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Economics

How does the price of related goods influence demand?

Related goods are goods that are bought with another good (complements), or instead of another good (substitutes). An example of complementary goods is movie tickets and beverages. Often, when someone buys a movie ticket, they will also buy a beverage. Notice that the strength of this relationship changes depending on the product: it is more common for someone to buy a drink on its own—without going to the movies—than for someone to buy a movie ticket and not buy a drink. The law of demand states that when the price of a good increases, the quantity demanded for that good decreases, and vice versa. So, if the price of movie tickets goes up, people will buy fewer movie tickets. For the theatre, this means that not only will they sell fewer movie tickets, but they will also sell fewer drinks: this shows the complementary relationship between movie tickets and drinks. An example of substitute goods are two competing movie theatres. If people usually go to the movies at MovieZone **or** CinePlace, but not both simultaneously, then these brands are strong substitutes for each other. The strength of this relationship is determined by consumer behavior. A consumer who wants to go to the movies is likely to prefer either MovieZone or CinePlace to a different activity, like going to a museum. So museums are a weak substitute for either of the theatres. As above, the law of demand dictates that if MovieZone raises the price of tickets, the quantity demanded will fall. This effect is amplified if consumers are very willing to switch to CinePlace. For this reason, firms aim to build brand loyalty, weakening the strength of the substitution relationship with competing brands. |If the price of MovieZone tickets | The quantity demanded for MovieZone tickets | The demand for drinks |The demand for CinePlace tickets | | :-: | :-: | :-: |:-:| | increases | decreases | also decreases |increases| | decreases | increases| also increases | decreases|

Biology

Why is water so important for metabolic reactions?

Water is indispensable for metabolism due to its role as a solvent, reactant, temperature regulator, and chemical stabilising properties. Without water, cells could not carry out the complex and coordinated reactions that sustain life. Let’s expand on these ideas. Water is essential for metabolic reactions because it acts as a universal solvent, allowing many substances to dissolve and interact within the cell. Most biochemical reactions occur in aqueous environments, and the reactants (substrates) must be dissolved in water to collide and react efficiently. Water facilitates the transport of ions and molecules, such as glucose, oxygen, and enzymes, to the right places in the cell where reactions take place. Without water, many metabolic reactions would slow down or stop altogether due to the lack of a suitable medium for molecular movement. Additionally, water actively participates in many chemical reactions. For example, in hydrolysis reactions, water is used to break down complex molecules into simpler ones, such as during the digestion of proteins, carbohydrates, and lipids. These reactions are fundamental to metabolism because they help provide cells with usable building blocks and energy. Conversely, in condensation reactions, water is released when smaller molecules are joined to form larger ones, such as in the synthesis of proteins or nucleic acids. Water, therefore, is not only a medium but also a reactant or product in key metabolic processes. Water also helps regulate temperature within cells and organisms. It has a high specific heat capacity, meaning it can absorb and release large amounts of heat with minimal temperature change. This property helps maintain stable internal conditions, which is crucial because enzymes that control metabolism function within a narrow temperature range. Sudden changes in temperature could denature enzymes or slow their activity, disrupting metabolic balance. Lastly, water's role in maintaining pH and ion balance is vital for metabolism. Many metabolic reactions are sensitive to changes in pH, and water helps buffer these changes by participating in equilibrium reactions. It also helps maintain proper concentrations of hydrogen ions (H⁺), hydroxide ions (OH⁻), and other electrolytes, which are necessary for processes like cellular respiration and nerve impulse transmission.

Psychology

What Is the directionality problem in psychology research?

The directional problem in psychology occurs when a correlation or relationship is found between two variables, and difficulty arises in determining which variable has caused the change in the other. For example, does having a “good memory” cause a person’s hippocampus to be larger, or does having a larger hippocampus result in having a stronger memory? The directional problem is sometimes referred to as bidirectional ambiguity. Bidirectional ambiguity raises a more general question about whether two variables are influencing each other simultaneously through mutual interaction. Issues with directionality most commonly occur when conducting correlational research. A correlational study examines the possible relationship between two or more variables. However, unlike a true experiment, the researchers do not manipulate variables; rather, they investigate naturally occurring or pre-existing variables.

Chemistry

What is the lattice energy trend across the periodic table?

Lattice energy increases as you move across a period in the periodic table, particularly when comparing ionic compounds of metals from successive groups. Lattice energy is defined as the energy required to completely separate one mole of an ionic solid into gaseous ions, essentially the energy needed to break apart the ionic lattice: :::center $\ce{M_aX_b}\textrm{(s)} \longrightarrow a \ce{\,M^{b+}}\textrm{(g)} + b\ce{\,X^{a-}}\textrm{(g)}$ ::: This value is always positive because energy must be supplied to overcome the electrostatic attractions holding the ions together. The magnitude of lattice energy directly reflects the strength of the ionic bonding in the compound; stronger ionic bonds require more energy to break, resulting in higher lattice energy values. This increasing trend across a period occurs because the charge on the cations increases as you move from left to right across the metal groups. For example, comparing oxides across Period 3: $\ce{Na2O}$ contains $\ce{Na+}$ cations, MgO contains $\ce{Mg^{2+}}$ cations, and $\ce{Al2O3}$ contains $\ce{Al^{3+}}$ cations. According to Coulomb's law, the electrostatic attraction between ions is directly proportional to the product of their charges and inversely proportional to the distance between them. As the cation charge increases from +1 to +2 to +3, the electrostatic attraction to the oxide anion $(\ce{O^{2-}})$ becomes progressively stronger. Additionally, moving across a period, the cations generally become smaller due to increasing nuclear charge pulling the electrons closer, which further increases the lattice energy since the ions can pack more closely together. This combination of higher charge and smaller ionic radius results in much stronger coulombic attractions and therefore significantly higher lattice energies as you progress across the periodic table.

Environmental Systems and Societies

What are environmental value systems?

An environmental value system is a model of our environmental perspectives. Like all systems, it includes inputs and outputs. $\hspace{7em}$ **Figure: An environmental value system** $\hspace{5.8em}$ **input $\rightarrow$ environmental perspective $\rightarrow$ output** **Inputs** that shape your environmental perspective include things you experience personally, like your education, media you engage with, holidays, events and family values. The worldviews of communities with which you come in contact also influence your environmental perspective, through their politics, ideologies, philosophies, religions and cultures. **Outputs** of your environmental perspective include the way you judge environmental issues, the position you take and the actions and choices that you make with regard to these issues. Inputs **change** throughout your lifetime, so your perspective will also change. You are also likely to have a different perspective on different environmental issues because the inputs will not be relevant to all of them. In IB ESS, we refer to three broad categories of perspectives, namely anthropocentric, ecocentric, and technocentric. Outputs will change in response to shifts in perspectives and emerging opportunities. A **systems approach** to environmental perspectives helps us understand why people hold certain beliefs and why they act in particular ways. It also helps shape environmental perspectives by changing the inputs, thereby affecting the outputs as well. Information campaigns by the government or NGOs can thus change people’s behaviour. The **old ESS syllabus**, with final exams scheduled for November 2025, employs the term environmental value system (EVS) to denote an individual’s perspective or a community’s worldview, as described above for the **new ESS syllabus**, with first exams in May 2026. Thus, the old exams might ask about how a technocentric EVS would solve an issue, whereas the new exams would ask how a technocentric perspective might solve it. Both syllabi recognise that environmental views are affected by various inputs and may result in a range of outputs.

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