Question

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Answer 1

Chemical reactions are the processes in which new substances with new properties are formed.

● Only a rearrangement of atoms takes place in a chemical reaction.

● The substances which take part in a chemical reaction are called reactants.

● The new substances produced as a result of chemical reaction are called products.

● In a chemical reaction, reactants are transformed into products.

● The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.

Note: Before burning in air, the magnesium ribbon is cleaned by rubbing with a sand paper. This is done to remove the protective layer of basic magnesium carbonate from the surface of magnesium ribbon so that it may readily combine with the oxygen of air.

Reactant: Substances which take part in a chemical reaction are called reactants.

In the example of formation of Magnesium Oxide by the reaction between Magnesium and Oxygen, Mg and O2 are reactants.

Product: New substance formed after a chemical reaction is called a product.

Example: MgO.

Characteristics of Chemical reactions

The conversion of reactants into products in a chemical reaction is often accompanied by some features which can be easily observed easily.

The important characteristics of chemical reactions are:

1. Evolution of a gas

2. Formation of a precipitate

3. Change in colour

4. Change in temperature

5. Change in state

Details:

(i) Evolution of gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.

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

(ii) Change in Colour: The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by a change in colour from purple to colourless.

The chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterized by a change in colour from orange to green.

(iii) Change in state of substance: The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because the wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas, carbon dioxide produced by the combustion of wax is a gas). There are some chemical reactions which can show more than one characteristics.

(iv) Change in temperature: The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).

The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature (which is a rise in temperature).

(v) Formation of precipitate: The chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.

BaCl2(aq) + H2SO4(aq) → BaSO4(s) (ppt) + 2HCl(aq)

Chemical Equation: Representation of chemical reaction using symbols and formulae of the substances is called Chemical Equation.

Example: A + B → C + D

In this equation, A and B are called reactants and C and D are called the products. The arrow shows the direction of the chemical reaction. Condition, if any, is written generally above the arrow.

Answer 2

Question) True/false and rewrite the correct statement.

(i) statement: Chemical equation is considered as the longest way to represent a chemical reaction.

answer: false

reason: $\large\rm \bullet$ it takes less time.

$\large\rm \bullet$ it takes less space.

$\large\rm \bullet$ it is easy to learn.

correct statement: chemical equation is considered as the shortest way to represent a chemical reaction.

(ii) statement: the products are written on right hand side of a chemical equation.

answer: true

example:

$\small\rm{ R-CHO + 2 Cu^{2+} + 5 \bar{O} H \longrightarrow RC \bar{O} + Cu_2 O + 3 H_2 O}$

Fehling’s solution A (aqueous copper sulfate) and Fehling solution B (alkaline sodium potassium tartrate) are mixed in equal amounts before the test. A reddish brown precipitate is obtained when an aldehyde is heated with Fehling’s reagent.

(iii) statement: chemical reactions take place more easily when reactants are in solid form.

answer: true

reason: higher the surface area, faster the reaction. surface areas of solids is greater than that of liquids and gases.

Question) what happens when

(i) Zinc metal is placed in hydrochloric acid.

answer:

When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced. The production of a gas is also an indication that a chemical reaction is occurring. Zinc reacting with hydrochloric acid produces bubbles of hydrogen gas.

(ii) silver chloride is exposed to sunlight.

answer:

When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas. This reaction is used in black and white photography.

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