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प्रश्न
State the conditions necessary for a chemical change or reaction.
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उत्तर
- Mixing (close contact): In some cases, a chemical reaction occurs when two substances are mixed in their solid states. For example., Iodine and phosphorus react explosively when brought into close contact.
- Solution: In some cases, a chemical reaction occurs when substances are mixed in either a molten or aqueous state. For example., Oxalic acid crystals and sodium carbonate react in water solution only.
- Heat: Some chemical reactions occur only on heating. For example., Copper carbonate decomposes on heating (Δ symbol for heating) into copper oxide and carbon dioxide.
\[\ce{CuCO3_{(s)} ->[\Delta] CuO_{(s)} + CO2_{(g)}}\] - Light: Some chemical reactions take place by the action of light. These are called photochemical reactions or photolysis. Molecules of the reactants absorb light energy to get activated and then react rapidly. For example., Plants form glucose from carbon dioxide and water in the presence of light.
\[\ce{6CO2 + 12H2O ->[Light] C6H12O6 + CO2 + 6H2O}\] - Electricity: Certain chemical reactions, like the decomposition of certain compounds, occur only when electricity is passed through the substance. For example., Electrolysis of acidulated water occurs only in the presence of electricity, to give hydrogen and oxygen.
\[\ce{2H2O ->[electricity] 2H2 + O2}\] - Pressure: Some chemical reactions take place only when the involved substances are subjected to high pressure. For example., Mercuric chloride and potassium iodide when rubbed in a mortar, give a scarlet-coloured substance called mercuric iodide.
\[\ce{HgCl2 + 2KI -> HgI2 + KCl}\] - Catalyst: Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur. The catalysts themselves do not take part in the reaction and so remain unchanged. For example., Ammonia reacts with oxygen to produce nitric oxide and water vapour in the presence of (platinum) a catalyst.
\[\ce{4NH3 + 5O2 ->[Pt][800° C] 4No + 6H2O}\]
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संबंधित प्रश्न
The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change − A), then it burns (Change − B). The following statements pertain to these changes. Choose the correct one.
Write the missing reactants and products and balance the equations.
\[\ce{KClO3 ->[heat]}\] ___________+ ____________
MATCH THE COLUMNS
| 1 . `H_2` + `CI_2` `→` 2HCI | a. displacement reaction |
| 2. `2FeCI_3`+ `H_2` `→` `2FeCI_2` +2HCI` | b. addition of an electronegative redical |
| 3. Fe + `CuSO_4` `→``FeSO_4` +Cu` | c. removal of an electronegative redical |
| 4. `2H_2O` +`4NO_2`+`O_2` `→` `4HNO_3` | d.reduction by adding hydrogen |
| 5. 3Fe +`3CI_2` `→` `2FeCI_3` | e.electrolysis of water |
| 6. `FeCI_3`+ 3NaOH `→``Fe(OH)_3``↓`+NaCI | F.double decomposition,precipitation reaction |
| 7.`2H_2O` `→` `2H_2``↑`+`O_2` | e. combination or synthesis reaction |
Write true or false .
A black residue is formed when sugar is heated .
Complete the statement by filling the gaps using appropriate term from the terms given in the bracket.
(slow, coloured, arrow, fast, smell, milky, physical, product, chemical, reactant, covalent, ionic, octet, duplet, exchange, sharing, equality sign)
Rusting of iron is a ______ chemical change.
Multiple choice:
Which of the following is not a characteristic of a chemical change?
Write your observation and a balanced equation in the case of the following substances being heated.
Ammonium dichromate.
Explain the Silver nitrate solution is kept in coloured reagent bottles in the laboratory.
Limestone contains ______ mainly.
Silver metal can displace hydrogen gas from nitric acid.
