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IG ChemistryPearson Edexcel International GCSE

Atomic Structure & Periodic Table4 simulations

Formulae, Mixtures & Quantitative Chemistry3 simulations

Practical Skills & Laboratory Equipment1 simulation

Bonding, Structure & Electrolysis3 simulations

Inorganic Chemistry3 simulations

Physical Chemistry6 simulations

Organic Chemistry & Materials4 simulations

AS ChemistryPearson Edexcel IAS · Grade 11

Atomic Structure, Amounts & Gases4 simulations

Bonding & Intermolecular Forces1 simulation

Energetics, Kinetics & Equilibria1 simulation

Inorganic & Redox ChemistryComing soon

Redox and the chemistry of Groups 1, 2 and 7.

Organic Chemistry1 simulation

Analysis & Practical SkillsComing soon

Mass spectra, infrared spectra and practical methods.

A2 ChemistryPearson Edexcel IA2 · Grade 12

KineticsComing soon

Rate equations, orders and activation energy.

Thermodynamics & EquilibriumComing soon

Entropy, energetics and equilibrium constants.

Acids, Bases & Buffers1 simulation

Redox & Transition Metals1 simulation

Advanced Organic Chemistry1 simulation

Analysis & Organic Synthesis1 simulation

IGCSE · States of matter

What is the change of state?

Drag a state into each circle. The arrow reads from the first circle to the second.

On a phone or tablet, tap a state and then tap a circle.

Your result Choose two states to begin

The direction matters: solid → liquid is melting, but liquid → solid is freezing.

Solid: ordered Liquid: close, irregular Gas: far apart

IGCSE · Electrolysis and metal extraction

Extracting aluminium by electrolysis

Watch ions move through molten aluminium oxide dissolved in cryolite. The graphite lining is the negative cathode; the carbon blocks are positive anodes.

1Break crust & feed Al₂O₃
2Dissolve in cryolite
3Electrolyse at ≈960°C
4Form CO₂ & consume carbon
5Tap molten aluminium

Drag to rotate · scroll or pinch to zoom · right-drag to pan

Before electrolysis: the Bayer process purifies bauxite into alumina, Al₂O₃. Cryolite dissolves alumina and lowers the operating temperature to about 960°C. Cell polarity: the carbon lining is the cathode; suspended carbon blocks are the anodes. Overall: 2Al₂O₃ + 3C → 4Al + 3CO₂

K_p Simulator

N₂O₄ ⇌ 2 NO₂ · Endothermic (ΔH° = +57.2 kJ/mol)
Only temperature changes K_p. Changing pressure (volume) or concentration shifts the equilibrium position but the K_p calculation always gives the same answer.

N₂O₄ ⇌ 2 NO₂

🌡

Temperature changes K_p

298 K

📦

Volume K_p unchanged ✓

30.0 L

⚗

Initial N₂O₄ K_p unchanged ✓

1.00 mol

Equilibrium gas mixture — mole fraction (bar width) shifts with volume & concentration; Kp computed below stays constant

N₂O₄ colorless

NO₂ brown

Live K_p derivation — updates with every slider move

① Equilibrium concentrations (mol/L)

[N₂O₄]_eq = 0.0271 [NO₂]_eq = 0.0125

↓ P = [c] × RT (R = 0.08206 L·atm·mol⁻¹·K⁻¹, RT = 24.45 L·atm·mol⁻¹)

② Partial pressures (atm)

P(N₂O₄) = 0.0271 × 24.45 = 0.6621

P(NO₂) = 0.0125 × 24.45 = 0.3059

↓ K_p = P(NO₂)² / P(N₂O₄)

③ K_p (atm)

K_p = (0.3059)² / 0.6621 = 0.09357 / 0.6621 = 0.1414 atm ← only T can shift this

④ Verify via K_c

K_c = [NO₂]² / [N₂O₄] = 0.005782 mol/L → K_p = K_c × RT = 0.005782 × 24.45 = 0.1414 atm ✓

Equilibrium Constant

K_p = 0.1414

atm

temperature-dependent only

Partial Pressures at Equilibrium

N₂O₄

0.662 atm

NO₂

0.306 atm

P_total = 0.968 atm

Dissociation α

18.8%

of N₂O₄ converted to NO₂

Drag the sliders — only temperature changes K_p; volume and concentration just shift the equilibrium position.

Electron Orbital Viewer

Point cloud weighted by |ψ(r)|² — denser regions have higher electron probability.
Hydrogen atom s-orbitals (a₀ = 1). Drag to rotate · scroll to zoom.

drag to rotate · scroll to zoom

1s orbital — lowest-energy state, spherically symmetric. Electron density peaks at the nucleus and decays exponentially outward. No radial nodes.

Electron Configuration Builder

Add electrons one at a time · Aufbau principle · Hund's rule applied within each sublevel
Covers elements H → Zn (Z = 1–30) · 4s fills before 3d per Aufbau order

— Z = 0

0 / 30 electrons

Electron Configuration

—

Chromium & Copper: Aufbau Exceptions

Cr (Z=24) and Cu (Z=29) deviate from the Aufbau principle because half-filled and fully-filled d subshells are extra stable.

Cr

Chromium Z = 24

Group 6 · Period 4 · [Ar] core = 1s² 2s² 2p⁶ 3s² 3p⁶

Aufbau predicts

[Ar] 4s² 3d⁴

4s

↑↓

3d

↑

↑

↑

↑

→

Actual

[Ar] 4s¹ 3d⁵

4s

↑

3d

↑

↑

↑

↑

↑

⚡ The 3d subshell is half-filled (3d⁵ — one electron per orbital, all spin-up). This maximises exchange energy and minimises electron-electron repulsion, making 4s¹ 3d⁵ lower in energy than the predicted 4s² 3d⁴. One electron is effectively "promoted" from 4s into the empty 3d orbital.

Cu

Copper Z = 29

Group 11 · Period 4 · [Ar] core = 1s² 2s² 2p⁶ 3s² 3p⁶

Aufbau predicts

[Ar] 4s² 3d⁹

4s

↑↓

3d

↑↓

↑↓

↑↓

↑↓

↑

→

Actual

[Ar] 4s¹ 3d¹⁰

4s

↑

3d

↑↓

↑↓

↑↓

↑↓

↑↓

⚡ The 3d subshell is fully filled (3d¹⁰). A completely filled d subshell is particularly stable. The atom reaches it by promoting one 4s electron into the last 3d orbital, giving 4s¹ 3d¹⁰ instead of the predicted 4s² 3d⁹.

Key takeaway

Both exceptions involve a one-electron shift from 4s → 3d to achieve an especially stable d subshell (half-filled 3d⁵ for Cr, fully-filled 3d¹⁰ for Cu). The same pattern recurs across the periodic table: Mo (Z=42), Ag (Z=47), Au (Z=79), and others. These exceptions are a frequent exam topic in AP, IB, and university chemistry.

Interactive Atomic Model

Oxygen-16 · 8 protons · 8 neutrons · 8 electrons · click any particle to inspect it

8O15.999

Shell 12e⁻Shell 26e⁻

Drag to turn · click a particle · click empty space for element info

Atomic number8 Mass number16 Overall charge0 Shell arrangement2, 6 Electron configuration1s² 2s² 2p⁴

SN1 vs SN2 Rate Simulator

Compare how substrate structure, concentration, nucleophile and solvent change the relative reaction rate.

R–X + Nu⁻ → R–Nu + X⁻

Substrate structure Solvent Nucleophile [R–X] concentration 1.0 mol dm⁻³ [Nu⁻] concentration 1.0 mol dm⁻³

SN1rate = k[R–X]

The slow step forms a carbocation, so nucleophile concentration is absent from the rate law.

SN2rate = k[R–X][Nu⁻]

Substrate and nucleophile collide in one slow step, so both concentrations affect the rate.

SN1

0.12

SN2

0.20

Relative rate on a shared scale

SN1 · two stepsR–X → R⁺ + X⁻slowR⁺ + Nu → R–Nu

SN2 · one stepNu⁻ + R–X → [transition state] → R–Nu + X⁻concerted

Rates are relative teaching values, not experimental rate constants.

Interactive calculation tools

Chemistry Calculators

Choose a syllabus unit, enter your values, and see every step of the calculation.

A2 · 1 calculator

Unit Formulae & EquationsGuided balancing

01

Guided Equation Balancer

Enter each chemical separately, then adjust only the coefficients until every element has equal atom totals.

Your equation · 自选方程式

Enter your own balancing problem

Type each reactant and product into its own Chemical box. Keep every coefficient in the separate Coefficient box.

Or load a guided example:

1Enter chemicalsFormulae only—no large numbers
2Adjust coefficientsUse the number controls
3Check and improveCompare each atom total

Reactants 反应物

→reacts to form

Products 生成物

◎

Build the equation first

Choose an example or type chemical formulae into the boxes.

Formula tips: use normal numbers for subscripts, for example H2O, Ca(OH)2 or Al2(SO4)3. Do not type a coefficient inside the chemical box.

Unit Acids & Bases Buffers and pH

01

Henderson–Hasselbalch Calculator

Find the pH of a buffer containing a weak acid and its conjugate base.

pH = pK_a + log₁₀ [A⁻][HA]

pK_a Acid dissociation value Weak acid, [HA] mol dm⁻³ Equilibrium concentration Conjugate base, [A⁻] mol dm⁻³ Equilibrium concentration

Calculated buffer pH 4.76

4.76 + log₁₀(0.20 ÷ 0.20) = 4.76

0 · acidic7 · neutral14 · alkaline

pH

Base : acid ratio = 1.00

Equal concentrations mean pH = pK_a.

Use concentrations in the same units. This equation applies to a buffer made from a weak acid and its conjugate base.

Self-test · 自我测试

Chemistry Quiz

Choose a level and topic to practise.

IGCSE · 40 topics · 200 questions

Edexcel International GCSE

Choose a topic

Class code 班级代码（可选）

Find a topic · 搜索专题

Your results

Saved on this device.

Interactive Periodic Table

Select any element to view its atomic data and electron configuration.

Find an element

Chemistry language · 化学词汇

Vocabulary

English terms, Chinese meanings and examples.

Search vocabulary

IGCSE Chemistry

Quick practice · 配对练习

Connect the meaning

Choose a term, then its matching definition. Five pairs per round. · 先选术语，再选对应定义。

0 / 5 matched

Level Unit

Choose a term to begin. · 先选一个术语。

Teacher resource library

Materials

Download assessments and revision guides.

1 resource

Practice assessment

Chemistry Exam 1 - Form A

Seven-page chemistry assessment covering solution equilibria, acids, bases and related calculations.

7 pagesPDF656 KB

Open PDF Download