Dogs are loyal companions.

Chinese
狗是忠实的伴侣。

The sun rises every morning.

Chinese
太阳每天早上升起。

The sun rises every morning. Chinese太阳每天早上升起。

What is the sub-atomic particle with the smallest relative mass?

The sub-atomic particle with the smallest relative mass is the electron.

Explain why atoms have the same number of protons as electrons.

Atoms have the same number of protons as electrons because of electrical neutrality. Protons carry a positive charge, while electrons carry a negative charge. In an electrically neutral atom, the number of protons (which have a positive charge) equals the number of electrons (which have a negative charge), resulting in a balance of charges. This balance ensures that the overall charge of the atom is neutral.

An aluminium atom can be represented as $_{13}^{27}$Al.

Identify aluminium’s mass number.

The notation $_{13}^{27}$Al represents an aluminium atom with a mass number of 27. The number 27 is the sum of protons and neutrons in the nucleus of the atom.

Calculate how many neutrons this atom of aluminium has.

To calculate the number of neutrons, subtract the atomic number (which represents the number of protons) from the mass number:

Number of neutrons = Mass number – Atomic number

For aluminium:
Atomic number (Z) = 13
Mass number (A) = 27

Number of neutrons = 27 – 13 = 14

Therefore, this atom of aluminium has 14 neutrons.

What does the bonding in iron consist of?

Iron consists of positive ions in a sea of delocalized electrons.

How do electrons contribute to iron’s ability to conduct electricity?

Electrons in iron are free to move, allowing electrical conduction.

What causes alloys like steel to be harder and stronger than pure iron?

Atoms of different sizes prevent the layers from sliding easily over each other.

What are the substances that react with iron to form rust?

Oxygen and water.

What is the process called where zinc is used to coat iron to prevent rusting?

Galvanising.

How does the zinc coating protect iron from rusting?

Zinc acts as a barrier to prevent oxygen and water from reaching iron.

How does zinc continue to protect iron from rusting even if the coating is scratched?

Zinc is more reactive than iron and continues to oxidize preferentially, protecting the iron.

What is the role of coke in the extraction of iron in a blast furnace?

Coke releases heat, reduces iron(III) oxide, and reacts with carbon dioxide to form carbon monoxide.

What happens when limestone decomposes in the blast furnace?

Limestone decomposes to calcium oxide, which reacts with silicon(IV) oxide to form slag.

Why is the temperature in the furnace maintained above the melting point of iron?

To keep iron molten, allowing it to separate from the slag.

What is the main chemical reaction of iron extraction in the blast furnace?

Fe₂O₃ + 3CO → 2Fe + 3CO₂.

Why is it important to have different atom sizes in an alloy?

Prevents the layers of atoms from sliding over each other, enhancing strength.

How does coke contribute to the reduction of iron ore?

Coke acts as a reducing agent by donating electrons to iron(III) oxide, reducing it to iron.

What is the purpose of adding limestone to the iron extraction process?

To remove impurities by forming slag with silicon(IV) oxide.

What chemical process occurs when zinc is exposed and iron is protected?

Zinc oxidizes by losing electrons, protecting iron through sacrificial protection.

## Understanding 1.2 SI Units: A Standardised System for Science The sources explain that in the past, various systems of measurement evolved around the world, similar to how different languages developed in different regions. This created difficulties when scientists from different places needed to share and compare their findings. To address this, scientists globally adopted the **Système Internationale (SI)**, a standardised system built upon the metric system. ### The Foundation of SI: Base Units SI is founded on **seven base units**, each meticulously defined and agreed upon at international conventions. These base units are: * **kilogram (kg)** for **mass** * **metre (m)** for **length** * **second (s)** for **time** * **ampere (A)** for **electric current** * **kelvin (K)** for **temperature** * **mole (mol)** for **amount of substance** — used primarily in A Level Physics. * **candela (cd)** for **luminous intensity**— not used in AS or A Level Physics. These base units serve as the building blocks for expressing all other physical quantities in a consistent and unambiguous manner. ### Building Upon the Foundation: Derived Units Any physical quantity that is not a base quantity can be expressed using **derived units**. These units are combinations of the base units, obtained through multiplication or division. The sources offer a table (Table 1.6) with examples of derived units. Here are a few examples: * **Speed:** Derived from length and time, it’s measured in metres per second (m/s or m s-1). * **Force:** Measured in newtons (N), which can be broken down into kg m s-2 using base units. * **Density:** Calculated by dividing mass by volume, it’s expressed in kg m-3. * **Pressure:** Determined by dividing force by area, its derived units are kg m-1 s-2. It’s important to note that derived units **cannot involve adding or subtracting base units**. For instance, you wouldn’t have a unit like “metre plus second”. ### Ensuring Consistency: Checking for Homogeneity For a physics equation to be valid, it must be **homogeneous**. This means every term in the equation must have the same base units. The sources offer several examples of checking equations for homogeneity. Homogeneity checks help: * **Identify errors in equations:** If the units don’t match, the equation is likely incorrect. * **Determine the units of unknown quantities:** If the equation is known to be correct, you can deduce the units of an unknown variable by analyzing the units of the other terms. ### Prefixes: Handling Large and Small Quantities To simplify the representation of very large or very small quantities, SI uses prefixes that denote multiples and submultiples of units. These prefixes are combined with base or derived units. Table 1.5 in the sources lists common prefixes and their corresponding multiplying factors. For example: * **millimetre (mm):** One thousandth of a metre (10-3 m). * **kilogram (kg):** One thousand grams (103 g). * **megawatt (MW):** One million watts (106 W). It’s crucial to be careful when converting units for areas and volumes, as the prefixes are applied to each dimension. For instance: * 1 mm2 = (10-3 m)2 = 10-6 m2 * 1 cm3 = (10-2 m)3 = 10-6 m3 ### Beyond Base and Derived Units: Named Units Some derived units, particularly those representing important physical concepts, are given special names for convenience and clarity. These named units can still be expressed in terms of base units if needed. Here are a few examples: * **newton (N):** The unit of force, equivalent to kg m s-2 in base units. * **joule (J):** The unit of energy, equivalent to kg m2 s-2. * **watt (W):** The unit of power, equivalent to kg m2 s-3. ### Conventions for Presenting SI Units: Clarity and Consistency The sources emphasize the importance of using proper conventions when writing symbols and units to avoid confusion. Key points include: * **Italic type for symbols, roman type for units:** For instance, *v* for velocity and m s-1 for its unit. This distinction is difficult to make in handwriting. * **Specific labeling for tables and graphs:** Column headings and axes should be labeled as “symbol/unit”. For example, *t*/s (time in seconds) or *v*/m s-1 (velocity in metres per second). * **Avoid informal labeling:** Don’t use phrases like “t in s” or “t(s)”. These conventions promote clear and unambiguous communication of scientific data. Overall, the SI system provides a robust and standardized framework for measuring and expressing physical quantities. Its widespread adoption ensures that scientists around the world can communicate and collaborate effectively.

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