Advanced

Tutor model

i62 💰:9.4

i52 💰:7.9

🏆 i67 💰:62.5

i70 💰:125.0

i68 💰:125.0

🏆 i66 💰:75.0

i65 💰:125.0

i63 💰:62.5

i61 💰:75.0

i60 💰:9.4

i60 💰:37.5

i58 💰:62.5

i57 💰:22.5

i57 💰:37.5

i56 💰:45.0

⚡ i55 💰:11.25

i55 💰:37.5

i54 💰:62.5

i52 💰:35.0

i51 💰:37.5

i49 💰:15.0

i49 💰:62.5

i48 💰:11.3

i47 💰:7.5

i44 💰:25.0

i43 💰:37.5

i42 💰:187.5

i40 💰:5.0

i39 💰:100.0

i38 💰:37.5

i37 💰:11.0

i36 💰:187.5

i34 💰:12.5

i33 💰:37.5

i30 💰:1.0

i29 💰:1.5

i28 💰:150.0

i23 💰:20.0

i20 💰:4.0

i18 💰:1.0

i16 💰:25.0

i13 💰:1.5

Keep session private

What would you like to learn?

Override for the tutor's lesson content generation calls. Auto uses the system default (Claude Sonnet 5 for lesson text, Opus 5 for diagrams).

Lesson generation model

System default

i58 💰:18.8

i31 💰:11.0

i25 💰:31.3

Tutoring abilities

When stomach acid causes painful heartburn, you swallow an antacid tablet to cancel out the burning acid.

Any substance that chemically cancels out or neutralizes an acid is called a **base**.

So what actually makes a substance a base, and how does it react with an acid?

In 1754, French chemist Guillaume-François Rouelle coined the term **base** to describe solid substances that serve as a foundation, reacting with liquid acids to form neutral crystals called salts.

A **base** is chemically defined as any substance that reacts with an acid to produce a **salt and water**.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      color: #1e2945;
      background: #ffffff;
      padding: 12px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: flex-start;
      gap: 12px;
      min-height: 100vh;
      user-select: none;
    }

    .container {
      width: 100%;
      max-width: 480px;
      display: flex;
      flex-direction: column;
      gap: 12px;
    }

    .header-row {
      width: 100%;
      display: flex;
      justify-content: space-between;
      align-items: center;
      gap: 8px;
    }

    .controls {
      display: flex;
      gap: 4px;
      background: #f1f5f9;
      padding: 3px;
      border-radius: 8px;
    }

    .btn-step {
      border: none;
      background: transparent;
      color: #64748b;
      font-size: 12px;
      font-weight: 600;
      padding: 5px 9px;
      border-radius: 6px;
      cursor: pointer;
      transition: all 0.2s ease;
    }

    .btn-step.active {
      background: #ffffff;
      color: #1e2945;
      box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    }

    .btn-play {
      border: 1px solid #e2e8f0;
      background: #ffffff;
      color: #1e2945;
      width: 28px;
      height: 28px;
      border-radius: 6px;
      cursor: pointer;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 12px;
    }

    /* Stage Area */
    .stage {
      position: relative;
      width: 100%;
      height: 180px;
      display: flex;
      justify-content: space-between;
      align-items: flex-end;
      padding: 30px 10px 10px 10px;
      overflow: visible;
    }

    /* Beakers */
    .beaker-wrapper {
      position: relative;
      width: 72px;
      height: 95px;
      z-index: 5;
      transition: transform 0.5s cubic-bezier(0.4, 0, 0.2, 1), opacity 0.4s ease;
    }

    .beaker-label {
      position: absolute;
      top: -24px;
      left: 50%;
      transform: translateX(-50%);
      font-size: 12px;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.5px;
      white-space: nowrap;
      z-index: 10;
    }

    .label-base { color: #b45309; }
    .label-acid { color: #e11d48; }

    .beaker {
      width: 100%;
      height: 100%;
      border: 2px solid #94a3b8;
      border-top: none;
      border-radius: 0 0 10px 10px;
      background: rgba(248, 250, 252, 0.85);
      display: flex;
      flex-direction: column;
      justify-content: flex-end;
      position: relative;
      overflow: hidden;
    }

    .liquid-base {
      width: 100%;
      height: 58px;
      background: rgba(254, 240, 138, 0.6);
      border-radius: 0 0 8px 8px;
      display: flex;
      flex-wrap: wrap;
      align-content: center;
      justify-content: center;
      gap: 3px;
      padding: 3px;
      transition: height 0.4s ease;
    }

    .liquid-acid {
      width: 100%;
      height: 58px;
      background: rgba(254, 205, 211, 0.6);
      border-radius: 0 0 8px 8px;
      display: flex;
      flex-wrap: wrap;
      align-content: center;
      justify-content: center;
      gap: 3px;
      padding: 3px;
      transition: height 0.4s ease;
    }

    /* Reaction Chamber */
    .chamber-wrapper {
      position: relative;
      width: 125px;
      height: 125px;
      z-index: 2;
    }

    .chamber-label {
      position: absolute;
      top: -24px;
      left: 50%;
      transform: translateX(-50%);
      font-size: 11px;
      font-weight: 700;
      color: #64748b;
      white-space: nowrap;
      text-transform: uppercase;
      letter-spacing: 0.5px;
    }

    .chamber {
      width: 100%;
      height: 100%;
      border: 2px dashed #cbd5e1;
      border-top: none;
      border-radius: 0 0 16px 16px;
      display: flex;
      flex-direction: column;
      justify-content: flex-end;
      background: #f8fafc;
      transition: border 0.3s ease;
      overflow: hidden;
    }

    .chamber-liquid {
      width: 100%;
      height: 0;
      background: #e0f2fe;
      border-radius: 0 0 14px 14px;
      transition: height 0.5s ease, background-color 0.4s ease;
      display: flex;
      flex-wrap: wrap;
      align-content: center;
      justify-content: center;
      gap: 4px;
      padding: 4px;
      overflow: hidden;
    }

    /* Particle Tags */
    .particle {
      font-size: 10px;
      font-weight: 700;
      padding: 2px 5px;
      border-radius: 10px;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      box-shadow: 0 1px 2px rgba(0,0,0,0.06);
      white-space: nowrap;
    }

    .p-cu { background: #fed7aa; color: #9a3412; }
    .p-o  { background: #fef08a; color: #854d0e; }
    .p-h  { background: #fecdd3; color: #be123c; }
    .p-cl { background: #bbf7d0; color: #15803d; }
    .p-h2o { background: #bae6fd; color: #0369a1; border: 1px solid #7dd3fc; }
    .p-salt { background: #e2e8f0; color: #334155; border: 1px solid #94a3b8; }

    /* Flow Streams */
    .stream {
      position: absolute;
      top: 36px;
      width: 4px;
      height: 48px;
      border-radius: 2px;
      opacity: 0;
      transition: opacity 0.3s ease;
      z-index: 4;
      pointer-events: none;
    }
    .stream-base {
      left: 88px;
      background: linear-gradient(to bottom, #fde047, #38bdf8);
      transform: rotate(35deg);
    }
    .stream-acid {
      right: 88px;
      background: linear-gradient(to bottom, #fda4af, #38bdf8);
      transform: rotate(-35deg);
    }

    /* Equations Section */
    .eq-box {
      width: 100%;
      background: #f8fafc;
      border: 1px solid #e6e6e6;
      border-radius: 10px;
      padding: 10px 12px;
      display: flex;
      flex-direction: column;
      gap: 6px;
    }

    .eq-row {
      display: flex;
      align-items: center;
      justify-content: space-between;
      font-size: 13px;
      font-weight: 600;
      color: #1e2945;
    }

    .eq-item {
      padding: 3px 8px;
      border-radius: 6px;
      transition: all 0.3s ease;
      text-align: center;
    }

    .eq-arrow {
      color: #94a3b8;
      font-weight: 700;
      padding: 0 4px;
    }

    .eq-chem {
      font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
      font-size: 12.5px;
      color: #475569;
    }

    /* Step states */
    .step-1 .beaker-wrap-base { transform: translate(0, 0) rotate(0deg); opacity: 1; }
    .step-1 .beaker-wrap-acid { transform: translate(0, 0) rotate(0deg); opacity: 1; }
    .step-1 .chamber-liquid { height: 0; }
    .step-1 .eq-reactants { background: rgba(34, 183, 255, 0.15); color: #0284c7; }

    .step-2 .beaker-wrap-base {
      transform: translate(32px, -38px) rotate(48deg);
      transform-origin: top right;
      opacity: 0.95;
    }
    .step-2 .beaker-wrap-acid {
      transform: translate(-32px, -38px) rotate(-48deg);
      transform-origin: top left;
      opacity: 0.95;
    }
    .step-2 .liquid-base { height: 20px; }
    .step-2 .liquid-acid { height: 20px; }
    .step-2 .stream { opacity: 0.85; }
    .step-2 .chamber { border: 2px dashed #22b7ff; }
    .step-2 .chamber-liquid { height: 55px; background: #e2e8f0; }
    .step-2 .eq-arrow { color: #22b7ff; }

    .step-3 .beaker-wrap-base { transform: translate(0, 0) rotate(0deg); opacity: 0.35; }
    .step-3 .beaker-wrap-acid { transform: translate(0, 0) rotate(0deg); opacity: 0.35; }
    .step-3 .liquid-base { height: 0; }
    .step-3 .liquid-acid { height: 0; }
    .step-3 .chamber { border: 2px solid #22b7ff; }
    .step-3 .chamber-liquid { height: 75px; background: #e0f2fe; }
    .step-3 .eq-products { background: rgba(34, 183, 255, 0.15); color: #0284c7; }

    @media (max-width: 380px) {
      .stage {
        padding: 26px 4px 6px 4px;
        height: 165px;
      }
      .beaker-wrapper {
        width: 62px;
        height: 85px;
      }
      .chamber-wrapper {
        width: 110px;
        height: 115px;
      }
      .btn-step {
        font-size: 11px;
        padding: 4px 6px;
      }
      .eq-row {
        font-size: 11.5px;
      }
      .eq-chem {
        font-size: 11px;
      }
      .particle {
        font-size: 9px;
        padding: 1px 4px;
      }
      .step-2 .beaker-wrap-base {
        transform: translate(24px, -32px) rotate(45deg);
      }
      .step-2 .beaker-wrap-acid {
        transform: translate(-24px, -32px) rotate(-45deg);
      }
      .stream-base { left: 74px; }
      .stream-acid { right: 74px; }
    }
  </style>
</head>
<body class="step-1">

  <div class="container">
    <div class="header-row">
      <div class="controls">
        <button class="btn-step active" id="step0">1. Reactants</button>
        <button class="btn-step" id="step1">2. Reacting</button>
        <button class="btn-step" id="step2">3. Products</button>
      </div>
      <button class="btn-play" id="playBtn" title="Play / Pause">⏸</button>
    </div>

    <div class="stage">
      
      <div class="beaker-wrapper beaker-wrap-base">
        <span class="beaker-label label-base">Base</span>
        <div class="beaker">
          <div class="liquid-base">
            <span class="particle p-cu">Cu²⁺</span>
            <span class="particle p-o">O²⁻</span>
          </div>
        </div>
      </div>

      <div class="stream stream-base"></div>

      
      <div class="chamber-wrapper">
        <span class="chamber-label">Reaction Chamber</span>
        <div class="chamber" id="chamber">
          <div class="chamber-liquid" id="chamberContent"></div>
        </div>
      </div>

      <div class="stream stream-acid"></div>

      
      <div class="beaker-wrapper beaker-wrap-acid">
        <span class="beaker-label label-acid">Acid</span>
        <div class="beaker">
          <div class="liquid-acid">
            <span class="particle p-h">H⁺</span>
            <span class="particle p-h">H⁺</span>
            <span class="particle p-cl">Cl⁻</span>
            <span class="particle p-cl">Cl⁻</span>
          </div>
        </div>
      </div>
    </div>

    
    <div class="eq-box">
      <div class="eq-row">
        <span class="eq-item eq-reactants">Base + Acid</span>
        <span class="eq-arrow">→</span>
        <span class="eq-item eq-products">Salt + Water</span>
      </div>
      <div class="eq-row eq-chem">
        <span class="eq-item eq-reactants">CuO + 2HCl</span>
        <span class="eq-arrow">→</span>
        <span class="eq-item eq-products">CuCl₂ + H₂O</span>
      </div>
    </div>
  </div>

  <script>
    const steps = ['step-1', 'step-2', 'step-3'];
    let currentStep = 0;
    let timer = null;
    let isPlaying = true;

    const btns = [
      document.getElementById('step0'),
      document.getElementById('step1'),
      document.getElementById('step2')
    ];
    const playBtn = document.getElementById('playBtn');
    const chamberContent = document.getElementById('chamberContent');

    function renderStage(idx) {
      document.body.className = steps[idx];
      btns.forEach((b, i) => b.classList.toggle('active', i === idx));

      if (idx === 0) {
        chamberContent.innerHTML = '';
      } else if (idx === 1) {
        chamberContent.innerHTML = `
          <span class="particle p-cu">Cu²⁺</span>
          <span class="particle p-o">O²⁻</span>
          <span class="particle p-h">H⁺</span>
          <span class="particle p-h">H⁺</span>
          <span class="particle p-cl">Cl⁻</span>
        `;
      } else {
        chamberContent.innerHTML = `
          <span class="particle p-salt">CuCl₂ (Salt)</span>
          <span class="particle p-h2o">H₂O (Water)</span>
        `;
      }
    }

    function setStep(idx) {
      currentStep = idx;
      renderStage(currentStep);
    }

    function nextStep() {
      currentStep = (currentStep + 1) % steps.length;
      renderStage(currentStep);
    }

    function startAuto() {
      isPlaying = true;
      playBtn.textContent = '⏸';
      clearInterval(timer);
      timer = setInterval(nextStep, 3200);
    }

    function stopAuto() {
      isPlaying = false;
      playBtn.textContent = '▶';
      clearInterval(timer);
    }

    btns.forEach((btn, i) => {
      btn.addEventListener('click', () => {
        stopAuto();
        setStep(i);
      });
    });

    playBtn.addEventListener('click', () => {
      if (isPlaying) {
        stopAuto();
      } else {
        nextStep();
        startAuto();
      }
    });

    // Initialize
    renderStage(0);
    startAuto();
  </script>
</body>
</html>

Most bases are metal oxides (like copper oxide, $\text{CuO}$), metal hydroxides, or metal carbonates.

If you stir different bases into water, a big difference appears — some dissolve easily, while others just sit at the bottom. What do we call the ones that dissolve?

An **alkali** is simply a base that dissolves in water — in other words, a **soluble base**.

When an alkali dissolves in water, it releases **hydroxide ions** ($\text{OH}^-$), which give the resulting solution a pH greater than 7.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: #ffffff;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      padding: 12px;
      user-select: none;
    }

    .controls {
      display: flex;
      gap: 6px;
      margin-bottom: 8px;
      width: 100%;
      max-width: 480px;
      justify-content: center;
    }

    .tab-btn {
      flex: 1;
      padding: 6px 10px;
      font-size: 13px;
      font-weight: 600;
      border: 1px solid #cbd5e1;
      border-radius: 8px;
      background: #f8fafc;
      color: #475569;
      cursor: pointer;
      text-align: center;
      transition: all 0.2s ease;
    }

    .tab-btn.active {
      background: #0284c7;
      color: #ffffff;
      border-color: #0284c7;
      box-shadow: 0 2px 4px rgba(2, 132, 199, 0.25);
    }

    .diagram-stage {
      position: relative;
      width: 100%;
      max-width: 440px;
      aspect-ratio: 1.1 / 1;
      margin: auto 0;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    /* Outer Circle: Bases */
    .circle-bases {
      position: absolute;
      inset: 2%;
      background: #f1f5f9;
      border: 2px solid #94a3b8;
      border-radius: 50%;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: flex-start;
      padding-top: 14px;
      transition: opacity 0.3s ease, border-color 0.3s ease, background-color 0.3s ease;
    }

    .bases-header {
      text-align: center;
      line-height: 1.2;
    }

    .bases-title {
      font-size: 15px;
      font-weight: 700;
      color: #334155;
      letter-spacing: -0.2px;
    }

    .bases-subtitle {
      font-size: 12px;
      color: #64748b;
      font-weight: 500;
    }

    /* Insoluble examples placed in upper region of Bases circle */
    .insoluble-group {
      display: flex;
      flex-wrap: wrap;
      justify-content: center;
      gap: 6px;
      margin-top: 10px;
      padding: 0 16px;
      z-index: 2;
      transition: opacity 0.3s ease;
    }

    .chip {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      border-radius: 6px;
      padding: 3px 8px;
      font-size: 12px;
      font-weight: 600;
      color: #334155;
      white-space: nowrap;
      box-shadow: 0 1px 2px rgba(0,0,0,0.04);
    }

    .chip sub {
      font-size: 9px;
      vertical-align: sub;
    }

    .chip .tag {
      font-size: 10px;
      color: #64748b;
      font-weight: 500;
      margin-left: 2px;
    }

    /* Inner Nested Circle: Alkalis */
    .circle-alkalis {
      position: absolute;
      bottom: 12px;
      width: 78%;
      height: 56%;
      background: #e0f2fe;
      border: 2px solid #0284c7;
      border-radius: 50%;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      padding: 8px 12px 14px 12px;
      z-index: 3;
      transition: all 0.3s ease;
    }

    .alkalis-title {
      font-size: 14px;
      font-weight: 700;
      color: #0369a1;
      line-height: 1.2;
    }

    .alkalis-subtitle {
      font-size: 11px;
      color: #0284c7;
      font-weight: 600;
      margin-bottom: 6px;
    }

    .soluble-group {
      display: flex;
      flex-wrap: wrap;
      justify-content: center;
      gap: 5px;
    }

    .chip-alkali {
      background: #ffffff;
      border: 1px solid #7dd3fc;
      color: #0369a1;
      border-radius: 6px;
      padding: 3px 7px;
      font-size: 11px;
      font-weight: 600;
      box-shadow: 0 1px 2px rgba(2, 132, 199, 0.1);
    }

    .chip-alkali sub {
      font-size: 9px;
      vertical-align: sub;
    }

    /* Dimming styles based on active filter */
    .stage-alkalis-only .insoluble-group,
    .stage-alkalis-only .bases-header {
      opacity: 0.25;
    }

    .stage-insoluble-only .circle-alkalis {
      opacity: 0.2;
    }

    /* Caption footer */
    .caption-box {
      margin-top: 8px;
      padding: 8px 14px;
      background: #f8fafc;
      border: 1px solid #e2e8f0;
      border-radius: 8px;
      text-align: center;
      width: 100%;
      max-width: 480px;
    }

    .caption-text {
      font-size: 13px;
      font-weight: 700;
      color: #0f172a;
    }

    .caption-sub {
      font-size: 11px;
      color: #64748b;
      margin-top: 2px;
    }
  </style>
</head>
<body>

  <div class="controls">
    <button class="tab-btn active" data-mode="all" onclick="setMode('all')">All Bases</button>
    <button class="tab-btn" data-mode="insoluble" onclick="setMode('insoluble')">Insoluble Bases</button>
    <button class="tab-btn" data-mode="alkalis" onclick="setMode('alkalis')">Alkalis (Soluble)</button>
  </div>

  <div id="stage" class="diagram-stage">
    
    <div class="circle-bases">
      <div class="bases-header">
        <div class="bases-title">Bases</div>
        <div class="bases-subtitle">Neutralise acids</div>
      </div>

      
      <div class="insoluble-group">
        <div class="chip">CuO <span class="tag">(solid)</span></div>
        <div class="chip">Fe<sub>2</sub>O<sub>3</sub> <span class="tag">(solid)</span></div>
        <div class="chip">CaCO<sub>3</sub> <span class="tag">(solid)</span></div>
      </div>

      
      <div class="circle-alkalis">
        <div class="alkalis-title">Alkalis</div>
        <div class="alkalis-subtitle">Soluble bases (form OH⁻)</div>
        <div class="soluble-group">
          <div class="chip-alkali">NaOH <span style="font-size:10px; opacity:0.8;">(aq)</span></div>
          <div class="chip-alkali">KOH <span style="font-size:10px; opacity:0.8;">(aq)</span></div>
          <div class="chip-alkali">Ca(OH)<sub>2</sub> <span style="font-size:10px; opacity:0.8;">(aq)</span></div>
        </div>
      </div>
    </div>
  </div>

  <div class="caption-box">
    <div class="caption-text">All alkalis are bases, but not all bases are alkalis.</div>
    <div id="subtext" class="caption-sub">An alkali is simply a base that dissolves in water.</div>
  </div>

  <script>
    function setMode(mode) {
      const stage = document.getElementById('stage');
      const subtext = document.getElementById('subtext');
      const buttons = document.querySelectorAll('.tab-btn');

      buttons.forEach(btn => {
        btn.classList.toggle('active', btn.getAttribute('data-mode') === mode);
      });

      stage.className = 'diagram-stage';

      if (mode === 'insoluble') {
        stage.classList.add('stage-insoluble-only');
        subtext.textContent = 'Insoluble metal oxides & carbonates do not dissolve in water.';
      } else if (mode === 'alkalis') {
        stage.classList.add('stage-alkalis-only');
        subtext.textContent = 'Alkalis dissolve in water to release hydroxide ions (OH⁻).';
      } else {
        subtext.textContent = 'An alkali is simply a base that dissolves in water.';
      }
    }
  </script>
</body>
</html>

This means that while every alkali can neutralize an acid, only the bases that dissolve in water earn the special title of **alkali**.

How can you tell whether a mystery substance in the lab is an insoluble base or an alkali?

## Worked Example: Classifying Mystery Bases

Suppose you test two solid compounds: black copper(II) oxide ($\text{CuO}$) and white sodium hydroxide pellets ($\text{NaOH}$).

Both react with hydrochloric acid to form water and a salt, confirming that both are bases.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Insoluble Base vs Soluble Alkali</title>
  <style>
    * {
      box-sizing: border-box;
      margin: 0;
      padding: 0;
    }

    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      color: #1e2945;
      background: transparent;
      padding: 12px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      min-height: 100%;
    }

    .main-grid {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 20px;
      width: 100%;
      max-width: 520px;
    }

    /* Stack vertically on narrow mobile screens to avoid cramped containers */
    @media (max-width: 420px) {
      .main-grid {
        grid-template-columns: 1fr;
        gap: 16px;
        max-width: 260px;
      }
    }

    .beaker-card {
      display: flex;
      flex-direction: column;
      align-items: center;
      background: transparent;
      padding: 4px;
    }

    .formula-tag {
      font-size: 13px;
      font-weight: 700;
      color: #1e2945;
      margin-bottom: 6px;
      text-align: center;
    }

    .beaker-container {
      position: relative;
      width: 100%;
      max-width: 155px;
      height: 150px;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    svg.beaker-svg {
      width: 100%;
      height: 100%;
      overflow: visible;
    }

    .status-badge {
      margin-top: 8px;
      text-align: center;
      line-height: 1.35;
    }

    .badge-title {
      font-size: 14px;
      font-weight: 700;
      display: block;
    }

    .badge-sub {
      font-size: 12px;
      font-weight: 600;
      display: inline-block;
      margin-top: 2px;
      padding: 2px 8px;
      border-radius: 6px;
    }

    .insoluble-title {
      color: #334155;
    }
    .insoluble-sub {
      color: #475569;
      background: #f1f5f9;
    }

    .alkali-title {
      color: #6b21a8;
    }
    .alkali-sub {
      color: #6b21a8;
      background: #f3e8ff;
    }

    .ion-label {
      font-size: 11px;
      font-weight: 700;
      font-family: system-ui, sans-serif;
    }
  </style>
</head>
<body>

  <div class="main-grid">
    
    <div class="beaker-card">
      <div class="formula-tag">CuO (Copper oxide)</div>
      <div class="beaker-container">
        <svg class="beaker-svg" viewBox="0 0 160 160">
          
          <path d="M 22 25 L 30 25 L 30 142 Q 30 152 42 152 L 118 152 Q 130 152 130 142 L 130 25 L 138 25" 
                fill="none" stroke="#94a3b8" stroke-width="3" stroke-linecap="round"/>
          
          
          <path d="M 33 65 Q 80 63 127 65 L 127 140 Q 127 149 118 149 L 42 149 Q 33 149 33 140 Z" 
                fill="#e0f2fe" opacity="0.85"/>
          <path d="M 33 65 Q 80 62 127 65" fill="none" stroke="#bae6fd" stroke-width="2"/>

          
          <text x="80" y="94" text-anchor="middle" fill="#0284c7" font-size="11.5" font-weight="600">Water</text>

          
          <path d="M 40 148 C 52 136, 70 134, 80 136 C 90 134, 108 136, 120 148 Z" fill="#1e293b"/>
          <circle cx="54" cy="143" r="2.5" fill="#0f172a"/>
          <circle cx="68" cy="140" r="2.5" fill="#334155"/>
          <circle cx="82" cy="139" r="3" fill="#0f172a"/>
          <circle cx="96" cy="141" r="2.5" fill="#334155"/>
          <circle cx="106" cy="144" r="2.5" fill="#0f172a"/>

          
          <rect x="34" y="112" width="92" height="18" rx="4" fill="#ffffff" stroke="#cbd5e1" stroke-width="1"/>
          <text x="80" y="125" text-anchor="middle" fill="#334155" font-size="9.5" font-weight="700">Solid stays undissolved</text>
        </svg>
      </div>
      <div class="status-badge">
        <span class="badge-title insoluble-title">Insoluble Base</span>
        <span class="badge-sub insoluble-sub">Not an Alkali</span>
      </div>
    </div>

    
    <div class="beaker-card">
      <div class="formula-tag">NaOH (Sodium hydroxide)</div>
      <div class="beaker-container">
        <svg class="beaker-svg" viewBox="0 0 160 160">
          
          <path d="M 22 25 L 30 25 L 30 142 Q 30 152 42 152 L 118 152 Q 130 152 130 142 L 130 25 L 138 25" 
                fill="none" stroke="#94a3b8" stroke-width="3" stroke-linecap="round"/>
          
          
          <path d="M 33 65 Q 80 63 127 65 L 127 140 Q 127 149 118 149 L 42 149 Q 33 149 33 140 Z" 
                fill="#d8b4fe" opacity="0.6"/>
          <path d="M 33 65 Q 80 62 127 65" fill="none" stroke="#a855f7" stroke-width="2"/>

          
          <g transform="translate(80, 77)">
            <rect x="-57" y="-9" width="114" height="18" rx="9" fill="#ffffff" stroke="#9333ea" stroke-width="1.2"/>
            <text x="0" y="3.5" text-anchor="middle" fill="#581c87" font-size="9.5" font-weight="700">Turns Purple (pH 14)</text>
          </g>

          
          
          <g transform="translate(56, 105)">
            <circle cx="0" cy="0" r="12" fill="#7c3aed"/>
            <text x="0" y="4" text-anchor="middle" fill="#ffffff" class="ion-label">OH⁻</text>
          </g>

          
          <g transform="translate(104, 105)">
            <circle cx="0" cy="0" r="11" fill="#2563eb"/>
            <text x="0" y="4" text-anchor="middle" fill="#ffffff" class="ion-label">Na⁺</text>
          </g>

          
          <g transform="translate(58, 134)">
            <circle cx="0" cy="0" r="11" fill="#2563eb"/>
            <text x="0" y="4" text-anchor="middle" fill="#ffffff" class="ion-label">Na⁺</text>
          </g>

          
          <g transform="translate(102, 134)">
            <circle cx="0" cy="0" r="12" fill="#7c3aed"/>
            <text x="0" y="4" text-anchor="middle" fill="#ffffff" class="ion-label">OH⁻</text>
          </g>
        </svg>
      </div>
      <div class="status-badge">
        <span class="badge-title alkali-title">Soluble Base = Alkali</span>
        <span class="badge-sub alkali-sub">Releases OH⁻ ions</span>
      </div>
    </div>
  </div>

</body>
</html>

Because $\text{NaOH}$ dissolves and frees $\text{OH}^-$ ions, it is an **alkali**, while the insoluble $\text{CuO}$ remains strictly an **insoluble base**.

Why do many students lose points when asked to measure the pH of a solid base in water?

A common mistake is assuming that adding any base to pure water will immediately create an alkaline solution with a high pH.

If a base is insoluble (like copper oxide), it cannot release hydroxide ions into the water, so the surrounding water stays at pH 7 — even though the solid itself will still neutralize an acid.

science

Defining Acids, Bases, and Soluble Alkalis. Define a base as a substance that neutralises an acid, and an alkali as a soluble base.

GCSE Chemistry: Acids, Bases and Salts (AQA/Edexcel/OCR)

Acids, Alkalis and Neutralisation - KS4 Chemistry

GCSE Chemistry Revision: Acids, Alkalis and Neutralisation

To secure mastery at GCSE Key Stage 4, begin with a hands-on or visual classification activity sorting substances (e.g., $\text{CuO}$, $\text{NaOH}$, $\text{Ca(OH)}_2$, $\text{HCl}$) into acids, insoluble bases, and alkalis. Reinforce the relationship using the nested Venn diagram heuristic ('All alkalis are bases; not all bases are alkalis'). Transition to particle theory by emphasizing the definition of an acid as a source of $\text{H}^+$ and an alkali as a source of $\text{OH}^-$, culminating in the universal ionic equation $\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$.

Defining Acids, Bases, and Soluble Alkalis

What Is a Base?

Bases vs. Alkalis