Chemistry questions in the General Aptitude Test are about the chemistry of ordinary life: why a matka cools water, what is in LPG, why a curry stain turns red with soap. They reward knowing a set of names and formulas and the reasons behind a few everyday effects. This lesson covers states of matter and separating mixtures, acids, bases and the pH scale, everyday compounds, metals and non-metals, corrosion and alloys, and the types of chemical reaction, with worked examples, a pH scale diagram and the traps set most often.
Everyday chemistry is a set of names joined to reasons. Baking soda is a mild base, so it soothes an ant sting; iron needs both air and water to rust, so paint stops it; ice is less dense than water, so lakes freeze from the top. Learn each fact with its reason and most questions answer themselves.
The traps are near-twins: brass and bronze, baking soda and washing soda, LPG and CNG, evaporation and boiling. Keep a table for each pair.
| Solid | Liquid | Gas | |
|---|---|---|---|
| Shape | Fixed | Takes the container's | Fills the container |
| Volume | Fixed | Fixed | Not fixed |
| Particles | Vibrate in place | Slide past each other | Move freely, attract weakly |
Sublimation is solid straight to gas (camphor, dry ice). Evaporation happens at any temperature, from the surface only, and cools what is left; boiling happens throughout, at one temperature. Pure ice melts at 0 °C, which is 273 K; −40 °C is 233 K.
| Everyday effect | Why |
|---|---|
| Ice floats | Water expands on freezing, so ice is less dense |
| LPG is liquid in the cylinder | Compressed under high pressure |
| Hot food smells travel faster | Particles move faster when hot, so they diffuse faster |
A pure substance, such as distilled water, has one kind of particle. A homogeneous mixture is the same throughout: salt water, air, brass. A colloid, such as milk, fog or smoke, has particles too small to settle but big enough to scatter light, so a beam through mist becomes visible: the Tyndall effect.
| Mixture | Method |
|---|---|
| Salt from sea water | Evaporation in the sun |
| Cream from milk | Centrifugation |
| Dyes in ink | Chromatography |
| Petrol, diesel, kerosene from crude oil | Fractional distillation |
| Camphor from salt | Sublimation |
| Iron filings from sand | A magnet |
| Oil from water | Separating funnel |
It runs from 0 to 14: 7 is neutral, below 7 acidic, above 7 basic. Acids turn blue litmus red; bases turn red litmus blue.
Example 1 · How many times more acidic?
Solution P has pH 3 and solution Q has pH 5. How many times greater is the hydrogen ion concentration in P?
Each step of 1 on the pH scale is a factor of 10.
P is 2 steps lower: .
100 times
| Natural acid | Found in |
|---|---|
| Citric | Lemons, oranges |
| Acetic | Vinegar |
| Lactic | Curd |
| Tartaric | Tamarind, grapes |
| Methanoic (formic) | Ant and bee stings |
| Hydrochloric | The stomach |
Key idea
Turmeric is a natural indicator: yellow in acid or neutral, reddish-brown in a base. That is why a curry stain turns red with soap and yellow again when rinsed.
Acid + base → salt + water, so hydrochloric acid and sodium hydroxide give sodium chloride. Acid + carbonate also gives carbon dioxide, the fizz when baking soda meets vinegar.
| Problem | Remedy |
|---|---|
| Indigestion | Milk of magnesia, a mild base |
| Ant sting | Baking soda paste |
| Acidic soil | Slaked lime, calcium hydroxide |
| Tooth decay, below about pH 5.5 | Basic toothpaste; fluoride strengthens enamel |
| Common name | Chemical | Use |
|---|---|---|
| Common salt | Sodium chloride, NaCl | Food |
| Baking soda | Sodium hydrogencarbonate, NaHCO₃ | Cooking, antacid |
| Washing soda | Sodium carbonate | Softens hard water |
| Bleaching powder | Calcium oxychloride | Kills germs in water |
| Liquid bleach | Sodium hypochlorite | Whitening, disinfecting |
| Gypsum | Calcium sulphate with water | Makes plaster of Paris |
| Quicklime / slaked lime | Calcium oxide / hydroxide | Whitewash, soil |
| Marble, chalk, limestone | Calcium carbonate | Building |
| Dry ice | Solid carbon dioxide | Cold transport |
| Laughing gas | Nitrous oxide | An early anaesthetic |
| Urea | Nitrogen fertiliser |
| Fuel or gas | Mainly |
|---|---|
| LPG | Butane and propane |
| CNG, biogas | Methane |
| Fizz in soft drinks | Dissolved carbon dioxide |
| Baking powder's rise | Carbon dioxide released on heating |
Watch out
Hard water has calcium and magnesium salts. Soap forms a scum with them and lathers poorly; detergents do not, so they still clean.
Metals are usually shiny, malleable, ductile and good conductors; non-metals are usually dull and brittle. The exceptions are the questions.
| Exception | Fact |
|---|---|
| Mercury | The only liquid metal |
| Bromine | The only liquid non-metal |
| Graphite | A non-metal that conducts, used in dry-cell electrodes |
| Diamond | The hardest natural substance |
| Sodium | So reactive it is kept under kerosene |
| Silver | The best conductor; copper is used for wiring because it is cheaper and drawn into wires |
| Aluminium | The most abundant metal in the crust, from bauxite |
Carbon is in every organic compound.
From most to least reactive: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, (hydrogen), copper, mercury, silver, gold. Metals above hydrogen release hydrogen from dilute acid; copper does not.
Example 2 · The iron nail
An iron nail sits in blue copper sulphate solution. The solution turns pale green and the nail turns brown. What does this show?
Iron has pushed copper out of its salt, forming green iron sulphate.
A metal can only displace one below it in the reactivity series.
Iron is more reactive than copper
Iron rusts only with both air and water; rust is hydrated iron(III) oxide. Silver tarnishes black (silver sulphide) and copper turns green (basic copper carbonate). Aluminium resists because a thin, tough oxide layer protects it.
| Protection or alloy | What it is |
|---|---|
| Galvanising | A zinc coat on iron |
| Stainless steel | Iron with chromium and nickel |
| Brass | Copper and zinc |
| Bronze | Copper and tin |
A chemical change forms a new substance with new properties: burning paper, rusting, cooking. Freezing water is a physical change.
| Type | Pattern | Example |
|---|---|---|
| Combination | A + B → AB | Magnesium burns to magnesium oxide; quicklime + water |
| Decomposition | AB → A + B | Limestone heated to quicklime; silver chloride in light |
| Displacement | A + BC → AC + B | Zinc + copper sulphate |
| Double displacement | AB + CD → AD + CB | Sodium chloride + silver nitrate |
Oxidation is gain of oxygen: copper turns black copper oxide in air. Rancidity is the oxidation of fats, which is why chips packets hold nitrogen. Exothermic reactions give out heat (respiration, quicklime with water); endothermic ones absorb it (photosynthesis).
Example 3 · Balancing an equation
Balance H₂ + O₂ → H₂O with the smallest whole numbers.
The right side has one oxygen, the left two, so write 2H₂O.
Now there are four hydrogens on the right, so write 2H₂: 2H₂ + O₂ → 2H₂O.
2, 1, 2
Matter
Acids and compounds
Metals and reactions
The physics of heat and states is in everyday physics, and chemistry in the air in pollution, climate and conventions.
68 questions, grouped by type, easy to hard. Pick an answer and check it. The worked solution opens when you ask for it.
The process by which a solid changes directly into a gas without first becoming a liquid is called
AnswerC · sublimation
Sublimation is the direct solid-to-gas change, skipping the liquid state entirely.
Camphor, naphthalene and dry ice all sublime, which is why they shrink away without leaving a puddle.
Evaporation is liquid to gas, condensation is gas to liquid, and fusion is the melting of a solid into a liquid.
Which of these is a pure substance rather than a mixture?
AnswerB · Distilled water
A pure substance has one kind of particle throughout and a fixed composition.
Distilled water is only H₂O. Air is a mixture of gases, sea water has dissolved salts, and milk is a mixture of fat, protein and water.
Which state of matter has a fixed volume but takes the shape of its container?
AnswerD · Liquid
A solid keeps both its shape and volume; a gas has neither and fills any container.
A liquid keeps its volume but flows to fit the container's shape.
Which of these is a homogeneous mixture, with the same composition throughout?
AnswerA · Brass
Brass is an alloy of copper and zinc mixed evenly at the level of atoms, so every part is alike.
Sand in water, oil and water, and granite all have visibly different parts: they are heterogeneous.
Worth noting
Alloys are mixtures, not compounds, even though they look like one material.
How is common salt obtained from sea water in the salt pans along the coast of Gujarat?
AnswerC · By letting the water evaporate in the sun
Salt is dissolved in sea water, so filtering cannot remove it.
In shallow pans the sun and wind evaporate the water and leave the salt behind.
Which of these is a colloid?
AnswerC · Milk
In a colloid the particles are bigger than in a true solution but too small to settle; milk has tiny fat droplets spread through water.
Salt water and sugar syrup are true solutions; chalk in water is a suspension that settles.
A beam of sunlight is clearly visible as it passes through the mist in a forest. This is an example of
AnswerB · the Tyndall effect
Mist is a colloid of tiny water droplets in air.
Colloidal particles scatter light sideways, so the path of the beam becomes visible. This is the Tyndall effect; a true solution does not show it.
At a dairy, cream is separated from milk by spinning the milk very fast. This method is called
AnswerC · centrifugation
Spinning pushes the denser parts of a mixture outwards and leaves the lighter parts nearer the centre.
Cream (fat) is lighter than the rest of milk, so it collects separately. This is centrifugation.
The different coloured dyes in the ink of a black sketch pen can be separated by
AnswerA · chromatography
In paper chromatography a solvent climbs up the paper and carries each dye a different distance.
The dyes separate into bands because they dissolve and stick to the paper to different extents.
Petrol, diesel, kerosene and bitumen are obtained from crude oil by
AnswerD · fractional distillation
The parts of crude oil boil at different temperatures.
In a fractionating column the vapours condense at different heights, so each fraction is collected separately.
Worth noting
Simple distillation works only when the boiling points are far apart; close boiling points need a fractionating column.
Ice floats on water because
AnswerA · water expands when it freezes, so ice is less dense than liquid water
Unlike most substances, water takes up more space as a solid: its molecules lock into an open pattern.
The same mass in a larger volume means a lower density, about 0.92 g/cm³ against 1 g/cm³, so ice floats.
Worth noting
This is why a lake freezes from the top down and fish survive in the water underneath.
The smell of hot, freshly cooked food reaches us from the kitchen faster than the smell of cold food. Why?
AnswerD · Particles move faster at a higher temperature, so they diffuse faster
Smell reaches us by diffusion: the particles spread through the air.
Heating gives the particles more kinetic energy, so more of them escape and they spread faster.
Worth noting
Hot air rises, it does not sink; the effect is about particle speed.
LPG is a gas at room temperature, yet it is stored as a liquid in a kitchen cylinder. How is this done?
AnswerB · By compressing it under high pressure
Pressing gas particles close together makes them attract each other enough to form a liquid.
So high pressure (and cooling) liquefies a gas. When the valve opens, the pressure drops and the liquid turns back into gas.
Worth noting
Heating does the opposite: it spreads particles apart. Lowering the pressure would keep LPG a gas.
Which statement about evaporation is correct?
AnswerA · It takes place at any temperature, from the surface of the liquid
Evaporation is escape of the fastest particles from the surface, and it goes on at all temperatures below boiling.
It is faster with more surface area, higher temperature and wind, and slower in humid air. It cools the liquid left behind.
Worth noting
Clothes dry slowly on a humid monsoon day because the air already holds a lot of water vapour.
What is the pH of pure water at 25 °C?
AnswerC · 7
Pure water is neutral, neither acidic nor basic.
On the pH scale from 0 to 14, neutral is 7; below 7 is acidic and above 7 is basic.
When a drop of dilute hydrochloric acid is put on blue litmus paper, the paper
AnswerD · turns red
Acids turn blue litmus red; bases turn red litmus blue.
Lemons and oranges taste sour mainly because they contain
AnswerB · citric acid
Citrus fruits get their sour taste from citric acid.
Acetic acid is in vinegar, lactic acid in curd and sour milk, and tartaric acid in tamarind and grapes.
Vinegar, used in pickles and salad dressings, is a dilute solution of
AnswerC · acetic acid
Vinegar is about 5–8% acetic acid (ethanoic acid) in water.
Which acid does the human stomach produce to help digest food?
AnswerD · Hydrochloric acid
Glands in the stomach wall release dilute hydrochloric acid.
It kills germs in food and provides the acidic medium the enzyme pepsin needs to digest protein.
Which of these has the lowest pH?
AnswerA · Lemon juice
The lower the pH, the more acidic the liquid.
Lemon juice is about pH 2, milk about 6.5, pure water 7 and soap solution about 10.
Worth noting
Soap solution is basic, so it has the highest pH of the four, not the lowest.
An ant sting hurts because the ant injects methanoic (formic) acid. Which is a sensible first-aid remedy to rub on the sting?
AnswerC · A paste of baking soda
An acid is neutralised by a base.
Baking soda (sodium hydrogencarbonate) is a mild base, so it neutralises the formic acid and eases the pain.
Worth noting
Lemon juice and vinegar are acids themselves and would not help; salt solution is neutral.
Milk of magnesia relieves acidity and indigestion because it
AnswerB · is a mild base that neutralises excess stomach acid
The stomach makes hydrochloric acid; too much of it causes pain and a burning feeling.
Milk of magnesia is magnesium hydroxide, a mild base, which neutralises the extra acid. Such medicines are antacids.
Tooth enamel starts to dissolve and decay begins when the pH in the mouth falls below about
AnswerA · 5.5
Bacteria in the mouth break down sugar left on the teeth and produce acids.
Below about pH 5.5, the enamel (calcium phosphate) starts to dissolve. Toothpastes are basic to neutralise this acid.
Worth noting
A pH of 7 is neutral; enamel is safe there. The damage begins only in clearly acidic conditions.
Plaster of Paris, used by doctors to set broken bones, is made by heating
AnswerD · gypsum
Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O.
Heating it to about 373 K drives off some water and gives plaster of Paris, CaSO₄·½H₂O, which sets hard again when mixed with water.
Worth noting
Heating limestone gives quicklime, not plaster of Paris.
When an acid reacts completely with a base, the products are
AnswerA · a salt and water
This is neutralisation: acid + base → salt + water.
For example, HCl + NaOH → NaCl + H₂O.
Worth noting
Hydrogen gas comes from an acid reacting with a metal; carbon dioxide comes from an acid reacting with a carbonate.
A soil test shows that a farmer's field is too acidic for the crop. What should be added to the soil?
AnswerC · Slaked lime (calcium hydroxide)
Acidic soil needs a base to raise its pH.
Slaked lime (or chalk) is a base and neutralises the excess acid.
Worth noting
Ammonium sulphate is a fertiliser that makes soil more acidic; common salt harms most crops.
A yellow curry stain on a white shirt turns reddish-brown when the shirt is scrubbed with soap, and turns yellow again after rinsing. Why?
AnswerB · Turmeric is a natural indicator that turns reddish-brown in a basic solution
Soap solution is basic.
Turmeric is yellow in acidic or neutral conditions and reddish-brown in basic ones. Rinsing removes the soap, so the colour returns.
Worth noting
The colour change reverses, which shows it is an indicator effect, not bleaching.
Which metal is a liquid at room temperature?
AnswerB · Mercury
Mercury melts at about −39 °C, so it is liquid at room temperature. It was used in thermometers.
An iron nail rusts only when it is exposed to
AnswerC · both air and water
Rust is hydrated iron oxide, formed when iron reacts with oxygen in the presence of water.
A nail in dry air, or in boiled water sealed with oil, does not rust.
Galvanised iron buckets and roofing sheets are iron coated with a thin layer of
AnswerD · zinc
Galvanisation means coating iron or steel with zinc.
Zinc is more reactive than iron, so even if the coating is scratched, the zinc corrodes first and protects the iron.
Worth noting
Tin plating is used on food cans, and chromium plating on taps and bicycle handles; neither is galvanising.
Which is the hardest natural substance known?
AnswerC · Diamond
Diamond is carbon with every atom bonded strongly to four others in a rigid three-dimensional network.
That makes it the hardest natural substance, used to cut glass and drill rock.
Among the non-metals, only one is found as a liquid under ordinary room conditions. Which is it?
AnswerA · Bromine
Bromine is the only non-metal that is liquid at room temperature; it is a red-brown liquid.
Chlorine is a gas and iodine a solid.
Worth noting
Mercury is also liquid, but it is a metal.
Which of these metals is the best conductor of electricity?
AnswerD · Silver
Silver conducts electricity best of all metals, with copper a close second.
Copper is used for wiring because it is almost as good and far cheaper.
Worth noting
Copper is the tempting answer because it is in every wire, but the question asks for the best conductor.
Which non-metal conducts electricity well enough to be used for electrodes in dry cells?
AnswerB · Graphite
Non-metals generally do not conduct electricity, but graphite, a form of carbon, is an exception.
Each carbon atom in graphite has one free electron, which lets current flow along its layers.
Worth noting
Diamond is also carbon, but it has no free electrons and does not conduct.
In a school laboratory, sodium metal is kept immersed in kerosene because it
AnswerA · reacts vigorously with oxygen and moisture in the air
Sodium is so reactive that in open air it catches fire and reacts violently with water.
Kerosene does not react with it and keeps air and moisture away.
Worth noting
Sodium is actually lighter than water, and it does not dissolve in kerosene.
Bronze, used for statues and medals, is an alloy of
AnswerB · copper and tin
Bronze is mainly copper with tin.
Copper and zinc make brass, iron and carbon make steel, and lead and tin make solder.
Worth noting
Brass and bronze are the pair most often swapped; brass has zinc, bronze has tin.
Aluminium is extracted from which ore?
AnswerA · Bauxite
Bauxite is the main ore of aluminium, mostly hydrated aluminium oxide. India has large deposits in Odisha.
Haematite is an iron ore, galena a lead ore and cinnabar a mercury ore.
Silver jewellery and utensils turn black after some time in air. The black coating is
AnswerC · silver sulphide
Silver reacts with traces of sulphur compounds, such as hydrogen sulphide, in the air.
The product is black silver sulphide. This tarnishing is a kind of corrosion.
Stainless steel does not rust easily because the iron in it is alloyed with
AnswerD · chromium and nickel
Stainless steel is iron with carbon plus chromium and nickel.
The chromium forms a thin, tough oxide film on the surface that stops the iron beneath from rusting.
Aluminium is more reactive than iron, yet aluminium window frames do not corrode away like iron. Why?
AnswerC · A thin, tough layer of aluminium oxide forms and protects the metal underneath
Aluminium reacts with oxygen at once, but the oxide it forms sticks firmly to the surface.
This film seals the metal from air and water. Rust on iron, by contrast, is flaky and falls off, exposing fresh iron.
Worth noting
The question states that aluminium is more reactive, so the answer must explain protection, not low reactivity.
An iron nail is left in blue copper sulphate solution. After a while the solution turns pale green and a brown coating forms on the nail. What does this show?
AnswerB · Iron is more reactive than copper and displaces it from the solution
Fe + CuSO₄ → FeSO₄ + Cu.
The brown coating is copper metal, and the pale green colour is iron sulphate. A more reactive metal displaces a less reactive one from its salt.
Worth noting
Rust is reddish-brown too, but rust would not change the solution from blue to green.
The chemical name of common salt is
AnswerA · sodium chloride
Common salt is sodium chloride, NaCl.
Washing soda, used to soften hard water and clean clothes, is
AnswerD · sodium carbonate
Washing soda is hydrated sodium carbonate, Na₂CO₃·10H₂O.
It removes the calcium and magnesium that make water hard.
Bleaching powder is commonly added to drinking water supplies in order to
AnswerC · kill germs
Bleaching powder (calcium oxychloride, CaOCl₂) releases chlorine in water.
Chlorine kills disease-causing bacteria, so the water becomes safe to drink.
The fizz in soda water and soft drinks is caused by dissolved
AnswerA · carbon dioxide
Carbon dioxide is dissolved in the drink under pressure.
When the bottle is opened, the pressure drops and the gas comes out as bubbles.
Compressed natural gas (CNG), used in buses and autorickshaws, is mainly
AnswerD · methane
Natural gas is mostly methane, CH₄, stored under high pressure as CNG.
It burns more cleanly than petrol or diesel, with less smoke and fewer particles.
'Dry ice', used to keep ice cream and medicines cold in transport, is
AnswerD · solid carbon dioxide
Dry ice is carbon dioxide frozen at about −78 °C.
It turns straight into gas without melting, so it leaves no liquid behind; that is why it is called dry.
What is the chemical formula of baking soda?
AnswerB · NaHCO₃
Baking soda is sodium hydrogen carbonate, written NaHCO₃.
NaCl is common salt and CaCO₃ is limestone or marble.
Na₂CO₃ is sodium carbonate, which is washing soda — a different substance with a similar name and a similar-looking formula.
Worth noting
Washing soda and baking soda are the pair most often swapped. The hydrogen in NaHCO₃ is the difference, and it is why baking soda releases carbon dioxide when heated.
The cooking gas LPG is mainly made up of
AnswerB · butane and propane
Liquefied petroleum gas is mostly butane and propane, obtained from petroleum.
Both gases have no smell, so a strong-smelling compound (ethyl mercaptan) is added to reveal leaks.
Worth noting
Methane is the main part of CNG and biogas, not LPG.
Which gas is known as laughing gas and was one of the first anaesthetics used in dentistry?
AnswerA · Nitrous oxide
Nitrous oxide, N₂O, causes a light-headed, cheerful feeling when breathed in, hence the name.
Worth noting
Nitrogen dioxide, NO₂, is a brown, poisonous air pollutant; the formulas differ by one letter.
A cake becomes soft and spongy when baked with baking powder because the baking powder releases
AnswerB · carbon dioxide gas
Baking powder is baking soda mixed with a mild edible acid such as tartaric acid.
With water and heat they react to give carbon dioxide, whose bubbles make the cake rise.
Worth noting
The acid in baking powder also removes the bitter taste that baking soda alone would leave.
Soap gives little lather in hard water. Hard water contains dissolved salts of
AnswerC · calcium and magnesium
Hard water has dissolved calcium and magnesium salts, mainly hydrogencarbonates, chlorides and sulphates.
These react with soap to form an insoluble scum instead of lather.
Why do most toothpastes contain a fluoride compound?
AnswerC · It strengthens tooth enamel and helps prevent decay
Fluoride becomes part of the enamel surface and makes it more resistant to acid.
Stronger enamel means less tooth decay.
Detergents clean well in hard water, but soaps do not. Why?
AnswerB · Detergents do not form an insoluble scum with calcium and magnesium ions
Soap molecules combine with calcium and magnesium ions to make an insoluble scum, so the soap is used up and little lather forms.
Detergent molecules stay dissolved with these ions, so they keep cleaning.
Worth noting
Detergents do not soften the water; they simply do not react with the hardness.
Liquid household bleach, used on white clothes and to disinfect toilets, is mainly a solution of
AnswerD · sodium hypochlorite
Household liquid bleach is a dilute solution of sodium hypochlorite, NaOCl.
It releases chlorine-based compounds that remove colour and kill germs.
Worth noting
It should never be mixed with acidic toilet cleaners, because that releases poisonous chlorine gas.
Which of these is a chemical change?
AnswerA · Burning a sheet of paper
A chemical change makes new substances and usually cannot be reversed simply.
Burning paper gives ash, smoke and carbon dioxide. Melting, dissolving and tearing change only the form.
Respiration in our cells releases energy from glucose. A reaction that gives out heat is called
AnswerD · exothermic
Exothermic reactions release heat; endothermic reactions absorb it.
Respiration, like burning, gives out energy, so it is exothermic.
Which of these is a physical change, not a chemical one?
AnswerA · Water freezing into ice
Freezing changes only the state; the ice is still water and melts back.
Souring milk, rusting and digestion all form new substances.
A magnesium ribbon burns with a dazzling white flame and leaves a white powder, magnesium oxide. What type of reaction is this?
AnswerA · Combination
2Mg + O₂ → 2MgO.
Two substances join to form one product, so it is a combination reaction (and also an oxidation).
When limestone (calcium carbonate) is heated strongly in a kiln, it gives quicklime and carbon dioxide. This is a
AnswerC · decomposition reaction
CaCO₃ → CaO + CO₂ on heating.
One substance breaks into two simpler ones, so it is a decomposition (here, thermal decomposition).
Zinc + copper sulphate → zinc sulphate + copper. This reaction is an example of
AnswerB · a displacement reaction
Zinc, the more reactive metal, takes the place of copper in the compound.
One element pushing another out of its compound is a displacement reaction.
Mixing solutions of sodium chloride and silver nitrate gives a white precipitate of silver chloride and a solution of sodium nitrate. What type of reaction is this?
AnswerB · Double displacement
NaCl + AgNO₃ → AgCl + NaNO₃.
The two compounds swap partners, so it is a double displacement; because a solid forms, it is also a precipitation reaction.
Packets of potato chips are filled with nitrogen gas. The main reason is to
AnswerA · stop the oil in the chips from being oxidised and turning rancid
Oxygen in air slowly oxidises fats and oils, giving a bad smell and taste (rancidity).
Nitrogen is unreactive, so replacing the air with it keeps the chips fresh (and the gas cushions them).
Worth noting
Nitrogen is not poisonous; we breathe it in every breath, as it is about 78% of air.
Shiny brown copper powder turns black when heated in air. This is because the copper has been
AnswerD · oxidised to copper oxide
2Cu + O₂ → 2CuO; copper gains oxygen.
Gain of oxygen is oxidation, and copper oxide is black.
Worth noting
Passing hydrogen over the hot black powder turns it brown again: that reverse step is reduction.
Which of these is an endothermic process, one that needs a continuous supply of energy?
AnswerC · Photosynthesis
Photosynthesis takes in energy from sunlight to make glucose from carbon dioxide and water.
Burning and respiration release energy, so they are exothermic.
When the equation H₂ + O₂ → H₂O is balanced using the smallest whole numbers, the coefficients of H₂, O₂ and H₂O are
AnswerD · 2, 1, 2
Oxygen: 2 atoms on the left, so 2H₂O is needed on the right.
Hydrogen: 2H₂O has 4 hydrogen atoms, so 2H₂ on the left: 2H₂ + O₂ → 2H₂O.
White silver chloride turns grey when left in sunlight. This reaction, once used in black-and-white photography, is a
AnswerB · decomposition caused by light
2AgCl → 2Ag + Cl₂ in sunlight.
Light splits the compound into silver (grey) and chlorine: a photolytic decomposition.
Worth noting
Sunlight supplies energy but is not an element, so it cannot displace anything.
Copper vessels left for a long time in moist air develop a dull green coating. The coating is mainly
AnswerC · basic copper carbonate
Copper reacts slowly with moist carbon dioxide and oxygen in the air.
The green film formed is basic copper carbonate, a mixture of copper carbonate and copper hydroxide. This is corrosion of copper.
Worth noting
Copper oxide is black, not green; it forms when copper is heated in dry air.
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