World & physics.
World & physics
What is chemistry?
Chemistry is about substances changing
Chemistry studies substances: what they are made of, how they behave and how they change. An iron bicycle left in damp air gives us a familiar example. Over time, its surface can develop rust.
Several ingredients meet
At the bicycle’s exposed surface, iron meets water and oxygen from the air. These substances can react together. Scratched paint exposes more iron, so that patch gives the reaction a place to begin.
Atoms rearrange into new substances
During rusting, electrons are transferred and atoms become arranged in new compounds containing iron and oxygen. Rust has different characteristics from the original metal, including a crumbly structure that offers little protection underneath.
Paint changes the conditions
A coat of paint acts as a barrier between the iron and its surroundings. Keeping water and oxygen away slows the reaction. This explains why protecting a scratch can help protect the bicycle.
The same lens reaches into life
The rusting bicycle shows chemistry’s central idea: what meets, under which conditions, determines what can change. Inside living cells, carefully organised chemical reactions build molecules, release energy and carry signals. That study is biochemistry.
Chemistry explains how atoms combine, separate and rearrange to make substances behave differently.
Ideas from OpenStax and the American Chemical Society · Introductory chemistry and food chemistry
Original plain-language explanation informed by the educational sources linked below.
A physical change can alter the arrangement or movement of molecules. Ice melting into liquid water is an example. A chemical reaction changes which substances are present by changing bonds between atoms.
Toast browning involves many reactions. A group called Maillard reactions includes interactions between sugars and amino acids. The resulting molecules contribute to colour and flavour.
Biochemistry follows chemical processes in living things. Materials chemistry helps explain substances used to build things, including the silicon inside a computer.
A dots-and-links picture gives a first view of atoms and bonds. More detailed models explain their shapes, electrical behaviour and reactions.
What is biology?
A seed becomes a plant
Biology is the study of living things and how they work. A bean seed provides a small example: under suitable conditions it takes in water, grows roots and eventually produces leaves.
Water starts the activity
A dry seed contains living tissue and stored nutrients. When it takes in water under suitable conditions, chemical activity increases. Its cells use stored resources to support the earliest growth.
Cells build more living tissue
The growing plant makes new cells and enlarges existing ones. Different cells take on different jobs. Roots absorb water and minerals, while leaves later capture light for photosynthesis.
The plant depends on its world
A plant grows differently when light, water or temperature changes. Insects may eat its leaves, and microbes may affect its roots. Understanding the plant also means understanding these relationships with its surroundings.
Living systems contain smaller systems
The bean plant connects several scales: chemical reactions help cells work; cells form roots and leaves; the whole plant interacts with its environment. Biology follows these connections across a living system.
Biology studies living systems, from cells maintaining themselves to organisms interacting with their surroundings.
Ideas from OpenStax and NIH · Biology, cells and the heart
Original plain-language explanation informed by the educational sources linked below.
In a human body, cells form tissues, tissues form organs, and organs work together in systems. A heart belongs to the system that circulates blood. This is one way to follow the connections between small parts and a whole living thing.
Some living things consist of one cell; others contain many cells. Biology also explores how populations change over generations and how living things interact in environments such as forests and oceans.
Biochemistry opens the molecules and reactions inside these living systems. Neuroscience studies the nervous system, including the brain. These subjects connect different levels of the same living world.
A human body is one example within biology. Different living things have different structures, needs and ways of life.
How is everything connected?
A bicycle is a system
A system is a group of parts that affect one another. A bicycle makes this easy to see: your legs, the pedals, the chain and the wheel work together to move you down the road.
Your foot starts the movement
Your foot pushes a pedal. The pedal turns a toothed wheel at the front of the bicycle. That wheel pulls the chain, so movement begins to travel through the connected parts.
The wheel receives the push
The moving chain turns the gear attached to the back wheel. The tyre pushes against the road, and the road pushes the bicycle forwards. Each connection helps pass the effect along.
A broken link changes everything
Now imagine the chain slips off. Your legs can still move the pedals, but that movement no longer reaches the back wheel through the chain. The bicycle has the same parts, with a connection missing.
Connections explain the whole
A bicycle works because its parts interact in a useful arrangement. Bodies, businesses and economies can also be understood by tracing which parts affect others and how a changed connection alters the whole system.
A system is a set of connected parts. A change in one part can travel through the rest.
A connected picture helps you ask questions across subjects. The detailed explanation of a political decision still needs people, institutions, evidence and circumstances. Several kinds of explanation can contribute to one event.
This is a learning map. Its links include physical mechanisms, social arrangements and ways of explaining. Their meanings are labelled separately.
What is everyday stuff made of?
The cup is made of matter
Matter is the material that makes up things such as a cup, water and air. A cup feels solid because enormous numbers of tiny constituents hold together in a stable structure.
Water fills the cup
Water is made of molecules. A molecule is a small group of atoms joined together. Each water molecule contains two hydrogen atoms and one oxygen atom, joined in a particular arrangement.
Warm water moves faster inside
When the water is heated, its molecules move more vigorously on average. They are already moving before heating. The added energy changes their motion, even though the individual molecules remain far too small to see.
Some water becomes vapour
Some molecules can escape from the liquid surface into the air. Heating can increase evaporation. The water changes its physical arrangement, while each escaping molecule still contains hydrogen and oxygen joined together.
Tiny behaviour becomes visible
The cup connects scales: atoms join into molecules, molecules move together, and you experience water that flows, warms and evaporates. Chemistry investigates the substances and their changes; physics helps explain the forces and energy involved.
Matter is the material in physical objects; its tiny constituents help explain its visible behaviour.
An atom contains a small nucleus and electrons. At this scale, everyday pictures of solid balls are limited. The atom lessons open the next layer.
Physics also studies light, fields, space and other phenomena. A map of everyday material is one starting point within that larger subject.
The building-piece picture is a first model of everyday matter. More detailed models are needed at atomic and smaller scales.
What does energy do?
Energy makes change possible
Energy is a quantity that helps us track the ability to cause physical change. When sunlight warms a pavement or your legs move a bicycle, energy is being transferred and transformed.
A leaf captures sunlight
A plant absorbs some sunlight. Through photosynthesis, it uses that energy to help make sugars from water and carbon dioxide. Energy becomes stored in the chemical arrangements of the products.
Food carries that energy onwards
Imagine eating bread made from the plant’s grain. Digestion breaks food into smaller molecules. Your cells use chemical reactions involving those molecules to supply energy for movement, repair and many other activities.
Your movement also produces heat
As your muscles work, some energy supports movement and some spreads as heat. Your body warms its surroundings. A machine follows the same broad accounting: useful output and energy spreading into the environment.
One journey crosses many subjects
Sunlight, plant chemistry, food production and your body belong to one connected story. Economics then asks how people organise the work and exchange the food. Energy provides a physical foundation for that activity.
Energy lets physical changes happen. Following its transfers connects sunlight, food, bodies and machines.
Energy is a measurable quantity that takes different forms. Tracking transfers gives a useful accounting method across electrical, chemical and mechanical changes.
In a food chain, energy passes through living things and is increasingly transferred as heat. Matter such as carbon and water also moves through the environment. These are related flows with different accounting.
This overview follows familiar energy transfers. The detailed mechanism depends on the system being studied.
How do tiny parts make a living body?
A living body keeps itself going
Living things carry out organised processes that help them persist, grow and reproduce. A person offers a familiar example: breathing, eating and circulation continually support the cells that make up the body.
Cells need incoming supplies
After a meal, digested nutrients enter the bloodstream. Breathing brings oxygen into the lungs. Circulation carries these supplies towards cells throughout the body, where they support many chemical processes.
Cells spend energy on upkeep
Cells use energy to maintain boundaries, build molecules, move substances and repair damage. This continuing activity helps preserve a living arrangement despite constant wear and changing conditions around the body.
Waste and heat leave the body
The body also releases carbon dioxide, other waste products and heat. Life involves a continuing flow through the system: resources enter, reactions occur, and products leave. Blocking a vital flow can disrupt the whole body.
Life links chemistry to behaviour
Those maintained cells form organs, muscles and the brain. Their organised activity enables movement, perception and choices. A human conversation rests on living processes that keep the speakers’ bodies functioning moment by moment.
Living things maintain and reproduce organised systems by exchanging matter and energy with their surroundings.
Organisation matters. Describing a list of molecules and describing how a cell maintains itself answer different parts of the same biological question.
The development and evolution of living systems are large subjects of their own. This card follows how a living body is organised today.
This picture describes cells and bodies at a broad level. The origin of life and the full details of each cell process require further explanations.
How can a system adjust itself?
A result comes back into the system
Feedback means that the result of an action affects the system’s next action. A room thermostat provides a simple example: it measures temperature and uses that information to control the heating.
Room temperature temperature is measured → Thermostat
The room is too cold
Suppose the thermostat is set to 20°C and measures 17°C. It switches the heating on. The measured temperature tells the controller that the room needs more warmth to reach its setting.
The heating changes the reading
As the room warms, the thermostat measures a higher temperature. The action has changed the very thing being measured. That returning information is the feedback connection in this example.
Reaching the target changes the action
When the room reaches the thermostat’s switching threshold, the heating turns off. As warmth later escapes, the reading falls and heating can start again. This repeated correction keeps temperature within a range.
Feedback can stabilise or amplify
The thermostat opposes a temperature change, which helps stabilise the room. Other feedback can amplify change: a rumour spreads, attracts attention and is shared again. The effect depends on how the returning result changes the next action.
Heat the room changes → Read temperature
Read temperature guides → Adjust heating
Adjust heating changes next heating → Heat the room
Feedback happens when an action’s result returns to influence what happens next.
Feedback that reduces a change can help keep a condition within a workable range. Delays and limits affect how smoothly the adjustment happens.
The same pattern can help organise questions about bodies, machines and organisations. In each case, identify what is measured, who or what responds, and how the response affects the next observation.
A thermostat and a person implement feedback in different ways. The shared pattern is useful when the parts and limits are specified.
Did we invent the world or its descriptions?
A map is a model
A model is a simplified representation used to understand something. A street map is one: it represents a city while leaving out most of what you would experience by walking through it.
The map keeps useful relationships
Suppose you need to reach a station. A map shows which roads connect, where you are and where the station is. Those relationships matter for choosing a route through the city.
Some details disappear
The same map may leave out steep hills, building colours or pavement closures. Leaving things out makes the map easier to read, but each omission limits which decisions the map can reliably support.
A different job needs a different map
A cyclist may need hills and cycle lanes. A train passenger needs stations and line changes. Both maps can describe the same city while highlighting different information for different purposes.
The model guides a checkable decision
You choose a route, try it and compare the journey with the map. A mismatch reveals something to update. Scientific models and mental models use this same broad habit: simplify, apply, observe and revise.
A model leaves out detail to make a particular relationship easier to understand or predict.
Our choice of words and diagrams shapes how we organise questions. Measurement and comparison help distinguish explanations with different practical consequences.
Philosophers debate whether successful scientific theories describe a world independent of minds, organise experiences, or do some combination of these. The debates involve further arguments.
The success of a useful model constrains some claims. The complete metaphysical interpretation remains a separate philosophical question.
What do we mean by real?
You experience a world through your mind
You see a cup on a table. Light reaches your eyes, and your brain processes the signals. The cup as experienced includes this biological work, which raises a philosophical question about the relation between experience and reality.
The view changes with the conditions
Move the cup into dim light and its colour may look different. Your experience changes with lighting, viewpoint and your visual system. These factors matter when interpreting what the cup looks like.
Other observations help us compare
Another person looks at the cup. A camera records it. A ruler measures its width. Comparing methods helps separate features that stay stable from appearances that depend strongly on the observer or conditions.
Philosophers interpret this relationship differently
Realists commonly hold that a world exists independently of our experience. Idealist traditions give mind a more fundamental role. The fact that perception involves processing leaves room for argument about these broader philosophical claims.
The whole picture includes the observer
Physics studies the cup and light. Biology studies eyes and brains. Psychology studies perception and interpretation. Philosophy examines what these findings allow us to claim. Connecting them helps keep observation, explanation and belief distinct.
Our experience is shaped by the mind; understanding reality also requires comparing observations and testing explanations.
Calling a category human-made raises questions about how people establish it. Whether everything that exists depends on mind is a further claim that different philosophies debate.
You can compare views by stating what each claims, what would count as evidence, and which questions it leaves open. Some disputes are about experience, some about measurement, and some about the meaning of our concepts.
This card organises an enquiry. It leaves competing philosophical accounts open to examination.