IELTS Academic Reading Practice Test 04
Dust: The Archive Beneath Our Feet
A Dust is usually treated as evidence of neglect, something to be swept away before it becomes visible. To environmental scientists, however, a layer of dust is a mixed record of the places through which air has travelled. It may contain mineral grains lifted from deserts, pollen released by plants, soot from combustion, fragments of synthetic fibres and microscopic remains of living organisms. Because these particles differ in size, chemistry and origin, their accumulation can reveal both sudden events and gradual environmental change. The challenge is that dust is not a single substance. Every sample is an assemblage whose components may have travelled different distances and settled at different rates. Researchers must separate these histories before interpreting the deposit as a whole.
B The longest records are found where particles settle and remain undisturbed. In polar ice, airborne material becomes trapped as annual snow is compressed. Lake sediments and peat bogs preserve comparable sequences, although their layers are often disturbed by burrowing organisms or flowing water. Researchers date such deposits using several techniques: identifiable volcanic ash can link distant sites to one eruption, while radioactive isotopes provide estimates for material from particular periods. No single clock is perfect, so independent methods are normally compared.
C Mineral dust can travel astonishing distances. Winds carry particles from the Sahara across the Atlantic, where some settle in the Amazon basin. These arrivals supply phosphorus, an element needed by plants but continuously removed from tropical soils by heavy rain. Satellite images reveal the moving plumes, yet satellites measure reflected light rather than collecting particles directly. Ground stations and chemical fingerprints are therefore required to establish where the material began and how much nutrient it delivered. Individual plumes also vary from year to year as rainfall, vegetation and wind patterns change in the source region. A dramatic satellite image cannot by itself represent the long-term average.
D Dust also records human activity. A sharp rise in microscopic charcoal may indicate burning, but interpreting it is difficult. Fire can result from lightning, deliberate land management or accidental ignition. Similarly, metal-rich particles may come from mining, industry or vehicle brakes. Researchers must compare the deposit with historical evidence and with particles from likely sources. A match in shape or chemistry can strengthen an interpretation; concentration alone rarely proves a cause.
E Inside buildings, dust follows a different cycle. Outdoor particles enter through windows, ventilation systems and on shoes, while occupants contribute skin cells, textile fibres and residues from cooking. Studies of household dust can identify exposure to lead, flame retardants and other substances. Nevertheless, the quantity found on a floor does not equal the dose entering a person’s body. Exposure depends on behaviour, particle size and the chemical form of the substance. Young children may face greater contact because they crawl and frequently place their hands in their mouths.
F Sampling methods can themselves distort the record. A vacuum cleaner captures a different range of particles from a passive collector left on a shelf. The height, duration and location of collection all affect results. Laboratories therefore use blank samples to detect contamination and standard reference materials to check measurements. Increasingly, researchers also record detailed metadata—the precise circumstances in which a sample was obtained—so that another team can judge whether two datasets are genuinely comparable. Preservation after collection matters as well: an unsealed container can gain modern particles, while damp storage may encourage biological growth. The archive continues to change unless it is carefully isolated.
G The ambition is not to make dust tell one simple story. Its value comes from combining imperfect clues across scales. An ice core may reveal centuries of atmospheric change, whereas dust from a bedroom may represent a few weeks of local exposure. Both are archives, but they answer different questions. Scientists who treat every particle as evidence rather than dirt must still distinguish what the deposit records faithfully from what has been moved, mixed or selectively preserved.
Where One Colour Ends and Another Begins
A The visible spectrum changes continuously; there is no physical line where green finishes and blue begins. Languages nevertheless divide this continuum into named categories. English speakers commonly use basic terms such as red, yellow, green and blue, but other languages organise the same visual information differently. This variation has made colour naming a valuable test case in a larger debate: does language merely label perceptions that humans already share, or can words influence what people notice and remember?
B An influential early survey compared colour terms across many languages. Its authors argued that basic colour vocabularies were not arbitrary. Languages with only a few general terms appeared to add new categories in a partly regular order as their vocabularies expanded. The proposal challenged the idea that cultures could divide colour in unlimited ways. Critics, however, questioned how the languages had been sampled and whether translating unfamiliar words into English categories had forced diverse systems into a predetermined pattern. They also noted that bilingual consultants might respond differently from speakers who used the language in a single local setting. Later surveys widened the sample, but disagreement over the universality of the proposed sequence continued.
C Later fieldwork showed why context matters. A word that seems to denote a colour may also refer to brightness, ripeness, moisture or the appearance of a valued material. Speakers may use one expression when describing cloth and another when discussing animals. Asking participants to name isolated coloured chips can therefore produce an incomplete picture. The controlled task is easy to compare across communities, but it removes the practical situations in which colour words acquire meaning.
D Experiments on categorical perception offer evidence that labels can affect speed of discrimination. Participants are shown two very similar colours and asked whether they match. Responses are sometimes faster when the colours fall on opposite sides of a boundary named in the participant’s language than when both have the same name. The physical distance between the colours may be equal. This result suggests that familiar verbal categories can make a difference more noticeable, especially when decisions must be made quickly. Researchers must still prevent participants from using brightness or saturation as unintended clues. Small differences in equipment or instructions can otherwise create an apparent language effect that another laboratory cannot reproduce.
E Yet the effect is neither absolute nor permanent. Infants can distinguish many colour differences before they learn words, and adults can learn new boundaries through training. When participants perform a demanding verbal task at the same time as a colour judgement, the advantage associated with named categories may weaken. Some researchers interpret this as evidence that language is recruited during the decision rather than having permanently rebuilt the visual system.
F Studies of memory add another complication. A remembered shade may drift towards the typical example of its named category. If a turquoise sample is labelled ‘blue’, a person may later select a slightly bluer version. However, memory is also shaped by attention, expectations and the purpose of the task. Professional painters and textile workers can develop unusually precise distinctions, sometimes supported by specialist vocabulary but also by repeated visual practice.
G Technology is changing the evidence available. Large digital image collections allow researchers to examine how colour words occur in natural descriptions rather than only in laboratory answers. At the same time, screens introduce problems: devices reproduce colour differently, ambient light alters appearance, and online participants may not be viewing calibrated displays. Bigger datasets do not automatically create cleaner measurements.
H The strongest conclusion is therefore modest. Human visual systems impose broad shared constraints, but languages highlight particular regions of the spectrum and provide habits for comparing them. Words do not imprison speakers inside separate perceptual worlds. Nor are they decorative labels with no cognitive effect. Colour research shows perception emerging from an interaction among biology, language, expertise and immediate task demands.
Concrete That Repairs Its Own Cracks
A Concrete is strong under compression, inexpensive and adaptable, which explains its dominance in modern construction. Its weakness is cracking. Small cracks may not threaten a structure immediately, but they allow water, oxygen and salts to reach steel reinforcement. Corrosion then expands the steel and breaks away surrounding concrete. Maintenance is costly partly because damage is difficult to inspect in bridges, tunnels and offshore structures. This has encouraged research into materials capable of sealing cracks with limited human intervention. Preventing every crack is unrealistic: concrete shrinks as it cures, responds to temperature changes and carries variable loads. The practical aim is to stop ordinary microcracks from developing into routes for more serious deterioration.
B Concrete already possesses a modest natural ability to heal. Water entering a narrow crack can react with unhydrated cement particles, producing new material, while dissolved calcium compounds may form crystals that block the gap. This autogenous healing works best in very small cracks and when moisture is available. It is unreliable for wider damage, so engineers have explored ways to amplify the process rather than depend on it alone.
C One approach places dormant bacterial spores and a nutrient source inside the concrete. When a crack admits water, the spores become active. Their metabolism creates conditions in which calcium carbonate precipitates, gradually filling the space. The bacteria are not intended to restore broken reinforcement or close a major structural fracture. Their purpose is to seal early cracks before aggressive substances penetrate deeply.
D Simply mixing living agents into wet concrete is ineffective because fresh concrete is highly alkaline and becomes mechanically dense as it hardens. Spores and nutrients may be crushed, damaged or consumed too soon. Researchers therefore protect them in lightweight aggregates, microscopic capsules or porous carriers. The protective system must survive mixing but open when a crack forms. A carrier that is too fragile releases its contents during construction; one that is too strong may never release them.
E Laboratory demonstrations are promising. Treated specimens often show reduced water flow after cracking, and some recover part of their resistance to liquid penetration. However, reports use different crack widths, curing conditions, bacterial species and measures of success. A surface that appears closed under a microscope may still permit some transport through its depth. Standardised testing is needed before results from separate laboratories can be compared confidently. Researchers must also distinguish sealing from recovery of mechanical strength. A mineral plug may block water effectively without returning a damaged specimen to its original load-bearing capacity. Both outcomes are useful, but they should not be reported as though they were identical.
F Cost remains an obstacle. Encapsulation adds manufacturing steps, and specialised nutrients can be expensive. Advocates argue that a higher initial price may be justified if structures last longer or require fewer disruptive repairs. Such claims depend on life-cycle assessment: the extra materials and energy used at construction must be weighed against maintenance avoided decades later. Field data covering that length of time do not yet exist for most systems.
G Environmental claims also require care. Cement production generates substantial carbon emissions, so extending the life of concrete could reduce the need for replacement. But a healing agent is not automatically sustainable. Its manufacture may consume energy, and some nutrient formulations have undesirable environmental effects. Researchers are investigating waste-derived nutrients and carriers with lower impacts, yet performance cannot be sacrificed merely to improve a laboratory carbon estimate.
H Bacterial concrete is best understood as one part of a broader maintenance strategy. Sensors may identify unusual strain, careful design may limit cracking, and conventional repair will remain necessary when damage is severe. Self-healing systems could be particularly useful where access is dangerous or expensive. Their success will depend less on a dramatic image of living buildings than on dependable performance, agreed test methods and evidence that benefits exceed costs over an entire service life.
Review each question for the accepted answer and passage evidence.
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