"total hardness" meaning in English

{
  "forms": [
    {
      "form": "total hardnesses",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "total hardness (plural total hardnesses)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Inorganic chemistry",
          "orig": "en:Inorganic chemistry",
          "parents": [
            "Chemistry",
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "The sum of calcium and magnesium ion concentrations in water, normally expressed in mmol/L"
      ],
      "id": "en-total_hardness-en-noun-w7dZgXC5",
      "links": [
        [
          "inorganic chemistry",
          "inorganic chemistry"
        ],
        [
          "calcium",
          "calcium"
        ],
        [
          "magnesium",
          "magnesium"
        ],
        [
          "water",
          "water"
        ]
      ],
      "raw_glosses": [
        "(inorganic chemistry) The sum of calcium and magnesium ion concentrations in water, normally expressed in mmol/L"
      ],
      "topics": [
        "chemistry",
        "inorganic-chemistry",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "total hardness"
}

{
  "forms": [
    {
      "form": "total hardnesses",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "total hardness (plural total hardnesses)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "en:Inorganic chemistry"
      ],
      "glosses": [
        "The sum of calcium and magnesium ion concentrations in water, normally expressed in mmol/L"
      ],
      "links": [
        [
          "inorganic chemistry",
          "inorganic chemistry"
        ],
        [
          "calcium",
          "calcium"
        ],
        [
          "magnesium",
          "magnesium"
        ],
        [
          "water",
          "water"
        ]
      ],
      "raw_glosses": [
        "(inorganic chemistry) The sum of calcium and magnesium ion concentrations in water, normally expressed in mmol/L"
      ],
      "topics": [
        "chemistry",
        "inorganic-chemistry",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "total hardness"
}