Monday, July 25, 2011

CBSE Class-7th Science Physical and Chemical Changes

Physical Change

A physical change is one in which the shape, size, appearance or state, of a substance may alter, but its chemical composition remains same. No new substance is formed. It is usually a change, which is reversible. By reversing the process, the original substance can be obtained. 

For e.g., by using a piece of gold, a goldsmith can make a large variety of ornaments. But all of them consist of the same substance, namely gold. A more common example is that of water, which can be converted into solid ice, liquid water, and gaseous water vapor. It can be reconverted to its previous state by various methods. Yet, in all the three forms, the chemical composition of water is not altered. No new substance with new chemical properties is formed. In all these, water consists of two atoms of hydrogen and one atom of oxygen.

Chemical Change 

A change in which the composition of a substance is altered is called as chemical change. As a result, the original properties get changed and one or more new substances are formed.Burnig of paper,rusting of iron,Cooking of food

The necessary conditions are: 

1) A minimum amount of energy needed to initiate a reaction (activation energy) should be supplied in the form of heat, light or electric current. In a chemical change, the reactants combine to form new products. For this process it is necessary to break the old bonds of the reactants and form fresh bonds, in order to give new products. 2) For the occurrence of any reaction, the molecules or atoms of the reactants must collide with one another, in order to break old bonds and form new bonds. 3) The speed with which the chemical reaction takes place is called the rate of the chemical reaction. This should be appreciable, to bring about the change.
Iron (Fe) + Oxygen (O2, from the air) + water (H2O)→ Rust (iron oxide) (Fe2O3.H2O)
Both Oxygen and water or water vapours are essential for rusting.

Very  Short type questions                         

Questions:What is the chemical name of Baking Soda?

(Ans)  Sodium hydrogen carbonate (NaHCO3).

Questions: What is Rust?

(Ans)  When iron oxide is hydrated, it is known as Rust(Fe2O3. XH2O)

Questions: What type of change is photosynthesis?

(Ans)  Photosynthesis is a chemical change.

Questions: What do you understand by physical properties of a substance?

(Ans)  Properties like size, shape, colour and state of a substance are known as physical properties.

Questions: What is a chemical change?

(Ans)  A change in which a new substance with different properties is formed, is known as a chemical 

Questions: What is malleability?

(Ans)  Malleability is a property by virtue of which a metal can be beaten into thin sheets without breaking.

Questions: Name four characteristics which are included in the physical properties of matter.

(Ans)  Properties such as shape, size, colour and state of a substance are called its physical properties.

Questions: Write the chemical formula of blue vitriol.

(Ans)   Crystals of copper sulphate pent hydrate (CuSO4.5H2O) are blue in colour. So, it is commonly 
known as blue vitriol.

Questions: Explain why rusting of iron objects is faster in coastal areas than in deserts.

(Ans)  In coastal areas, there is more moisture in air due to the presence of sea. In deserts, there is shortage 
of water and hence, air is almost dry. For rusting, both air (Oxygen) and moisture are essential. So, rusting is 
faster in coastal areas than in deserts.

Questions: What are the two important conditions for rusting? 

(Ans)  Presence of both water and oxygen are required to form rust on iron. 

Questions: What is freezing of water?

(Ans)  Freezing is the process in which water turns to ice when cold enough (below 0 °C temperature).

Questions: What is freezing point of water?

(Ans)  The freezing point is 0 °C at which water freezes.

Questions: What do you understand by melting?

(Ans)  The process of turning a solid to a liquid.

Questions: How melting and freezing related?

(Ans)  Melting (converts solid into liquid) is opposite to freezing (converts liquid into solid).

Questions: What is evaporation?

(Ans)  It is the process by which water molecules continuously change to vapor.

Questions:At what temperature water evaporates and what do we call it?

(Ans)  Evaporation of water occurs when water is heated to a temperature of 100 °C, which is called as 
boiling point of water.

 Questions:What is symbol?

(Ans) The symbol of an element is a short way of representing an element.

Questions: What Does A Symbol Represent?  

(Ans) A symbol represents one atom of an element.   Example: H stands for one atom of hydrogen

Short type questions                              

Questions: What is rusting? How can we prevent rusting?

(Ans)   When a piece of iron is left in open for some time, it acquires a film of brownish substance called rust.  The process is known as rusting.
We can prevent rusting by following methods:
(1) By painting or greasing: - Apply a coat of paint or grease regularly on iron articles.

(2) By galvanizing:-Deposit a layer of zinc or chromium on iron. The process is known as galvanization.

Questions:  Why setting of curd is regarded as a chemical change 

(Ans)  Curd is formed by adding some sour substance to milk and keeping it undisturbed for some hours. Some useful bacteria help in the setting of curd. Curd cannot be converted into milk. It is a different substance than milk. Hence, formation of curd is a chemical change.

Questions: What happens when magnesium oxide is dissolved in water?

(Ans)  When magnesium oxide is dissolved in water, magnesium hydroxide is formed.     
Mg O +H2O ---------> Mg(OH)2

Questions: What happens when baking soda is treated with vinegar?

(Ans)  Ethanoic acid reacts with sodium hydrogen carbonate to evolve brisk effervescence of carbon dioxide.
CH3COOH + NaHCO3        ------------> CH3COONa + CO2 + H2

Questions:  Is the burning of candle a physical change?

(Ans)  When a candle burns, both physical and chemical changes take place.
On burning candle, the wax melts but can be solidified again on cooling. This shows that melting of wax is a physical change.
Burning of candle also produces light and some gases like carbon dioxide. Hence, burning of wick of the candle is a chemical change.

Questions:  What happens when a piece of iron metal is placed in copper sulphate solution?(3 Marks)

(Ans)  A more reactive element displaces a less reactive element from its compound. For example when a piece of iron metal is placed in copper sulphate solution, then iron sulphate solution and copper metal is formed.Iron acquire redish brown colour that deposit  is copper.

Questions:   Which one is better technique to obtain sugar from sugar solution- crystallization or evaporation to dryness? 

(Ans)  Crystallization is a better technique than evaporation  because it has no dissolve impurities .that may left in evaporation  So Salt obtain from evaporation is mixed in boiled water . Then solution is filtered  to remove dissolved impurities . The filtrate is left undisturbed for few hour . Sodium chloride aggregate and form pure crystal of salt 

Long type questions                                      

Questions:  What is Valency?

Ans: Valency is the combining capacity of an atom. It is equal to the number of electrons the atom loses, or gains or shares when it combines with one or more atoms.   Depending on whether the atom loses, gains or shares electrons, the valency may be positive or negative or neutral respectively.   Example: Valency of sodium is + 1, because it can lose one electron. It is represented as Na+. Valency of oxygen may be - 2 if it accepts 2 electrons i.e., O2-.

Questions: What is Chemical formula ?

Ans: The chemical formula of a substance is the symbolic representation of the actual number of atoms present in one molecule of that substance.  
Example 1: Formula of one molecule of oxygen is O2. It shows that one molecule of oxygen is made up of 2 atoms of it.  
Example 2: Formula of one molecule of water is H2O. It shows that one molecule of water is made up of 2 atoms of hydrogen and one atom of oxygen.  
 Example 3: Formula of one molecule of sulphuric acid is H2SO4. It shows that one molecule of sulphuric acid is made up of 2 atoms of hydrogen, 1 atom of sulphur and 4 atoms of oxygen.

Questions: What is radicals ?

Ans:  when the atoms of different elements combine in a group they are called radicals. They behave as a single unit and the valency of radicals is the overall net charge of the group. The radicals maintain their identity in the chemical changes but are incapable of independent existence  Hydroxide - OH-   Sulphate - SO4-   Nitrate - NO3-   Sodium Na1+ , Cu 2+ 

Negative radicals
Positive Radicals 

Questions:  What is the difference between physical change and chemical change? Give example.

(Ans) The difference between physical change and chemical change is as follows-Physical change
(1) No new substance is formed. A substance undergoes a change in its physical properties.
(2) Physical change is generally reversible.
Example—Melting of ice, lighting of bulbChemical Change
(1) One or more new substances are formed by chemical reaction.
(2) Chemical change is irreversible.
Example—Burning of coal, photosynthesis

Questions:  How is alloy prepared?

(Ans) An alloy is prepared by mixing various metals in molten state in required proportions and then cooling this mixture to the room temperature. The properties of alloys are different from the properties of the constituent metals, e.g., Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust like pure iron.If one of the metals in an alloy is mercury, it is called an amalgam

Questions:   Describe how crystals of copper sulphate are prepared?

(Ans) A cup full of water is taken in a beaker and a few drops of dilute sulphuric acid are added into it. The water is heated. When it starts boiling, copper sulphate powder is added slowly while stirring continuously till no more powder can be dissolved. The solution is filtered and allowed to cool down. Crystals of copper sulphate slowly form at the bottom of the beaker.

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