non-metals are called metalloids.  Examples are boron, silicon, germanium etc.

Compounds

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

Difference between mixtures and compounds