1. Give two examples for each of the following cases: (a) Physical changes which are reversible. (b) Physical changes which are not reversible. (c) Chemical changes. 2. Answer the following:- a. List the reactants of each reaction. b. Balance the equation Mg(s)+O2(g)→MgO(s) c. Complete the equation:- MgO + H2O → 3. Define the terms:- a. Rusting b. Crystallisation c. Galvanisation d. Displacement reaction 4. Differentiate between physical and chemical reactions. 5. Iron pillar near the Qutub Minar in Delhi is famous for the following facts. Which of these facts is responsible for its long stability? (a) It is more than 7 metres high. (b) It weighs about 6000 kg. (c) It was built more than 1600 years ago. (d) It has not rusted after such a long period
Answers
Answer:
Three classifications of chemical reactions will be reviewed in this section. Predicting the products in some of them may be difficult, but the reactions are still easy to recognize.
A composition reaction (sometimes also called a combination reaction or a synthesis reaction) produces a single substance from multiple reactants. A single substance as a product is the key characteristic of the composition reaction. There may be a coefficient other than one for the substance, but if the reaction has only a single substance as a product, it can be called a composition reaction. In the reaction
2 H2(g) + O2(g) → 2 H2O(ℓ)
water is produced from hydrogen and oxygen. Although there are two molecules of water being produced, there is only one substance—water—as a product. So this is a composition reaction.
A decomposition reaction starts from a single substance and produces more than one substance; that is, it decomposes. One substance as a reactant and more than one substance as the products is the key characteristic of a decomposition reaction. For example, in the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate),
2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(ℓ)
sodium carbonate, carbon dioxide, and water are produced from the single substance sodium hydrogen carbonate.
Composition and decomposition reactions are difficult to predict; however, they should be easy to recognize.
Answer:
1)
(a) Folding of paper
Melting of ice
(b)Tearing of paper
Breaking of glass
(c)Reaction between vinegar and baking soda
Burning of a matchstick
2)
(a) • Mg and O2
• Mg and H2O
(b)2Mg(s)+O2(g)→2MgO(s)
(c) MgO + H2O → Mg(OH)2
3)
(a)Rusting of iron: if a piece of iron is left open for sometime, it acquires a coating of brownish substance. This substance is called rust and the process is called rusting.
Rusting takes place in the presence of both oxygen and water (or water vapour). One simple way to prevent rusting is to apply a coat of paint or grease over the metallic items.
Another way is to coat a layer of a metal like chromium or zinc on iron.
(b) Galvanisation: The process of coating a thin layer of zinc on iron objects is called galvanisation.
(c) Crystallisation: It is the process of getting crystals of pure substance from their solutions.
(d) Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. Both metals and non-metals take part in displacement reactions.eg Fe2O3 + 2Al → Al2O3 + 2Fe
4) . Physical Change // Chemical Change
•When a substance undergoes a physical change, its composition remains the same despite its molecules being rearranged.// When a substance undergoes a chemical change, its molecular composition is changed entirely. Thus, chemical changes involve the formation of new substances.
•Physical change is a temporary change. //A chemical change is a permanent change.
•A Physical change affects only physical properties i.e. shape, size, etc. //Chemical change both physical and chemical properties of the substance including its composition
•A physical change involves very little to no absorption of energy. //During a chemical reaction, absorption and evolution of energy take place.
•Some examples of physical change are freezing of water, melting of wax, boiling of water, etc.// A few examples of chemical change are digestion of food, burning of coal, rusting, etc.
•Generally, physical changes do not involve the production of energy. //Chemical changes usually involve the production of energy (which can be in the form of heat, light, sound, etc.)
•In a physical change, no new substance is formed. //A chemical change is always accompanied by one or more new substance(s).
•Physical change is easily reversible i.e original substance can be recovered.// Chemical changes are irreversible i.e. original substance cannot be recovered.
5)
(d) It has not rusted after such a long period